chemistry hydrogen and its compounds
DESCRIPTION
JEE preparationTRANSCRIPT
INTRODUCTION
Hydrogen is the first element in the periodic table and is also the lightest element known. Its
atomic form exists only at high temperatures. In the normal elemental form, it exists as a diatomic molecule, i.e. .
Unique Position of Hydrogen in the periodic table
A proper position could not be assigned to hydrogen either in the Mendeleev’s periodic table or
Modern periodic table because of the following reason:
In some properties, it resembles alkali metals and in some properties it resembles halogens. So
hydrogen can be placed both in group 1 and group 17 with alkali metals and halogen
respectively.
Resemblance with alkali metals
1. Electronic configuration.
Hydrogen contains one electron in the valence shell like alkali metals H :
Li :
Na :
K :
Rb :
2. Electropositive Character
Like alkali metal, hydrogen also loses its only electron to form hydrogen ion, i.e,
3. Oxidation state
Like alkali metals, hydrogen exhibits an oxidation state of +1 in its compounds.
4. Reducing agent
Alkali metals act as reducing agents because of their tendency to lose valence electron. Hydrogen is also a very good reducing agent as evident from the following reactions:
5. Combination with electronegative elements
Just like alkali metals hydrogen combines with electronegative elements such as halogen, oxygen, sulphur, etc to form compounds with similar formulae
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Difference from Alkali Metals
1. Ionization enthalpy
Ionization enthalpy of hydrogen (1312 kJ mol1) is very high in comparison with the ionization enthalpy of alkali metals.
2. Existence of
It has been established that ion does not exist freely in a aqueous solution. This is because of
the fact that has a very small size as compared to normal atomic and ionic size
(which range from 50 to 220 pm). Thus it exists in aqueous solution in the form of hydrated proton with a formula, . However, for the sake of simplicity hydrated proton is represented by
hydronium ion, .
On the other hand, the alkali metal ions mostly exist as hexahydrated ions.
3. Difference in halides
Hydrogen halides are different from the halides of alkali metals although they have similar molecular formulae. For example(i) Pure HCl is a covalent compound while NaCl is an ionic compound. (ii) HCl is a gaseous compound while NaCl is a solid at ordinary temperature.
Resemblance With Halogens
1. Electronic configuration.
Just like halogens, hydrogen needs one electron to attain the configuration of nearest noble gas.
2. Atomicity.
Like halogens, hydrogen also exists in a diatomic state. The atomicity of hydrogen as well as halogens is two.
3. Electrochemical nature.
During electrolysis of LiH, CaH2, etc, in molten state hydrogen is evolved at the anode indicating its electronegative nature. In this respect, hydrogen shows resemblance with halogens which are also liberated at the anode during electrolysis.
4. Oxidation state.
Just like halogens, hydrogen also exhibit state of -1 in some of its compounds such as metal hydrides.
5. Combination with alkali metals.
Just like halogen, hydrogen also combines with alkali metals to form salts with similar formulae.
6. Combination with non-metals
Just like halogens hydrogen also react with non-metals such as carbon, silicon, germanium, etc, to form covalent compounds.
7. Ionization energy
Ionization energy of hydrogen is comparable to the ionization energies of halogens as shown
below:
Element : H F Cl Br Ionization energies ( ) : 1312 1681 1255 1121
Difference from halogens
1. Less tendency of hydride – formation.
Although hydrogen forms hydride ion (H) like halogens, yet its tendency to form hydride ion is very less in comparison with the halogens. It is quite clear from the fact that halogens form halides with very large number of metals but hydrogen form hydrides with only a small number metals like sodium and calcium, etc.
2. Absence of unshared electrons
There is no unshared pair of electron in hydrogen molecule whereas halogen molecules
have six unshared electron pairs as shown below:
H H Cl Cl F F
3. Nature of oxides.
The oxides of halogen are acidic in nature whereas oxide of hydrogen is neutralH2O
Neutral oxide
Cl2O7,I2O5
Acidic oxides
4. Nature of compounds
The compounds of hydrogen with halogens, i.e. hydrogen halides (HF, HCl, HBr, HI) are low boiling covalent compounds whereas alkali metal halides (LiF, NaCl, KBr, CsI) are high melting ionic solids.
Conclusion
From the above discussion, it is quite evident that there is a marked resemblance in the properties of hydrogen with alkali metals as well as with halogens. Therefore, it is very difficult to place it either with the elements of group 1 or those of group 17. In other words position of hydrogen in the periodic table is anomalous. It is due to this reason hydrogen is some times called rogue element.
ISOTOPES OF HYDROGEN
It has been found by mass spectrograph that hydrogen has three isotopes namely; protium, deuterium and tritium. The relative abundance of three isotopes of hydrogen is as under
(a) Protium or hydrogen
It is represented by the symbol H. Its atomic number is 1 and mass number is also 1. It has one proton (but no neutron) in its nucleus and one electron in its 1s orbital. Naturally occurring hydrogen contains 99.985% of this isotopes.
(b) Deuterium or heavy hydrogen
It is represented by the symbol D or . It’s atomic number is 1 and mass number is 2. It has one
proton and one neutron in its nucleus and one electron in its 1s orbital. Naturally occurring hydrogen has 0.015% of this isotopes mostly in the form of HD.
(c) Tritium
It is represented by the symbol T or . Its nucleus has one proton and two neutrons and there is
one electron in its 1s orbital. It is an extremely rare isotope. Out of molecules of ordinary
hydrogen there is just one molecule of tritium. This isotope of hydrogen is radioactive in nature
and emits low energy - particles
PROTIUM 11H
(Non – Radioactive 99.985%
DUTERIUM 21H
(Non – Radioactive) 0.015%
TRITIUM 31H
(Radioactive) 1510 %
It may be noted that three isotopes of hydrogen have same chemical properties because of the same electronic configuration . However due to different mass numbers they have different rates of chemical reactions. For example, reaction between protium and chlorine is 13.4 times
faster than that between deuterium and chlorine. Similarly electrolysis of ordinary water
occurs more rapidly than of heavy water .Difference in properties arising due to the
difference in mass number is referred to as isotopic effect.
Uses of Tritium
Tritium is used to make thermonuclear devices and for carrying out researches in fusion reactions as a means of producing energy. Tritium gas is usually stored by making , which on heating to
673 K releases .
Illustration 1. The decay product of tritium is
(A) 1H1 (B) 1H2
(C) 2He3 (D) 2He4
Solution: (C)
Exercise 1. Which of the following statements concerning protium, deuterium and tritium is not true?
(A) They are isotopes of each other
(B) They have similar electronic configurations
(C) They exist in the nature in the ratio 1:2:3
(D) Their atomic masses are in the ratio 1:2:3
DIHYDROGEN, H2 MOLECULE.
(i) The term dihydrogen is used for the mixture of diatomic molecules such as and HD containing H and D atoms with respect to their natural abundance.
(ii) For diatomic molecules containing only protium atoms the term used is diprotium.
(iii) For diatomic molecules containing only deuterium atoms , the correct term is
dideuterium.
Occurrence of Hydrogen
Hydrogen is most abundant element in the universe. About half the mass of sun and stars is accounted for by hydrogen. Jupiter and Saturn mostly consist of hydrogen. On earth, it occurs in free state in small traces in volcanic and natural gases. However in combined state it is widely distributed as water.
Preparation of Dihydrogen
Some of the important methods of preparation of dihydrogen are given below:
1. By the action of water on metals.
Various metals react with water at different temperatures to
displace hydrogen from it. For example,
(a) Very active metals like Na, K, Ca react at room temperature to form H2 along with
corresponding hydroxides.
Reaction with these metals is highly vigorous and H2 liberated immediately catches fire which
can cause accident. In order to slow down the reaction, amalgam of these metals with
mercury are used. (b) Metals like Mg, Al, Zn, etc, react with water at boiling temperature to form and
corresponding oxides.
(c) Metals like Fe, Sn, etc, react with steam at high temperature.
The reaction between iron and steam is used for the large scale preparation of dihydrogen by
Lane’s Process.
2. By the action of acids on metals
Metals lying above hydrogen in the activity series displace hydrogen by reaction with dilute HCl or dilute .
In the laboratory, dihydrogen is prepared by the treatment of zinc granules with dil. .
3. By the action of strong alkalies on metals
Metals like Zn, Sn or Al react with strong alkalies such as NaOH or KOH liberating H2 gas
4. Electrolysis of water
Dihydrogen of high degree of purity is prepared by the electrolysis of water in the presence of small amount of acid or base.
Laboratory Preparation of Dihydrogen
In the laboratory, dihydrogen is prepared by the action of diluted sulphuric acid on granulated zinc according to the arrangement shown in the figure
Conc. 2 4H SO
2H gas
Beehive Shelf
Granulated Zinc
Woulf’s Bottle
Thistle Funnel
dil. 2 4H SO
Water
Illustration 2. Why conc. and pure Zn is not used in preparation of dihydrogen?
Solution: (i) Concentrated is not used because it acts as acid as well as oxidizing
agent and consequently gets reduced during the process to liberated
gas
(ii) Pure zinc is not used in the preparation of dihydrogen because it is non-porous in nature, and therefore, the reaction between Zn and is slow.
The presence of impurities, however, makes zinc porous, which helps in
constituting electrochemical couple and speeds up the reaction.
Exercise 2. Hydrogen has three isotopes, the number of possible diatomic molecules
will be
(A) 3 (B) 6
(C) 9 (D) 12
Commercial Preparation of Dihydrogen
From steam and water gas- Lane’s process
Dihydrogen is manufactured by passing alternate currents of steam and water gas over red hot
iron at about 1025-1075 K. The method, in fact, consists of two stages.
(a) Oxidation stage
It involves passing of super heated steam over iron fillings heated to about 1025—1075K. Iron gets oxidized to and in turn it reduces water to dihydrogen gas.
Water gas
Steam
Porous Iron
Furnace
2 2CO +H O
2H gas
(b) Reduction stage.
When whole of iron is oxidized to , the supply of steam is cut off and stream of water gas
is passed so as to cause reduction of to back to iron. Fe3O4 + 2H2 +CO 3Fe + 2H2O + CO2
Water gas The oxidation and reducing periods are alternatively carries out using two or more furnaces.
Bosch’s Process
The maximum quantity of commercial dihydrogen is prepared by this method. It involves the
following steps:
(a) Preparation of water gas.
