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Chemical equilibrium
Equilibrium state [dynamic equilibrium]The equilibrium system is apparently a
stationary system but in reality dynamic .
Example: If water in a closed vessel is heated, we notice two reversible or
opposite processes. These are vaporization and condensation processes . At the
beginning of heating, vaporization takes place faster which associated with an
increase in the vapor pressure (vapour pressure is the pressure due to water
vapour in air at a certain temperature).
The vaporization process continues until the vapour
pressure equals saturated water vapour pressure
which is maximum water vapour pressure (which is
the maximum water vapour pressure in air at a
certain temperature).
A state of equilibrium is reached between the rate of vaporization and
condensation. At this time the number of water molecules which evaporate from
the liquid is equal to the number of water molecules in the condensed vapour.
vaporiztion
condensationwater(liquid) water(vapour)
Similar to the equilibrium established in physical systems, equilibrium in many
chemical reactions takes place chemical reactions can be divided into two types :
1. Complete (irreversible) reactions.
2. Reversible reactions.
1. Irreversible reaction: is the chemical reaction takes place in one direction only
which is forward, because the product can’t react together reforming reactant
once more, because one of the product escapes from the medium as gas evolved
or precipitate deposit. As in the following reactions.
AgCl 3
NaNO
3
AgNO NaCl
2 H
2ZnCl
dil 2HCl Zn
Question explain why thermal decomposition of copper nitrate is irreversible
reaction?
Cu(NO3)2 heat CuO + 2NO2 + O2 because oxygen and nitrogen dioxide escape
from the medium so the product cant react together reforming reactant once
more.
2. Reversible reaction: is the chemical reaction takes place in the two directions,
forward and backward reactions because all the product and reactant are found in
the medium and can react together reforming reactant once more, because none
of them escapes from the medium. For example the reaction between acetic acid
and ethyl alcohol to form ethyl acetate and water as following:
Chemical reactions
Irreversible reaction
Reversible reaction
5 53 2 3 2 2
CH COOH C H OH CH COOC H H O
So we can define the chemical equilibrium as the following:
It is a dynamic state in which the rate of forward reaction equals the rate
of backward reaction.
The rate of reaction is :
The concentration of reactant or the product per unit time and measured in
mole/liter.
Note that:
1- Some reactions are very fast (instantaneously) as all ionic
compounds ( reaction of sodium chloride with silver nitrate
2- Some reactions are slow as reaction of caustic soda with oil to form
soap
3- Some reactions are very slow as iron rust take many months
So there are some factors affect rate of reaction.
1. Nature of reactant:
a. Surface area
Powder reacts faster than large particles Why? Because powder has larger surface
area.
Experiment:
In a beaker contains 250 cm3 dilute hydrochloric acid add one gram of zinc block,
notice the speed of the reaction, collect amount of hydrogen gas produced in two
minutes.
Repeat the above experiment but use one gram of zinc in the form of powder, and
notice the speed of the reaction, collect the amount of hydrogen produced in two
minutes, then compare between the two experiments.
Conclusion: the rate of reaction in the 2nd stage takes place faster as the number of
zinc atom in case of powder increases so the rate of reaction with hydrochloric
acid increases.
Question: write scientific explanation:
The chemical reaction takes place faster if the reactants are finely divided or
pours?
Factors affecting
rate of reaction
Nature of reactant concentration temperature
Surface area Kind of bonds
catalyst
Ionic
covalent
pressure light
Because as the substance is in the form of powder or pourus the number of
molecules exposed to other reactant increase and the reaction takes place faster
due to the increase of surface area.
b- Type of bonding in reactants:
In case of ionic compounds the rate of reaction takes place faster than covalent
compounds, because in case of ionic compound there is exchange of ions so we
reach to equilibrium state faster.
1- Concentration: of reactants:
The rate of reaction takes place faster in cases of high concentration
because there is a great chance for collision takes place according to the law
of mass action ------------ to understand that study the following experiment
Steps:
Add iron (III) chloride solution (faint yellow) gradually to ammonium thiocynate
solution (colourless).
Observation:
The colour of the solution turns blood red due to formation of iron (III)
thiocyanate.
3 4 4 3int
3 3 ( )colourlessfa red
FeCl NH SCN NH Cl Fe SCN yellow blood
If an excess amount of iron (III) chloride is added, the red blood colour of the
solution increases indicating formation more iron (III) thiocynate
Conclusion: If the concentration of reactants (iron III)chloride increases, the rate of
forward reaction increases i.e Fe(SCN)3 is increased.
The law of mass action:- At a constant temperature, the rate of
chemical reaction is directly proportional to the product of multiplication of the
reactant concentrations , each is raised to the power of the number of
molecules .
This law discovered by Waag and Guldberhg
You can understand the law of mass action from the above experiment
by gradual addition of iron (ІІІ) chloride solution (faint yellow colour ) to
thiocyanate solution (colourless) ,the colour of the reaction mixture becomes
red blood due to the formation of iron (ІІІ) thiocyanate .
The reaction can be represented by the following equilibrium :
3 4 4 3 blood
3 3 ( )red
FeCl NH SCN NH Cl Fe SCN
If an excess amount of iron (ІІІ) chloride is added, the red colour of the solution
increases indicating formation of more iron(ІІІ) thiocyanate.