It is prepared by the action of steam with red hot coke
Water gas also be obtained by action of steam on hydrocarbons in the presence of catalyst. This process is also called steal reforming of hydrocarbons
(b) Separation of Hydrogen.
The water gas formed by any of the above methods, is mixed with steam and is passed over heated and at 770K when CO is oxidized to . Gaseous mixture of and
is then bubbled into cold water under pressure when dissolves leaving behind dihydorgen
gas which escape out.
This reaction is called water gas-shift reaction.
Electrolysis of Water
This process involves the electrolysis of acidified or alkalined water. In general, the electrolysis is
carried out using iron as cathode, nickel coated iron as anode and 15-20% NaOH solution as
electrolyte. The reaction taking places are
Cathode: Reduction of water occurs
Anode: Oxidation of OH occurs
Properties of Dihydrogen
Physical Properties
1. It is colourless, tasteless, odourless gas.
2. It has extremely low solubility in water. Since its molecules are non-polar.
3. Its density is approximately 1/4th that of air. So lightest substance known.
4. It can be liquefied at high pressures and very low temperature.
5. It is highly combustible and should be handled with care.
Chemical Properties
Because of high dissociation energy (435.9 kJ ) dihydrogen is not very reactive. However, it
forms compounds with large number of elements under appropriate experimental conditions.
Some of the chemical properties are:
1. Neutral character
It is neutral to litmus.
2. Combustibility
It is highly combustible gas. It burns in air with a pale blue flame to form water.
3. Reaction with metals.
It reacts with metals like Na, Ca, Li, etc, to form respective hydrides. In such reactions it acquires the oxidation state of -1 and acts as an oxidizing agent.
In case of metals like Pt, Pd, Ni, etc, hydrogen forms interstitial hydrides in which hydrogen atoms occupy the interstitial in the metallic crystals. This property is refereed to as occlusion. The occluded hydrogen can be liberated from the metals by strong heating. 4. Reaction with non-metals
The chemical reaction of dihydrogen non-metals have been summarized as follows:
Carbon :
Halogen :
Dioxygen :
Dinitrogen :
5. Reaction with metal oxide
Reduces such as iron and metal less active than iron.
6. Reaction with carbon monoxide
Dihydrogen reacts with carbon monoxide at 700 K in the presence of a catalyst to
produce methanol.
7. Reaction with unsaturated hydrocarbons
Unsaturated hydrocarbons like ethene and ethyne react with dihydrogen
to form saturated hydrocarbons.
H2C CH2 + H2Ni or Pt
473 KH3C CH3
EthaneEthene
8. Hydrogenation of Vegetable oils. When dihydrogen (under pressure) is passed through edible vegetable oils such as ground – nut or cotton seed oil at about 473 K in presence of finely nickel as a catalyst, they undergo hardening and change into edible fats known as Vanaspati Ghee.
The above process is known as hydrogenation or hardening of oils.
Uses of Dihydrogen
Dihydrogen is used in various industries as listed below:
(i) In the preparation of ammonia which is a starting material for the manufacture of various
fertilizers such as urea, ammonium sulphate, calcium ammonium nitrate, etc.
(ii) In the hydrogenation of vegetable oils.
(iii) In the manufacture of synthetic petrol. (iv) In the oxy-hydrogen torch for welding, if the temperature around 2500C is desired.
(v) In the atomic hydrogen torch for welding, if the temperature around 4000C is desired.
(vi) Liquid hydrogen mixed with liquid oxygen is used as rocket fuel in space programmes.
Illustration 3. The metal which gives hydrogen on treatment with acid as well as sodium
hydroxide is
(A) Fe (B) Zn
(C) Cu (D) none of these
Solution:
Hence (B)
Exercise 3. When electric current is passed through an ionic hydride in the molten
state
(A) Hydrogen is liberated at the anode
(B) Hydrogen is liberated at the cathode
(C) No reaction takes place
(C) Hydride ion migrates towards cathode
HYDRIDES
Dihydrogen combines with a number of elements to form binary compounds called hydrides. Their general formula being where M represents the element and x the number of hydrogen
atoms. Depending upon the physical and chemical properties, the hydrides have been divided
into the following three broad categories:
1. Ionic or salt-like or saline hydrides
2. Metallic or Interstitial hydrides
3. Molecules or Covalent hydrides
Saline Hydrides or Ionic hydrides
These are binary compounds of hydrogen and elements which are more electropositive than
hydrogen such as alkali metals, alkaline earth metals (except Be), etc. Saline hydrides are
formed by the transference of electron from metal to hydrogen. Some common examples of this category are: etc. The general characteristic of these hydrides are as
follows:
(i) They are crystalline solids having white or greyish colour.
(ii) They have high melting and boiling points.
(iii) They have high density and high heat of formation.
(iv) They conduct electricity in molten state liberating dihydrogen gas at anode which confirm the
presence of hydride in then.
At anode:
(v) They react vigorously with water and other protonic solvents such as ethanol and ammonia to
liberate dihydrogen gas. Thus they act as strong bases.
Illustration 4. Ionic hydrides are frequently used to remove traces of water from organic
compounds. What is the underlying basis of this process?
Solution: H is a strong Bronsted base and thus it reacts with water easily.
Covalent Hydrides or Molecular Hydrides
These are binary compounds of hydrogen and elements of comparatively high electronegativity such as p- block elements. In these hydrides, H atoms are bonded to the other atoms by covalent bonds. Some examples of covalent hydrides are, HCl, , , etc. The general formula
of covalent hydrides can be written as where n is the number of outershell electrons of X
atom. However, elements of group 13 are exception to this formula. The elements of group 13 such as B, Ga form polynuclear hydrides which are electron deficient compounds.
, etc, are some examples. Some of the general characteristic of covalent hydrides
are as follows:
(i) These hydrides consist of individual covalent molecules with relatively weak interparticle forces
(Vander waal’s force of attraction). Hence they generally soft, with low melting and boiling
points.
(ii) They are poor conductors of electricity.
(iii) Being covalent in nature, they are more soluble in organic solvents.
(iv) They undergo thermal decomposition into their respective elements.
(v) They are covalent in nature, & are more soluble in organic solvents. (vi) Some of them react with water to liberate .
(vii) Along any given row of periodic table, the covalent hydrides become increasing acidic in
moving from left to right.
Interstitial Hydride or Metallic Hydrides
These are binary compounds of hydrogen and transition elements. These hydrides are generally formed by the (a) transition metals of group 3, 4, 5 of d- block; (b) Cr metal of group 6 and
(c) f – block elements. It may be noted that elements of group 7, 8, 9 of d – block do not form hydrides at all. This inability of metal, of group 7, 8, 9 of periodic table to form hydrides is referred to as hydride gap of d – block. In these compounds H atoms are supposed to occupy interstitial position in the metal lattices. Some scientists consider these compounds as simply solid solutions of hydrogen. The composition of these hydrides may not correspond to simple whole number ratio and therefore, they are also called non-stoichimotric hydrides. Their composition is also found to vary with the conditions of temperature and pressure. Some examples of interestial hydrides of elements of group 3 to 5 are , , , , CrH, , , , VH, NbH, , TaH etc.
Some examples of non-stoichimetric hydrides are
, etc.
Some general characteristics are as follows:
(i) They are generally powders or brittle solids having dark or metallic appearances.
(ii) They are good conductors of electricity. The conductivity, however, decreases with increase in
temperature.
(iii) They have high thermal conductivity.
(iv) Most of these hydrides are harder than parent metals.
(v) They generally undergo reversible decomposition into H2 gas and metal.
Besides three main categories of hydrides some other types of hydrides are also known. Two of
these are described as follows:
(a) Polymeric Hydrides
They are formed by the elements having electronegativity range between 1.4 and 2.0. They
consist of molecules held together in two or three dimensions by hydrogen bridges. Some
common examples are
They are amorphous solids and stable up to 525 K. Above this temperature they begin to evolve
hydrogen gas.
(b) Complex Hydrides
These are the compounds which contain hydride ions co-ordinated to metal atom ions.
Some common examples are (lithium aliminium hydride), (sodium borohydride)
etc. They are generally very good reducing agents.
Exercise 4.
Which of the following is an ionic hydride? (A) (B)
(C) (D)
HYDROGEN ECONOMY
Hydrogen as fuel
Hydrogen is another proposed alternative energy source. Some advantages of hydrogen are:
(i) Hydrogen is aboundantly available in the combined form as water.
(ii) Use of hydrogen as fuel provides pollution free atmosphere because its combination product
is water.
(iii) Time required for regeneration of hydrogen is much shorter as is clear from the following diagram:
Combination Transportation Electrolysis
Water in Rivers, lake
oceans H2
Days and Weeks for redistributionn Water in
Atomsphere
Energy
Fuel
O2 as by product
(iv) An automobile engine burning hydrogen is about 25 to 50% more efficient than an automobile
engine burning gasoline.
(v) Heat of combustion per gram of hydrogen is more than twice that of jet fuel.
(vi) Hydrogen – oxygen fuel cells provide other possibilities of powering motor vehicles.
(vii) Hydrogen is excellent reducing agent and can replace coal in many industrial processes
involving reduction because it produces less atmospheric pollution than carbon.
The changes in out way of life by adopting widespread uses of hydrogen listed above refer to
hydrogen economy.
Obstacles of Hydrogen Economy
Although hydrogen looks as very good future fuel but some of the tough problems must be solved
before we adopt hydrogen economy. The problems are as described below:
(i) Availability.
Hydrogen is not available as much. It does not occur in free state is nature. Therefore, cheap
production of Hydrogen is basic requirement of hydrogen economy. The most likely future source
of hydrogen is water. Hydrogen might be generated at an appropriate site by using solar energy
and then transporting it as fuel.
(ii) Storage and Transportation.
Hydrogen gas has explosive flammability which causes problem to its storage and transportation.
Hydrogen can be stored in vacuum insulated cryogenic tanks (already in use for space
programmes in USA) Liquid hydrogen can be transported by road and rail tankers. It can also be
stored in underground tanks and transported pipelines. Another promising solution to this
problem is the use of Fe-Ti alloy which act like a sponge to absorb hydrogen and results in the
formation of the silvery power. Heating the power safely releases hydrogen gas. The other small storage units are alloys like , , etc. Such storage systems are safer
than storage of hydrogen as gas or liquid.
(iii) Platinum Scarcity
In oxygen-hydrogen fuel cells, a lot of platinum is required as catalyst. In each succeeding year
the demand of platinum exceeds the supply. This will cause problems for fuel cells which are
highly energy source for automobiles.
Uses of Liquid Hydrogen As Fuel
Liquid hydrogen has already been used as rocket fuel. The chemical reaction involved is:
Both reactions H2 and O2 are stored as liquid in separated tanks. The tank hold of
liquid hydrogen. The oxygen tank carries of liquid oxygen. During the “lift off”
operations, these properties power shuttle’s main engine for about 8.5 min. Here, liquid hydrogen
is consumed at the rate of nearly 3000L/sec.
Illustration 5. Activated hydrogen is obtained by
(A) electrolysis of heavy water
(B) reaction of water with heavy metals
(C) thermal decomposition of water
(D) passing silent electric discharge through hydrogen at low pressure
Solution:
Hence (D) is correct answer.WATER
Water is an important hydride of oxygen, which is principal (about 75%) constituent of
earth’s surface. It is most abundant, omnipresent and easily obtainable of all chemical compounds. It is a significant component of animal and vegetable matter and play a vital role in their process. It ranks next to oxygen in importance of out existence. It constitutes about 65% of human body and about 95% by weight of some plants. It can be easily transformed from liquid to solid or from liquid to gaseous states and vice versa.
Distribution of Water
The distribution of water over the earth’s surface is not uniform
The desert regions have no permanent surface water while the oceans cover about 78% of the earth’s surface. They contain 97% of the available water. Out of the total surface water only 2.7% is fresh water and rest is locked in polar ice caps, glaciers or under the ground and is not readily available.
Structure and Aggregation of Water Molecules
In water molecule, the two H atoms are bonded to O atoms by two covalent bonds. The oxygen atom assumes hybrid state. Each of the covalent bonds are formed by the axial overlap of 1s-
orbital of H atom and hybrid orbital of O atom. The two bond pairs and two lone pairs of
electrons around oxygen atom assume tetrahedral arrangement. Consequently, the molecules has a bent structure. Due to relativity greater repulsive interactions of lone pairs the bond angle around the O atom decreases from 109.28 to 104.5 as shown in the figure.
OOH H
Lone Pair
H 104.5
H
O
1.030 (a)
(b) (c)
Polar nature of H2O
Since water molecule has bent shape, therefore bond moments of two O – H bond causes the molecule to behave as permanent electrical dipole. The dipole moment of
molecule has been found to be 1.84 D which confirms its polar nature, as shown in the figure.
Aggregation of Water Molecules
In gaseous state, the individual covalent molecules exist as such. However in liquid state,
large aggregates of units are formed because of their association through intermolecular hydrogen bonds as shown below
O
H
O
H
O
H
H H H
The extent of association, however, depends upon the conditions of temperature and pressure.
The intermolecular hydrogen bonding is responsible for the abnormally high freezing point, boiling
point, heat of fusion and heat of vaporization as compared to the hydrides of the other elements
of oxygen family.
In ice, a solid state of water, each molecule is tetrahedrally surrounded by four neighboring
molecules with their oxygen atoms occupying the corners of tetrahedron. There are four H
atoms around each O atom. Two of the four H atoms are bonded by covalent bonds (bond length 100 pm) whereas the other two are linked through hydrogen bonds (176 pm) as shown in the figure. This gives highly three dimensional structure having large vacant spaces, which may be compared to open cage.
Due to open cage like structure, ice has a relatively larger volume for a given mass of liquid water. Consequently, density of ice is less than water and it floats over water.
As the temperature is raised beyond 273 K, open cage like structure dismentalling due to cleavage of some H-bonds and ice melts. The breaking of H-bond causes aggregation of
molecules to have closer resulting in the decrease in volume and thereby increase in density. This continues till 277 K. When the density becomes maximum. Beyond 277 more and H-bonds cleaves and expansion of liquid water starts occurring due to increased K.E. of molecules and the density again starts decreasing however, it remains higher than ice. Hence density of water is maximum at 277 K.
This property of maximum density at 277 K is a boon for
the survival of aquatic animals during winter months
because when the upper layer of sea water freezes.
The frozen water does not sink to the bottom but keeps
floating at the surface due to its lesser density. This
provides thermal insulation to the water below it.
It is very interesting to note that nine different crystalline
forms of ice have been found to be exist under different
pressure conditions. Each one has different melting point.
At one bar pressure ice has normal hexagonal form.
However at low temperature it adopts cubic form.
OO
O
HH
O
H
H
H
H
H
O
H
H
276pm
176pm
100pm
Properties of Water
Physical properties
Water has some unique features which arise due to intermolecular H-bonding. Some important
physical constants of water are given in table.
Property Magnitude
Molecular mass 18.015
Melting Point (K) 273.2
Boiling Point (K) 373.2
Temperature of maximum Density (K) 277.1
Maximum Density 1.000
Density 0.997
Heat of Vaporisation (373 K)(kJ/ ) 40.66Heat of Fusion (kJ/ ) 6.01
Specific Heat 4.177
Ionization Constant
Some of the important physical properties are discussed below:
(i) The freezing point, boiling point, heat of fusion and heat of vapourization water are abnormally higher than those of the hydrides of the other elements of the same group (16) such as etc. This is due to the presence of intermolecular hydrogen bonding
in molecules which is, however absent among the molecule of etc. (ii) Water has a higher specific heat, thermal conductivity and surface tension than most other
liquids. These properties allow water to play a vital role in the biosphere. For example, the high heat of vapourization and the high heat capacity of water are responsible for moderation of the climate and body temperature of living organisms.
(iii) Water because of its high dielectric constant (78.39) has the ability to dissolve most of the inorganic (ionic) compounds and its, therefore, regarded as a universal solvent. Whereas solubility of ionic compounds takes place due to ion-dipole interactions (i.e. solvation of ions), the solubility of covalent compounds such as alcohols, amines, urea, glucose, sugar etc, takes place due to the tendency of these molecules to form hydrogen bonds with water.
Illustration 6. The melting points of most of the solid substances increase with an increase of pressure acting on them. However, ice melts at a temperature lower than its usual melting point when the pressure is increased. This is because
(A) pressure generates heat (B) the chemical bonds break under pressure (C) ice is less denser than water (D) ice is not a true solid
Solution: (C). On increasing pressure, coordination number of ions in solids increases thus thereby increasing their melting point but in ice pressure does not increase the coordination number of oxygen & melting point does not increase but decrease under pressure.
Exercise 5. Water boils & freezes at 100C & 0C respectively, find the reason for it from the following:(A) water dissolves anything however sparingly the dissolution may be (B) water is a polar molecule(C) boiling & freezing temperature of water were used to define
temperature scales. (D) liquid water is denser than ice
Chemical Properties of Water
1. Action towards litmus
Pure water is neutral to litmus.
2. Decomposition
Water is quite stable and does not dissociate at high temperature. The dissociation into its
elements is only 0.02 % even at 1500K.
The small conductivity of pure water reveals its dissociation into and ion.
3. Acid-base reactions
Water is amphoteric substance because it can act as acid as well as base as shown -
However, the pH of water at is 7 and its is neutral towards litmus.
4. Hydrolytic reactions
Water can hydrolyse many non-metallic oxides, halides and also some metallic phosphides,
carbides and nitrates.
Illustration 7. Which of the following is formed by the action of water on sodium peroxide? (A) (B)
(C) (D)
Solution: Na2O2 + H2O ¾¾¾® 2NaOH + 1/2O2 Hence (C).
Exercise 6. Water is oxidized to oxygen by (A) (B)
(C) (D)
HEAVY WATER
Heavy water is the oxide of heavy hydrogen (deuterium) and is also called deuterium oxide. It is represented by the formula or . The credit of discovery of heavy water goes to Urey
who first proved that 6000 parts of ordinary water contains about 1 part of heavy water. Lewis and Donald (1933) were able to isolate a few ml of heavy water by the prolonged electrolysis of alkaline water.
Preparation of Heavy Water
The main source of heavy water is ordinary water. From which it is prepared either by prolonged electrolysis or by fractional distillation.
Prolonged Electrolysis of Ordinary Water
This method involves multistage electrolysis of
ordinary water containing NaOH. The cell used for
electrolysis was designed by Brown, Daggat and
Urey. It is a cylindrical vessel made of steel which
acts as cathode. The anode is a perforated
cylindrical sheet. The electrolysis is carried out in
different stage as described below and in actual
practice large number of electrolysis cells are used.
_
Gases
Steel Cathode
Nickel anode Water
Electrolytic cell for preparation of heavy water
First Stage
In this stage thirty electrolytic cell are used. Each cell is filled with about 3% solution of NaOH.
The electrolysis is carried out for about 72 hrs, using a current of 110 volts. The volume reduces
to about 1/6th of the original volume taken. The gases evolved are discard the
volume left contain about 2.5% of heavy water.
Second Stage
This stage involves the electrolysis of residue left from the first stage using 6 electrolytic cells.
The gaseous evolved are burnt and water formed is returned to the first stage cell. The residual
liquid contains about 12% of heavy water.
Third Stage
This involves the electrolysis of residue of second stage. The content of heavy water is raised to
about 60%. The gases evolved are burnt to get water that is fed to 2nd stage cells.
Fourth Stage
This stage involves the electrolysis of residue of third stage and here, nearly 99% of heavy water
is obtained. The gases evolved are burnt as usual, and sent to third stage cells.
Fifth stage
The 99% heavy water from fourth stage is made free from alkali and other impurities by distillation and distillate is electrolysis. Here, the gases evolved are and which are burnt to get 100% pure water. A flow sheet diagram of the process is shown below in the figure.
2 2H ,O
FIRST STAGE 30 cells
SECOND STAGE 6 cells
FIFTH STAGE
BURNERS
BURNERS
BURNERS
BURNERS
2D O
2 2D ,O FOURTH STAGE
THIRD STAGE
Flow sheet diagram for producing D2O
By Fractional Distillation of Ordinary Water
This method involves partial separation of heavy water from ordinary water. Advantage is taken
of the small difference in the boiling points of protium oxide (373.2K) and deuterium oxide
(374.3K). Since the difference in boiling points is very small, a long fractionating column (about
13 m) is used for distillation and the process is repeated several times. The lighter fraction
is distilled first while heavier fraction is left behind. The heavier fraction becomes rich in
.
Properties of Heavy Water
(i) Heavy water is colourless, tasteless and odourless liquid.
(ii) All physical constants of heavy water are higher than the corresponding values of ordinary water. Some of the physical constants of heavy water are given below in table.
Property
Molecular mass 18.015 20.028
Melting Point (K) 273.2 276.8
Boiling Point (K) 273.2 374.4
Temperature of maximum Density (K) 277.1 284.4
Maximum Density 1.000 1.106
Density 0.997 1.104
Heat of Vaporisation (373 K)(kJ/ ) 40.66 41.61Heat of Fusion (kJ/ ) 6.01 6.28
Specific Heat 4.177 -
Ionization Constant
Chemical Properties of Heavy Water
Heavy water is chemically similar to ordinary water. The chemical reactions of heavy water are
slower than those of ordinary water. Some of the important reactions of heavy water are listed
below:
1. Reaction with metals
Alkali metals and alkali earth metals reacts with heavy water to form heavy hydrogen .
2. Reaction with metal oxides
reacts slowly with metal oxides to form corresponding deutroxides
3. Reaction with non-metallic oxides
Non-metallic oxides react with to form corresponding deutro acid,
4. Reactions with carbides, nitrides, phosphides, arsenide etc.
5. Electrolysis
A solution of heavy water containing , when electrolysed evolve heavy hydrogen at
cathode
6. Exchange reactions
When compounds having mobile hydrogen react with heavy water, hydrogen is exchanged
by deuterium partially or completely.
7. Deutero hydrates
Heavy water like ordinary water may be associated with salts as water of crystallization, giving deutro hydrates, e.g., .
8. Deuterolysis
Water brings hydrolysis of certain inorganic salts. gives similar reactions which are termed
deuterolysis.
9. Biological and Physiological effects
It has been established that heavy water of high concentration retards the growth of plants and
animals. It has been confirmed by Lewis that tobacco do not grow in heavy water. Pure heavy
water kill small fishes, tadpoles and mice, when fed on with it.
Taylor has shown that heavy water has germicide and bactericide properties. Water containing small quantity of acts as a tonic and stimulates vegetable growth. Certain moulds have
been found to develop better in heavy water in comparison to ordinary water.
Uses of Heavy Water
The following are the important uses of heavy water:
1. As a neutron moderator.
Fission in uranium – 235 is brought in by slow speed neutrons. The substances which are used
for slowing down the speed of neutrons are called moderators. Heavy water is used for this
purpose in nuclear reactors.
2. For the preparation of deuterium.
Heavy water on electrolysis by its decomposition with metals produces deuterium.
3. As a tracer compound.
Heavy water is commonly used as a tracer compound for studying various reactions
mechanisms. It has also used for studying the structure of some oxyacids of phosphorus such as as to determine the number of ionisable hydrogen atoms.
Production in India
Various units have been set in India to manufacture heavy water. These are situated at Nanital,
Trombay, Rourkel, Namrup and Neyveli.
Illustration 9. Heavy water is a compound of
(A) hydrogen & heavier isotope of oxygen
(B) heavier isotope of hydrogen & heavier isotope of oxygen
(C) oxygen & heavier isotope of hydrogen
(D) none of the above
Solution: (C). D2O is heavy water.
Exercise 7.
Dielectric constant of D2O is
(A) equal to that of water (B) slightly less than that of water
(C) slightly more than that of water (D) half of water
HARD AND SOFT WATER
Water is classified into categories depending upon its behaviour towards soap solution. These
are: soft water and hard water.
(a) Soft water
Water which produces lather with soap solution readily is called soft water. Distilled water and
rain water are common examples of soft water.
(b) Hard Water
Water which does not produce lather with soap solution easily is called hard water. Sea water,
tap water are common examples of hard water.
Cause of Hardness of water
Hardness of water is due to the dissolved impurities of the salts like bicarbonates, chlorides and
sulphates of calcium and magnesium. Water gets contaminated by these salts when it passes
through the ground and rocks. Hard water does not produce lather with soap solution readily
because the cations present in hard water react with soap (which is a mixture of
sodium salts of higher fatty acids like stearic acid, palmitic acid, oleic acid, etc) to form a
precipitate of calcium and magnesium salts of fatty acids.
(M = Ca or Mg)
Thus, no lather is produced until all the calcium and magnesium ions have been precipitated.
This leads to the consumption and hence, wastage of lot of soap. Hard water is, therefore, not fit
for washing purposes
Types of Hardness
The hardness of water is of two types: temporary hardness and permanent hardness.
(i) Temporary hardness.
It is due to the presence of soluble bicarbonates of calcium and magnesium. Such water is also
said to posses carbonate hardness. The term temporary indicates that most of the hardness can
be removed by simply boiling the water. The bicarbonates of calcium and magnesium are formed
in water by dissolution of carbonates of calcium and magnesium in the presence of atmospheric
carbon dioxide.
(ii) Permanent hardness.
It is due to the presence of chlorides and sulphates of calcium and magnesium. Such water is
also said to posses non-carbonate hardness. The term permanent indicates that type of hardness
can not be removed by boiling of water.
Softening of Hard Water
The process of removal of metallic ions responsible for hardness of water is
known as softening of water. A number of methods are available to soften water depending upon
the nature of dissolved mineral salts as described below:
Removal of Temporary Hardness
Temporary hardness can be removed by the following methods:
(i) Boiling
Temporary hard water is taken in large boilers and boiled for about fifteen minutes.
Consequently, the bicarbonate of calcium and magnesium present in the water decompose into
their insoluble carbonates which settle at the bottom of the tank as precipitate which are removed
by filtration or decantation.
(ii) Calcium hydroxide (or Clarke’s) method
Calculated quantity of lime (calcium hydroxide) is added to temporary hard water. The soluble
bicarbonates are converted into insoluble carbonates which settle at the bottom of the tank and
are removed by filtration.
Removal of Permanent Hardness
Permanent hardness of water can be removed by the following methods:
1. By chemical additives
(a) Addition of washing soda.
In this method, ions can be precipitated by the addition of calculated amount of
washing soda
(b) Addition of sodium polymetaphosphate (calgon process)
In this method ions are rendered ineffective by the addition of sodium
polymetaphosphate. The trade name of which is calgon (meaning calcium gone). The general
formula of sodium polymetaphosphate is , where the value of n is sometime as large as
1000. However, commonly, calgon is represented by sodium hexametaphosphate .The
of hard water reacts with calgon to form soluble complexes.
The complexes of calcium and magnesium so formed remain dissolved in water but they do not
cause hindrance in the formation of lather. This is because calcium and magnesium ions are not
free to react with soap but these have been tied up in stable complexes. This is also known as sequestraction of ions.
2. Ion Exchange method
This is a modern method of softening of water. In this method, the ions present in the hard water
are exchanged for less damaging ions from the exchangers. There are two main types of ion-
exchanger as:
(a) Inorganic cation exchangers (Permutit Method)
These are complex inorganic salts like hydrated sodium-aluminum silicate
which have interesting property of exchanging cations such as calcium and magnesium ions in
hard water for sodium ions. These complex salts are known as zeolites which can be either a
naturally occurring or an artificially synthesized substances. Their technical name is permutit. For
artificial synthesis of permutit, a mixture of soda ash , sand and alumina
is fused together. The product is washed with water to remove soluble impurities leaving behind a porous mass of permutit.
Hard Water
NaCl Solution
Soft Water Out
Permutit process for softening of hard water
Coarse Sand
Permutit
The permutit is loosely packed in a big tank over a layer of coarse sand. Hard water is introduced
into the tank from the top. Water reaches the bottom of the tank and then slowly rises through the
layer of permutit in the tank. The cations present in hard water are exchanged for sodium ions.
(M = Ca or Mg)
where Z =
As the process is continued, zeolites gets exhausted because of its conversion into calcium and
magnesium zeolite. The exhausted resin is regenerated allowing about 10% solution of sodium
chloride to percolate through it when the following reaction occurs.
(M = Ca or Mg)
The regenerated or reactivated resin can be used again and again for a quite longer period.
(b) Organic ion exchangers
These are complex organic molecules having giant hydrocarbon frame work with either acidic
group or basic group attached to them. The resins with acidic
group are capable of exchanging the ions for the cations and are called cation exchangers.
They are represented as resin. The resins with basic group are capable of exchanging their
ions for other anions and are called anion exchangers. They are represented as
resin.
Method of Removal of Hardness By Organic Exchangers
First of all, the hard water is passed through a bed of cation exchange resin. The cations present in hard water are exchanged with of resin as:
Hard Water
Dil. HCl For
Regeneration
Cation Exchange
Resin
Waste Waste
Anion Exchange
Resin
Dil. NaOH For
Regeneration
24H,Cl, SO
Gravel Gravel
Soft Water
Removal of hardness by the organic ion - exchanges
The water which comes out of the bottom of first tank is richer in ions.
This water is then passed through a bed of anion exchange resin where anions contained in water are exchanged with ions as
Therefore, the ions (formed in the first tank) combine with the ions (formed in the
second tank) to produce water.
Thus, water obtained by this method is free from all types of cations as well as anions. It is known
as deionised or demineralised water.
This method is particularly suitable for obtaining pure water for laboratory purposes.
Regeneration of resins
The exhausted resin in the first tank is regenerated by treatment with moderately concentrated
hydrochloric or sulphuric acid.
Similarly, the exhausted resin in the second tank is regenerated by treatment with moderately
concentrated solution of sodium hydroxide.
Exercise 8. One of the following is an incorrect statement point it out(A) permanent hardness can be removed by boiling water (B) hardness of water effects soap consumption(C) temporary hardness is due to bicarbonates of Ca & Mg. (D) permanent hardness is due to the soluble of Ca & Mg.
Degree of Hardness of Water
Degree of hardness of water is defined as number of parts of mass of (Calcium
carbonate), equivalent to various calcium and magnesium salts present in one million parts by
mass of water. It is expressed in ppm (parts per million).
Illustration 11. Calculate the degree of hardness of a sample of hard water which is found to contain 36 mg of per kg of water.
Solutions: Our objective is to find mass of equivalent to in one million parts
of water. Now, g of water contains = 36 mg
g of water contains = 36 mg = 36 g
From the formula, 1 mol of = 1 mol of
or 120 g of = 100 g of
or 36 g of =
Thus, degree of hardness = 30 ppm.
It may be noted degree of hardness up to 100 – 150 ppm in water required for our daily needs
such as cooking, bathing, washing of clothes, etc, is tolerable. But if degree of hardness exceeds
this limit, then water is not suitable for domestic use.
HYDROGEN PEROXIDE H2O2
Hydrogen peroxide is another hydride of oxygen. Unlike water it is highly unstable, and therefore,
does not exist in nature as such. Its, importance, in recent years has increased due to its use as
rocket fuel.
Preparation of Hydrogen Peroxide
1. From sodium peroxide (Merck’s method)
Calculated amount of sodium peroxide is gradually added to an ice-cold solution of 20%
in small lots with constant stiring.
Upon cooling, crystals of separate out and the resulting solution contains about
30% . The solution also contains some dissolved , but it does not interfere with the
reactions of . A pure sample of may, however, be prepared by vacuum distillation.
2. From barium peroxide-Laboratory method of preparation.
Hydrogen peroxide is prepared from barium peroxide by the following methods:
(a) By the action of dilute sulphuric acid. A paste of hydrated barium peroxide is
prepared in ice – cold water and then added slowly to an ice-cold solution of 20% .
The white precipitate of is removed by filtration leaving behind a dilute solution (5%)
of . In this method, anhydrous barium peroxide can not used since the precipitated
forms a protective layer around unreacted barium peroxide thereby preventing the
further reaction
Illustration 12. A certain sample of hydrogen peroxide is 1.5 M solution. It is to be labelled as X
volumes. Calculate the value of X.
Solution:
Or mass of per litre =
Now 68 g of give at STP = 22.4 L
Thus 1 L of which contains 51 g of produces at STP = 16.8 L
Hence value of X is = 16.8 L Or The given sample is 16.8 volume .
Limitation
Hydrogen peroxide prepared by this method contains appreciable quantities of ions (in the
form of dissolved barium persulphate) which catalyse the decomposition of .Therefore,
prepared by this method cannot be stored for a long time. Further, also acts as a catalyst for decomposition of .
(b) By the action of carbon dioxide or carbonic acid. When a rapid stream of is bubbled
through a thin paste of in ice-cold water, and are produced:
The insoluble barium carbonate is removed by filtration leaving behind a dilute solution of
(c) By the action of phosphoric acid. Hydrogen peroxide can also be prepared by the action of
phosphoric acid on barium peroxide:
This method has the advantage over method since almost all the heavy metal
(e.g. Pb etc) impurities present in and which catalyse the decomposition of are
removed as insoluble phosphates. As a result, the resulting solution of has good
keeping properties.
Manufacture of Hydrogen Peroxide
1. By electrolysis of 50% H2SO4
Hydrogen peroxide is manufactured by the electrolysis of a cold 50% solutions of in an
electrolytic cell using platinum as anode and graphite as cathode. The reactions taking place are:
At cathode:
At anode :
Peroxydisulphuric acid formed around anode is withdrawn and then distilled with water under reduced pressure. The low boiling distils over along with water leaving behind high boiling
which is recovered and recycled.
or,
Modification
Recently, it has been observed that if instead of 50% an equimolar mixture of H2SO4 and
ammonium sulphate is electrolysed, a more concentrated solution of is obtained.
At cathode:
At Anode:
Ammonium persulphate formed around anode is withdrawn and distilled with water to give
This process is now used only for the laboratory preparation of , i.e.;
Deutroperoxide
2. By autoxidation of 2-ethylanthraquinol
This is a new method and is widely used in U.S.A. In this process, air is bubbled through a 10%
solution of 2-ethylanthraquinol in benzene and cyclohexane when 2-ethylanthraquinol is oxidized to 2-ethylanthraquinone and is formed according to the following equations.
C2H5
OH
OH
2-Ethylanthraquinol
O2
H2/Pd
C2H5
O
O
+ H2O2
2-ethylanthraquinone
The thus formed (about 1%) is extracted with water and the aqueous solution is
concentrated by distillation under reduced pressure to give 30% (by weight) solution. 2 – Ethylanthraquinone so produced can be reduced back to the starting to the material i.e., 2-ethylanthraquinol by hydrogen using palladium catalyst.
C2H5
O
O
2-Ethylanthraquinone
H2
Pd
C2H5
OH
OH
2-Ethylanthraquinol
Thus, we find that the raw material are and only.
Concentration of hydrogen Peroxide Solution
Hydrogen peroxide produced by any of the above method is in the form of dilute solution. The solution can not be concentrated by simple heating because it readily decomposes below its boiling point as
The dilute solution of hydrogen peroxide is concentrated carefully by the following steps:
(i) Slow evaporation on water bath
The dilute solution of hydrogen peroxide is taken in evaporating dish which is heated carefully on
water bath. Slow evaporation continues causing the escaping of water molecules till
approximately 50% solution of is obtained.
(ii) Evaporation in vacuum desicator
The 50% solution is placed in vacuum desicator over concentrated sulphuric acid. The water
vapours are absorbed by sulphuric acid leaving approximately 90% solution of .
(iii) Distillation under reduced pressure
The 90% solution of hydrogen peroxide is then subjected to distillation under reduced pressure
(10 – 15mm). During this process, water distils over at 303 – 313 K and about 99% pure
hydrogen peroxide is left behind.
(iv) Removal of last traces of water
The 99% solution of hydrogen peroxide is cooled in a freezing mixture of solid carbon dioxide and
ether. As a result of this, crystals of hydrogen peroxide separate out which are removed, dried
and remelted. This gives completely pure hydrogen peroxide.
Storage of Hydrogen Peroxide
Decomposition of hydrogen peroxide into water oxygen is catalysed by traces of metal impurities,
strong bases and exposure to light. Because of this property of , the concentrated solution of
can be dangerous as uncontrolled rapid decomposition can result in an explosion. Thus,
(i) It is stored in wax lined coloured bottles as rough glass surface also causes its
decomposition.
(ii) A small amount of stabilizer like phosphoric acid, or glycerol or acetanilide must to added to
retard its decomposition.
Structure of Hydrogen Peroxide
As established by X – ray studies, hydrogen peroxide molecules has a non – planar structure.
The molecules dimensions in gas phase and that in solid phase have given in figure. (a) and (b)
respectively. In the crystal, the dihedral angle reduces to on account of
hydrogen bonding. The two oxygen atoms are joined by a single electron – pair bond. The O – O Linkage is also called peroxide linkage.
H
95 pm
147.5 pm
94.8
H
H
98.8 pm
145.8 pm
101.9
H
(a) Gas phase (b) Solid phase
90.2 111.5
Properties of Hydrogen Peroxide
Some of the physical properties of hydrogen peroxide are listed below:
(i) Pure hydrogen peroxide is a colourless, syrupy liquid.
(ii) It has odour like that of nitric acid.
(iii) Its aqueous solution has a bitter taste.
(iv) It is soluble in water, alcohol and ether in all proportions.
(v) Its density is 1.44 g . The high density is due to association of its molecules by
intermolecular hydrogen bonds.
(vi) Its boiling points is 423 K at 670 mm pressure and 358 K at 68 mm pressure. Its melting point
is 272 K.
Chemical Properties
(a) Decomposition
It is a unstable liquid readily decomposes on heating or on long standing to give water and
dioxygen. It is an example of disproportionation decomposition is suppressed by addition of
glycerol, acetanilide or phosphoric acid.
(b) Acidic behaviour
Pure hydrogen peroxide is a weak acid ( at 298 K). it ionizes in water as:
Its acidic character can be shown its ability to neutralize bases such as NaOH, etc, to
form corresponding peroxides
Hydrogen peroxide has an interesting chemistry because of its ability to act as oxidizing as well
as reducing agent both in acidic and basic solutions.
Oxidising nature of H2O2
can acts as oxidizing agents in acidic as well as basic medium as described below.
Illustration 13. “Hydrogen peroxide is a strong oxidizing agent both in acid and alkaline
medium”. Justify giving suitable reactions.
Solution: Oxidising agent
Reducing agent
Illustration 14. What happens when is treated with
(a) acidified potassium permanganate
(b) lead sulphide
(c) alkaline potassium ferrocyanide
(d) acidified ferrous sulphate
(e) sulphurous acid.
Solution: (a) It reduces acidified solution. The pink colour of is
discharged.
(b)
(c) It reduces alkaline potassium ferricyanide to potassium ferricyanide.
(d) It oxidizes acidified ferrous sulphates to ferric sulphate.
(e) It oxides sulphurous acid to sulphuric acid
Illustration 15. What happens when?
(i) Chromium hydroxide is treated with hydrogen peroxide in the presence of
NaOH.
(ii) Hydrazinc reacts with hydrogen peroxide.
(iii) Hydrogen peroxide reacts with .
(iv) Sodium hypochlorite reacts with .
Solution: (i) is converted into yellow coloured
(ii) Hydrazine is oxidised to and .
(iii)
(iv)
Bleaching Action
The bleaching action of hydrogen peroxide is due to the nascent oxygen which it liberates on
decomposition.
The nascent oxygen combines with colouring matter which, in turn, gets oxidised. Thus, the bleaching action of is due to the oxidation of colouring matter by nascent oxygen. It is used
for the bleaching of delicate materials like ivory, feather silk, wool etc.
Addition reactions
Hydrogen peroxide reacts with alkenes to form glycols. CH2
CH2
+ H2O2
CH2OH
CH2OH
Strength of Hydrogen Peroxide
1. As percentage
Some time the actual concentration of hydrogen peroxide in a solution in expressed as percentages of in solution. Thus, 40% solution of hydrogen peroxide means 40 grams of
hydrogen peroxide are present in 100 grams of solution.
2. As volume strength
The most common method of expressing the concentration of hydrogen peroxide solution is, in terms of the volume of oxygen which a solution of hydrogen peroxide gives on decomposition by heat. For example, “10 volume” hydrogen peroxide refers to a solution of hydrogen peroxide whose, 1 litre will give 10 litre of oxygen at STP.
Calculation of Strength of H2O2
It is possible to calculate the strength of hydrogen peroxide from its “volume strength”. Let us calculate the concentration in g/litre of “30 volume” solution of hydrogen peroxide. A 30 volume solution of hydrogen means that 1 litre of the solution gives 30 litre of oxygen at STP. According to its decomposition reaction.
Now 1 litre of hydrogen peroxide gives 30 L of oxygen at STP From the above equation 22.4 L of oxygen is obtained from =
30 L of oxygen at STP is obtained from =
Thus, 1L of sample contains = 91.07g
or Strength of the 30 volume solution of hydrogen peroxide is 91.07
Illustration 16. Calculate the strength in , molarity and normality of 40 volume of hydrogen
peroxide solution.
Solution:
40 volume means, 1L of the solution produces 40 L of at STP
Now, 22.4 L of at STP is produced from = 68
40 L of oxygen at STP is obtained from =
=
Illustration 17. Aqueous solution of an inorganic compound (x) shows the following reactions. (i) It decolourises an acidified solution accompanied by the solution of
oxygen. (ii) It liberates iodine from an acidified KI solution.(iii) It gives a brown precipitate with alkaline solution with evolution
oxygen. (iv) It removes black stains from old oil paintings.
Identify (x) and give chemical equations for the reactions at step (i) to (iv).
Solution:
Exercise 9. Hydrogen peroxide solution (20 ml) reacts quantitatively with a solution of (20 ml) with dilute . The same volume of the
solution is just decolorized by 10 ml of in neutral medium
simultaneously forming a dark brown precipitate of hydrated . The
brown ppt. is dissolved in 10 ml of 0.2 M sodium oxalate under boiling condition in the presence of dilute . Calculate the molarity of .
Exercise 10. In basic medium, acts as an oxidizing agent in its reactions with
(A) (B)
(C) (D)
ANSWERS TO EXERCISES
Exercise 1: (C)
Exercise 2. (B)
Exercise 3. (A)
Exercise 4. (C)
Exercise 5: (C)
Exercise 6. (D)
Exercise 7: (B)
Exercise 8. (A)
Exercise 9. m = 0.1
Exercise 10: (A)
MISCELLANEOUS EXERCISES
Exercise 1: How is dihydrogen obtained from? (A) dilute sulphuric acid (B) sodium hydroxide (C) water Give one equation in each case.
Exercise 2: Although resembles chemically yet it is a toxic substance. Explain.
Exercise 3: What is the difference between hydrolysis and hydration?
Exercise 4: What is meant by autoprotolysis of water?
Exercise 5: Why is not used in the laboratory preparation of dihydrogen?
Exercise 6: What are advantages of using hydrogen as a fuel over gasoline or coal?
Exercise 7: How is a solution of concentrated? Why it can not be concentrated by distillation at ordinary pressure?
Exercise 8: (a) Name the blue compound formed in the reaction of with acidified
in ether? Give reaction.
(b) What is the principle in the bleaching action of ?
Exercise 9: The following statements are true under certain conditions. Mention condition in each case in a few words. (i) Hydrogen reacts with nitrogen to form ammonia. (ii) Hydrogen can be prepared from water at ordinary temperature. (iii) A metal will liberate hydrogen by its reaction with dilute
Exercise 10: Complete the following reactions
(i)
(ii)
(iii)
(iv)
(v)
ANSWERS TO MISCELLANEOUS EXERCISES
Exercise 1: (a) More electropositive elements than hydrogen (eg, Zn, Fe, Mg) react with dilute mineral acids (dil, HCl & H2SO4) to liberate dihydrogen gas.
(b) Metals like Be, Zn, Sn, Al etc react with boiling alkali solution liberating dihydrogen.
(c)
Exercise 2: Heavy water is injurious to human being, plants and animals since it slows down the rate of reactions occurring in then. Thus heavy water does not support life so well as does ordinary water.
Exercise 3: Interaction of H+ and OH ions of H2O with anions and cations of the salt respectively to give the original acid and the original base is called hydrolysis eg.
Hydration, on the other hand, means addition of H2O to ions or molecules to form hydrated ions or hydrated salts for eg.
Exercise 4: Auto protolysis of water means that two molecules of water reacts with each other through protons transfer i.e. one acts as the acid while the other acts as the base. The molecule which accepts a proton is converted into H3O+ while that which loses a proton is converted into OH ion.
Exercise 5: HNO3 is a strong oxidizing agent. It oxidizes the H2 produced.
Exercise 6: Advantages of using as fuel. (i) Abundantly available. (ii) Pollution free as the product is water. (iii) High calorific fuel. (iv) Excellent reducing agent. (v) Time required for regeneration of H2 fuel is short.
Exercise 7: Concentration of H2O2 is carried out carefully in a number of stages as follows. (i) Evaporation on water bath – slow evaporation of water continues until the
solution contains about 50% H2O2. (ii) Dehydration in a vaccum desiccators50% solution of H2O2 obtained from above is taken in a disk and placed in a vaccum desiccator containing conc. H2SO4. The inside presence is reduce as a result of low, water readily evaporates and the water vapours thus produced are absorbed by conc. H2SO4.The H2O2 obtained is 90% pure.
(iii) Distillation under reduced pressure The 90% solution of H2O2 as obtained above is subjected to distillation under reduced pressure (10 – 15 mm). During this process, water distils over between 303 – 313 K leaving behind almost pure 99% H2O2.
(iv) Removal of least traces of water
The last traces of water in H2O2 are removed by freezing it in a freezing mixture consisting of dry ice (solid CO2) and ether when crystal of H2O2 separate out. These crystals are removed, dried and melted to give pure H2O2.It can not be concentrated by distillation at ordinary pressure because H2O2
decomposes much below its boiling point to give H2O & O2.
Exercise 8: (a)
Chromium pentoxide
O
O
Cr
O
OO
(b) The bleaching action of H2O2 is due to the nascent oxygen which it liberate on decomposition
The nascent oxygen combines with colouring matter which in turn gets oxidizing. Thus the bleaching of H2O2 is due to the oxidation of colouring matter by nascent oxygen.
Exercise 9: (i) At high pressure (200 atm) and in presence of catalyst Fe, Mo. (ii) Very active metals like Na, K, Ca etc can react with H2O at room temperature. (iii) Metals which are more electropositive than hydrogen.
Exercise 10: (i) CaO (s) + H2O ¾® Ca(OH)2 (aq)
(ii) Na2O (s) + H2O ¾® 2NaOH (aq)
(iii) 3Fe (s) + 4H2O ¾® Fe3O4 + 4H2
(iv)
(v) Co(g) + H2O (g)
SOLVED PROBLEMS
Subjective:
Board Type Question
Prob 1. Why is hard water softened before its use in boilers?
Sol. Hard water cannot be used in boilers because on heating it gives precipitates of CaCO3
and MgCO3 alongwith CaSO4 which forms scales in boilers. To avoid the scales formation, hard water is softened before its use in boiler.
Prob 2. What volume of oxygen at NTP will be obtained by the complete decomposition of 100 ml of H2O2 marked 30 volume?
Sol. 1 ml H2O2 of 30 volume liberates 30 ml O2 at NTP. 100 ml H2O2 of 30 volumes liberate 30 100 ml O2 at NTP or 3 litre O2 at NTP.
Prob 3. Cu does not liberate H2 from dil H2SO4. Explain.
Sol. Cu is placed below H in electrochemical series having oxidation potential 0.34 V hence Cu can not transfer electron to H+ ions of H2SO4 to liberate H2.
Prob 4. A small amount of phosphoric acid or glycerol or acetanilide is added in H2O2 during its storage. Explain.
Sol. All the three additives act as negative catalyst for the decomposition of H2O2 and thus decomposition of H2O is checked off.
Prob 5. Which isotope of hydrogen is used as a trace in organic reactions?
Sol. Hydrogen has three isotopes, Viz, H, D and T. Due to difference in masses, the rate constants of three isotopes with the same substance are different. In other words, both D and T show isotope effect. But since T is not only radioactive but is also least abundant hydrogen isotope. Therefore D is used as a trace to study the mechanism of organic reactions.
IIT Type Question
Prob 6. H2O2 is a very good solvent for ionic compounds. Explain.
Sol. The dielectric constant of pure H2O2 is 93.7 (which also increases on dilution (97 for 90% pure; 120 for 65% pure) which is appreciably high to weaken the electrostatic forces of attraction among ions and to pass them in solution state.
Prob 7. Anhydrous is not used for preparing why?
Sol. Anhydrous is not used because the formed during the reaction forms a
protective layer around the unreacted and the reaction stops after some time.
Prob 8. Statues coated with white lead on long exposure to atmosphere turns black and the original colour can be restored on treatment with . Why?
Sol. On long exposure to atmosphere, white lead is converted into black PbS due to the action of present in the atmosphere. As a result, statues turn black
On treatment of these blackened statues with , the black PbS get oxidized to white
and the colour is restored
Prob 9. Show with suitable example hydrogen acts as reducing agent as well as oxidizing agent?
Sol. In the reaction of with metals to form metal hydrides, it acts as an oxidizing agent.
Here Na has been oxidized to Na+ while is reduced to hydride ion.
In the reaction of heated cupric oxide with dihydrogen to form and copper metal, dihydrogen acts as a reducing agent
Here, CuO is reduced to Cu while dihydrogen is oxidized to .
Prob 10. Ozone is used for purifying air in crowded place such as cinema halls, tube railways, tunnels etc explain?
Sol. is easily decomposes to produce thereby purifying air at
crowed places.
Prob 11. Why is sodium chloride less soluble in heavy water than in ordinary water?
Sol. The dielectric constant of is less than that of , therefore ionic compounds like
NaCl are less soluble in than in .
Prob 12. Why electrolysis of ordinary water occurs faster than heavy water?
Sol. This is due to lower bond dissociation energy of protium bonds in H – O – H, which is less than deuterium bonds in D – O – D, so electrolysis of occurs much faster
than that of .
Prob 13. Name the class of hydrides to which H2O, B2H6, NaH and LaH3 belong. What is understood by ‘Hydride Gap’?
Sol. H2O covalent hydride, B2H6 covalent hydride, NaH ionic hydride, LaH3 metallic hydride.Hydride Gap: The metals of 7, 8, 9 groups do not form hydride. This is called the hydride gap.
Prob 14. Concentrated sulphuric acid can not be used for drying . Why?
Sol. This is because conc. on absorbing water from moisture generates so much heat that hydrogen catches fire.
Prob 15. A mixture of hydrazine and with Cu(II) catalyst is used as rocket propellant. Why?
Sol. The reaction between hydrazine and is highly exothermic and is accompanied by a large increase in the volumes of the product and hence this mixture is used as a rocket propelled
Objective:
Probl 1. HCl is added to the following oxides. Which one would give ?
(A) (B)
(C) (D) none of the above
Sol. Since none of the oxides is a peroxide. Hence none of them would give .
Probl 2. When the same amount of zinc is treated separately with excess of sulphuric acid and excess of sodium hydroxide, the ratio of volumes of hydrogen evolved is (A) 1 : 1 (B) 1 : 2 (C) 2 : 1 (D) 9 : 4
Sol.
The ratio of volumes of evolved in both the cases is 1 : 1.
Probl 3. Which of the following is the true structure of H2O2? (A) H O O H (B)
O O
H
H(C)
O O
H
H
(D)
O
H
H
O
Sol. Oxygen atom is hybridized having two large pair of electrons. Therefore the
structure of hydrogen peroxide is
O O
H
H
Probl 4. When electric current is passed through an ionic hydride in the molten state (A) hydrogen is liberated at the anode (B) hydrogen is liberated at the anode(C) no reaction takes place (D) hydride ion migrates towards cathode
Sol. Ionic hydride contains ion which liberates at the anode.
Probl 5. Which of the following can not be oxidized by ?
(A) KI + HCl (B)
(C) PbS (D)
Sol. is more powerful oxidizing agent than . So reduces to .
Probl 6. The reaction Ag2O + H2O2 ¾¾® 2Ag + H2O + O2 takes place in(A) basic medium (B) bleaching agent (C) neutral medium (D) both in acidic and basic medium
Sol. on oxidation gives only in basic medium
Probl 7. Which of the following compounds turns white on treatment with . (A) HgS (B) PbS (C) NiS (D) CuS
Sol.
Probl 8. The strongest base is (A) (B)
(C) (D)Sol. Basic character decreases as the size of central atom increases in hydrides of group
15 members
Probl 9. Semi water gas is a mixture of (A) (B)
(C) (D) none
Sol. Mixture of CO + + is called semi water gas.
Probl 10. A hydride of nitrogen which is acidic is (A) (B)
(C) (D)
Sol. (hydrazoic acid) is the acidic hydride of nitrogen
True/False
Probl 11. Tritium can be obtained from natural sources.
Sol. False
Probl 12. H2O2 act as antichlor.
Sol. True
Probl 13. Sodium carbonate can remove only temporary hardness of water.
Sol. False
Probl 14. Ortho hydrogen has lower energy and is more stable than para hydrogen.
Sol. True
Probl 15. Bleaching action of H2O is due to its reducing nature.
Sol. False
Fill in the Blanks
Probl 16. When hydrogen peroxide is treated with acidified K2Cr2O7 solution, a ……….. colour appears.
Sol. Blue green
Probl 17. Carbon monoxide combines with hydrogen when heated to 150 450oC in presence of ZnO + Cu to form …………..
Sol. CH3OH
Probl 18. The mixture of hydrazine hydrate and ………… with a copper (II) catalyst is used as rocked propellant.
Sol. H2O2
Probl 19. Chemical name for permutit is …………….
Sol. Zeolite
Probl 20. H2O2 is used as …………….. in toothpaste.
Sol. Antiseptic
ASSIGNMENT PROBLEMS
Subjective:
Level – O
1. Discuss briefly the properties and uses of different types of hydrides.
2. What is the meant by water gas shift reaction? Describe it use for the preparation of dihydrogen.
3. Differentiate between (a) hard and soft water (b) temporary and permanent hardness of water. Briefly explain how to remove temporary and permanent hardness of water.
4. Explain the following (i) Soft water lathers with soap but not with hard water. (ii) Temporary hard water becomes soft on boiling. (iii) Water can extinguish most fires but not petrol fire. (iv) Hard water is softened before use in boilers.
5. Explain, why the density of ice is less than water?
6. Write short notes on (i) Ortho and para hydrogen (ii) Bosch’s process (iii) calgon (iv) Ion-exchange method for removal of hardness of water.
7. How is heavy water prepared from normal water? Discuss the importance of heavy water in
nuclear reactors. What is the action of heavy water on (i) Sodium (ii) NaOH (iii) (iv)
8. What is the difference between hydrolysis and hydration? What are the ways in which water molecules are bonded to the anhydrous salt to form hydrate?
9. (a) What are interstitial hydrides? Discuss their important uses. How do they differ from molecular hydrides.
(b) Distinguish between salt like and covalent hydrides. Discuss briefly the characteristic of salt like hydrides.
(c) What is understood by hydride gap.
10. Describe permutit process for softening of hard water.Calculate the hardness of a water sample which contains 0.001 mole of dissolved per litre of the solution.
11. A solution of ferric chloride acidified with HCl is unaffected when hydrogen is bubbled through it, but is reduced when zinc is added to some acidified solution. Explain.
12. Why should a bottle of H2O2 be cooled before opening it?
13. What should be the necessary requirements for portable water i.e., water for drinking purposes.
14. Explain why hydrogen is best placed separately in the periodic table of elements?
15. What is understood by hydrogen economy?
16. Metals like platinum & palladium can adsorb large volumes of hydrogen under special conditions. Such adsorbed hydrogen by the metals is called?
17. Find out the order of the reaction for the decomposition of hydrogen peroxide .
18. When is reacted with & either, the ethereal layer becomes blue, why?
19. Write the correct order of the O – O bond length in , and .
20. What would happen when a small quantity of H2O2 is added to a solution of FeSO4?
Level- I
1. In the preparation of hydrogen in the laboratory from granular zinc. Why the following acids cannot be used. (a) Conc. (b) conc. HCl (c) nitric acid
2. Hydrogen peroxide acts both an oxidizing agent and as a reducing agent in alkaline solution towards certain first row transition metal ions. Illustrate three properties of using chemical equations.
3. 20 ml of a solution containing 0.2 g of impure sample of reacts with 0.316 g of (acidic). Calculate (a) Purity of .
(b) Volume of dry evolved at C and 750 mm P.
4. Justify the statement “An aqueous solution of hydrogen peroxide is weakly acidic”.
5. Explain. In the preparation of , the use of phosphoric acid has an advantage over dilute sulphuric acid.
6. Given reason (a) Marine species cannot survive in distilled water. (b) Distilled water in called deionised water.
7. Give ion electron equation for the following reactions(a) oxidation of ferrous ions to ferric ions by .
(b) oxidation of iodide ion to iodine by .
(c) oxidation of acidified per magnate ion . (d) reduction of alkaline ferricyanide ions to ferrocyanide ions.
8. What happens when? (i) Hydrolith is treated with water. (ii) Heavy water reacts with aluminium carbide. (iii) Concentrated caustic potash solution is spilled on granulated zinc.
9. One litre of a sample of hard water contains 1 Mg of CaCl2 and 1 Mg of MgCl2. Find the total hardness of in terms of parts of per parts of water of mass.
10. Explain (a) act as a bleaching agent.(b) Concentration of hydrogen peroxide is difficult.
11. What happens when?(a) Barium peroxide is treated with cold dilute sulphric acid (b) Sodium peroxide is treated with cold dilute sulphuric acid and the resulting mixture is
cooled below 273K. (c) Barium peroxide is treated with phosphoric acid. (d) Hydrogen peroxide is treated with sodium carbonate. (e) Hydrogen Iodide is added to hydrogen peroxide.
12. Explain
(a) Why hydrogen peroxide is stored in coloured plastic bottle?
(b) H2O2 prepared from hydrated barium peroxide and dilute sulphuric acid can not be
stored for a long time.
13. Two liquids (A) and (B) are made of same elements and are diamagnetic. Liquid (A) on
treatment with KI and starch gives blue coloured solution, however. Liquid (B) is neutral to
litmus and does not gives response to starch iodine paper. Identify (A)
and (B).
14. 50 ml of an aqueous solution of H2O2 was treated with an excess of KI solution in dil. H2SO4,
the liberated iodine required 20 ml of 0.1 N Na2SO3 solution for complete reaction. Calculate
concentration of H2O2 in g/litre.
15. Complete the following
(i)
(ii)
(iii)
Level- II
1. The process is endometric .
Yet salt like sodium hydride are known. How do you account for this?
2. Find the degree of hardness of a sample of water containing 12 mg of (mol. Mass 120) per kg of water.
3. Explain the following(a) A small of amount of acid or alkali is added before electrolysis of water. (b) The electrolysis of water for manufacturing hydrogen gas is always carries out in the presence of acid ( ) or alkali (KOH). Yet no ions are discharged.
4. Explain the following (a) A solution of ferric chloride acidified with HCl is unaffected when hydrogen is bubbled through it, but gets reduced when zinc is added to some acidified solution. (b) When sodium hydride is fused state is electrolysed, hydrogen is discharged at anode.
5. Explain the following (i) Why are the melting and boiling points of are higher than those of ordinary water?
(ii) Presence of water is avoided in preparation of from . (iii) Hydrogen peroxide gives acidic properties.
6. Explain the following (i) A mixture of hydrazine and is used as a rocket propellant. (ii) Hydrated barium peroxide is used in the preparation of hydrogen peroxide instead of the anhydrous form.
7. What happens when?(a) Hydrogen peroxide is added to ferrous ammonium sulphate solution. (b) Sodium hydride reacts with diborane. (c) Benzene is treated with in presence of .
8. A 5 ml solution of liberates 0.508 g of iodine from an acidified KI solution. Calculate the
strength of solution in terms of volume strength at STP.
9. To a 25 ml solution excess of acidify solution of potassium iodide was added. The iodine liberated required 20 ml of 0.3 sodium thiosulphate solution. Calculate the volume strength of solution.
10. 10 ml of a solution of labeled ’10 volume’ just decolourises 10 ml of potassium per
magnate solution acidified with dilute acid. Calculate the amount of potassium per magnate in the given solution.
Objective:
Level– I
1. Volume of same weight of ice is………….than/to the same weight of water(A) More (B) Less (C) Equal (D) not related
2. Which of the following hydrides is covalent compound? (A) LiH (B) NaH (C) (D)
3. Which of the following hydroxides is amphoteric?
(A) (B)
(C) (D)
4. The O – O – H bond angle in in gas phase is
(A) (B)
(C) (D)
5. Amongst the one with the highest boiling point is
(A) because of hydrogen bonding
(B) because of higher molecular weight
(C) because of hydrogen bonding
(D) because of lower molecular weight
6. Polyphosphates are used as water softening agents because they (A) form soluble complexes with anionic species (B) precipitate anionic species(C) form soluble complexes with cationic species (D) precipitate cationic species
7. The volume strength of 1.5 solution is(A) 4.8 (B) 5.2 (C) 8.8 (D) 8.4
8. The volume of 10 volume solution that decolourizes 200 ml of 2N solution in acidic medium is (A) 112 ml (B) 336 ml (C) 200 ml (D) 224 ml
9. An oxide which gives on treatment with dilute acid is
(A) (B)
(C) (D)
10. Acidified solution of chromic acid on treatment with yield
(A) (B)
(C) (D)
11. The correct increasing order of the acidity of is
(A) (B)
(C) (D)
12. Ammonium per sulphate on heating under reduced pressure gives (A) (B)
(C) (D)
13. The hybridization on the oxygen atom in is
(A) (B) sp
(C) (D)
14. In proton ( ) exist as
(A) (B)
(C) (D) all the above
15. The reactions manifests
(A) acidic nature of (B) alkaline nature of
(C) oxidizing action of (D) reducing nature of
Assertion/Reason Type Questions
Codes:(a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true. (e) A and R both are false.
16. Assertion (A): On passing H2 gas into aqueous Fe3+ solution, resulting solution gives test of Fe3+. Reason (R): H2(g) reduces Fe3+ to Fe2+ but Fe2+ IS oxidized to Fe3+ by atmospheric oxygen.
17. Assertion (A): Drinking of heavy water (D2O) mixed water (H2O) could prove fatal. Reason (R): There is slower rate of transfer of D+ compared with that of H+ ion in acid-base reaction involved enzyme catalysis.
18. Assertion (A): Hydride (H) is a conjugate base of hydrogen (H2). Reason (R): Every negative ion is an electron-pair donor and is thus, a Lewis base.
19. Assertion (A): Ionic hydrides are the ready-made source of H2.Reason (R): LiH is an ionic hydride.
20. Assertion (A): H2O2 decomposes Na2CO3 to give CO2. Reason (R): H2O2 has two protons and is thus a strong acid.
Level-II
1. Hydrogen will not reduce (A) heated cupric oxide (B) heated ferric oxide (C) heated stannic oxide (D) heated aluminium oxide
2. Hydrolysis of one mole of peroxodisulphuric acid produces(A) two moles of sulphuric acid B) two mole of peroxomonosulphuric acid (C) one mole of sulphuric acid and one mole of peroxomonosulphric acid (D) one mole of sulphuric acid, one mole of peroxomonosulphuric acid and one mole of
3. Which contains both polar and non-polar bonds? (A) (B) HCN
(C) (D)
4. The critical temperature of water is higher than that because the molecule has (A) fever electrons than oxygen (B) two covalent bonds (C) V – shape (D) dipole moment
5. When zeolite which is hydrated sodium aluminium silicate is treated with hard water the sodium ions are exchanged with (A) (B)
(C) (D) all of the above
6. Hydrogen gas is liberated by the action of aluminum with a concentrated solution of
(A) NaOH (B)
(C) (D) none
7. Heavy water is obtained by (A) boiling water (B) distillation of
(C) pronged electrolysis of (D) heating
8. The compound that can work both as an oxidizing agent and a reducing agent is (A) (B)
(C) (D)
9. In acidic medium, acts as a reducing agent in its reaction with
(A) (B)
(C) (D)
10. The decomposition of can be checked by the addition of (A) alkali metal oxides (B) benzene (C) acetamilide (D)
11. The metal which displaces hydrogen from a boiling caustic soda solution is (A) Mg (B) Fe (C) AS (D) Zn
12. acts as an oxidizing agent in (A) neutral medium (B) acidic medium
(C) alkaline medium (D) both acidic & alkaline medium 13. Decomposition of is favoured by
(A) traces of acids (B) alcohol (C) acetanilide (D) MnO
14. The maximum possible number of hydrogen bonds a water molecule can form in ice is (A) 1 (B) 2 (C) 3 (D) 4
15. and form by (A) electrovalent bond (B) covalent bond (C) coordinate bond (D) none of these
16. Select correct statements: (A) H2 is more rapidly adsorbed on Mo surfaces than D2 (B) H2 reacts over 13 times faster with Cl2 than D2 because H2 has lower energy of activation. (C) both are true (D) none is true
17. H2 reacts much faster with Cl2 than D2, because (A) rate of diffusion of H2 is greater than D2 (B) H2 has lower energy of activation than D2 (C) both statements are correct (D) none of the statements is correct
18. Which is true statement about D2O and H2O? (A) D2O has lower dielectric constant than H2O (B) NaCl is more soluble in D2O than in H2O (C) Both of the above are correct(D) none of the above is correct
19. In the following compounds H is covalent bonded in case of (A) BaH2 (B) CaH2 (C) SiH4 (D) NaH
20. Select the correct statement: (A) Ammonia is more soluble in aqueous ammonium chloride than in pure water (B) Solid ammonium fluoride and ice are miscible in all proportions (C) both the statements are correct(D) none of the statements is correct
ANSWERS TO ASSIGNMENT PROBLEMS
Subjective:
Level - O
4. (i) The hard water contains bicarbonates cruddy white ppt. chlorides and sulphate of Ca and Mg. When hard water is treated with soap solution, Ca+2 & Mg+2 ions present in water react with anions of fatty acids present in soaps to form curdy white ppt. As a result it does not produce lather with soap.
(ii) When temporary hard water is boiled, bicarbonates of calcium and magnesium respectively. These insoluble carbonate are removed by filtration and the water is rendered soft.
(iii) Petrol being lighter than water, floats over the surface of water. Hence the petrol fire can not be extinguished by water.
(iv) The salt of Ca & Mg slowly is long time form a layer on the wall of boilers, making is a bad conductor of heat. So making it unsuitable for use.
5. Ice has open cage like structure with a number of vacant spaces in the crystal lattice. As a result, density of ice is lower than that of water.
6. (i) When the spins of the nuclei are in the same direction (parallel spins), dihydrogen is called ortho hydrogen and when the spins are in the opposite direction (anti parallel spins), dihydrogen is called para hydrogen. (iii) Calgon is the trade name for sodium hexametaphosphate Na2[Na4(PO3)6]. It softens hard water by exchanging its Na+ ions with Ca+2 ions present in hard water.
7. (i)
(ii)
(iii)
(iv)
8. Water has the ability to combine with some metal salts to form compounds known as hydrates. There are three categories of hydrates. (a) Water molecules form complex ions by combining with metal ions through co – ordinates bonds.
e.g.
(b) Water molecules may be hydrogen bonded to certain oxygen containing anions. e.g. CuSO4.5H2O in which four water molecules are coordinated to a central Cu2+ ion while the fifth water molecules is hydrogen bonded to sulphate groups. Thus, it can also
be represented as
(c) 7, 8 & 9th group elements does not form hydride thus it is called as hydride gap
10. 0.1 ppm
11. Reaction of Zn and HCl gives nascent hydrogen which is more powerful reducing agent than ordinary hydrogen and thus ferric chloride solution is reduced.
12. H2O2 being unstable liquid, decomposes into water and oxygen either on standing or on heating. Bottle of H2O is therefore cooled before opening to lower the vapour pressure of contents in it otherwise bumping of liquid may take place.
13. It should be free from suspended impurities, bacterial impurities and also dissolved impurities which are harmful to body.
14. It electronic configuration resembles alkali metals (group I) and halogen group VII and it shows reactions and properties similar as well as dissimilar to the elements of these two groups.
15. The production, storage and transportation of energy in the form of liquid or gaseous hydrogen as a fuel in view of the facts that fossil fuels are limited, electricity can’t be stored and use of nuclear power is restricted.
16. Occlusion
17. First order reaction
18. Cr(OH)2
19. H2O2 > O3 > O2
20.
or
Level – I
1. (a) Cons. is not used because a part of the acid gets reduced to .
(b) liberated by conc. HCl on Zn will be impure as it contains fumes of volatile HCl. formed is insoluble in conc. HCl and forms coating on zinc and the reaction stop after some time.
(c) acts as an acid well as oxidizing agent.
2. Chromium hydroxide is oxidised by in presence of NaOH into sodium chromate.
Potassium ferricyanide is reduced to ferricyanide in presence of KOH by .
3. (a) 85% (b) 124.79 ml
5. When phosphoric acid is used in the preparation of from , it plays the double role.
It liberates and also acts as preservator by retarding its decomposition.
8. (i) Hydrolysis of occurs with evolution of
(ii) Deuteromethane is evolved
(iii) Zn dissolves in caustic potash solution evolving hydrogen
9. 1.95 ppm.
13. A is and B is .
14. 0.68g
15. (i)
(ii)
(iii)
Level - II
1. This is due to the reason that high lattice energy released is more that compensates the energy needed for the formation of ions from gas.
2. 100 ppm.
3. (a) Pure water being a weak electrolyte and feebly ionized & bad conductor of electricity. (b) Sulphate & potassium ions are not discharged as the discharge potential of is much
higher than ions and the discharge potential of ions is much higher than ions.
4. (a) Molecular (ordinary) hydrogen is not so reactive as hydrogen at the moment of formation. Zinc reacts with the acid to produce nascent hydrogen. Which reduces ferric chloride into ferrous chloride.
(b) Sodium hydride is an electrovalent compound in which hydrogen is present as an anion, H on electrolysis it is discharged at anode.
5. (i) has more molecular mass and greater degree association than and thus shows higher mid point and boiling point.
(ii) Water reacts with to produce NaOH which increases decomposition of .
(iii) reacts bases to form peroxides (salts) & water
6. (i) The reaction is highly exothermic. It brings large increase in volume also.
(ii) If anhydrous barium peroxide is used, formed forms an insoluble protective coating on the surface of solid barium peroxide. This prevents further reaction of the acid.
7. (i) (ii) Sodium borohydride is formed
(iii) Benzene in presence of is converted into phenol.
8. 4.48 volume.
9. 1.344
10. 0.564 g
Objective:
Level – I
1. A 2. C 3. C
4. D 5. A 6. C
7. D 8. D 9. B
10. C 11. B 12. A
13. D 14. D 15. C
16. C 17. A 18. B
19. B 20. C
Level – II
1. D 2. C 3. C
4. D 5. B 6. A
7. C 8. D 9. B
10. C 11. D 12. D
13. D 14. D 15. A
16. C 17. B 18. A
19. C 20. C