When the rate of the backward reaction (r2) equals the forward reaction (r), in the
above reaction an equilibrium state is established and both reaction rates are
expressed as follows :
r1 α * FeCl3] [NH3SCN]3
r1 = k1[ FeCl3] [NH3SCN]3
r2 α *Fe(SCN)3 ] [ NH4Cl ]3
r2 = k2 [Fe(SCN)3 ] [ NH4Cl ]3
The brackets [ ] represent the concentration ( mole / liter unit ) whereas k1 and k2
are rate constants for the forward and back ward reactions , respectively .
At equilibrium r1 = r2
k1 [ Fecl3][ NH4SCN]3 = k2[Fe(SCN)3][NH4cl]3
3
3 3
3
3 4
[ ( ) ][ ]
[ ][ ]
Fe SCN NH Cl
FeCl NH SCN
1c
2
k=K =
k
The product of 1
2
K
K is a constant value referred to as Kc and is known as the
equilibrium constant of the reaction :
Example :-
Calculate the equilibrium constant of the reaction : 2 2I + H 2HI
Provided that the concentration of І2 ,H2 and HІ at equilibrium are 0.221 , 0.221
and 1.563 mole/liter , respectively .
Solution :-
2 2
2[ ]
[ ][ ]
HIKc
H I
2(1.563)=50
0.221 0.221x
Please note that
(a) The small values of equilibrium constant (less than one) means that the
concentration of the products are less than the concentration of the reactants
which represent that the reaction is not proceed well towards the formation of the
products and that the reversible reaction has an effective role . As an example is
the solubility of silver chloride in water
+ -
(s) (aq) (aq)AgCl Ag + Cl
Kc= [Ag+][Cl-] =1.7x 10-10
(B) The value of Kc in the above reaction indicates the insolubility of silver chloride
in water . KC is so small so backward is more favored
(c) The high values of equilibrium constant indicate that the reaction proceeds
almost to its end .This means that the forward reaction is the predominant
reaction, as in the case of the reaction of chlorine with hydrogen .
2 (g) 2(g) (g)H +Cl 2HCl K c=4.4x1032
(D) The concentration of water(liquid) , solid substances and precipitates should
not appear in the equilibrium constant equation ,because their concentrations
remain constant whatever their quantities. In general the concentration of water
or solvent is considered as a constant because it does not significantly varied in the
reaction .
3- Temperature: the rate of reaction is directly proportional to any rise in
temperature because temperature activates the molecules so a great chance for
active collision takes place this rapid the reaction. This called collision theory,
chemical reaction takes place faster at active collision, it is found that a reaction
can be doubled if the temperature increased by 10℃
note that (1) pressure cooking pots used to obtain high temperature in short time
(2) also increasing temperature increases pressure so it is undesirable to heat
butagas cylinders because it may explode due to the rise in pressure
Activation energy: it is the minimum amount of energy required to start
the reaction.
Activated molecules : Are those molecules having kinetic energy equals to
or more than activation energy
Experiment to illustrate the effect of temperature on a reaction at equilibrium:
This can be explained according to the following equilibrium equation .
reddish brown
cooling2 2 4colourlessheahing
2 NO N O + heat
We deduce from this experiment that if an exothermic reaction has reached the
equilibrium state , decrease in temperature force the reaction to proceed in the
forward direction in order to liberate heat.
3- Effect of pressure :
If the reactants or products are in the gaseous state , the concentration is
expressed by using their partial pressure. For example , ammonia gas is prepared
in industry from its elements according to the following reaction:
high pressure,cooling
2(g) 2(g) 3(g)N +3H 2NH , H= -92kJ
Four molecules react to form two molecules , i.e. , the formation of ammonia gas
is accompanied by a decrease in the number of molecules and consequently a
reduction in the volume. It was found that by applying pressure and cooling , the
rate of ammonia formation increases. We can conclude from the above example
that by increasing the pressure or cooling on a gaseous reaction under equilibrium
, a shift in the direction of reducing the pressure (or the direction in which volume
is less) takes place.
In this case , the equilibrium constant is expressed by the symbol Kp to indicate
that the concentrations of the substances are expressed by the partial pressure:
2
3
3
2 2
p (NH )Kp=
p(N )xp (H )
The equilibrium constant of the above reaction can also be expressed in terms of
molar concentration .
Example: Calculate the equilibrium constant (Kp) of the reaction :
2(g) 2(g) 2(g)N + 2O 2NO
Provided that the pressures are 2 , 1 and 0,2 atmosphere for the gases NO2, O2 and
N2, respectively .
Solution:
2 2
2
2 2
2 2
p (NO ) (2)Kp = = = 20
p(N )xp (O ) 0.2x(1)
The equilibrium constant =20
From the above observations and others , Le Chatelier ,a French scientist ,
formulated a rule named after him . This rule predicts the effect of different
factors such as concentration , temperature and pressure on the systems under
equilibrium .
In the following reaction N2 + O2 2NO - energy.
What is the effect of heat, concentration of reactant and pressure on the
formation of nitric oxide.
Answer
The effect of heat according to Le Chateleir increasing of heat will shift the
reaction forward in the direction of formation of nitric oxide so concentration of
nitric oxide increases.
Le Chatelier’s principle :
The changes in any of the conditions of a chemical equilibrium such as concentration ,
pressure or temperature cause shift of the equilibrium in the direction which will oppose
this change .
The effect of concentration of reactant: If we add more of nitrogen and oxygen the
concentration of nitric oxide will increase.
The effect of pressure: the pressure at the both side is equal so it has no effect.
(5) Effect of catalysts :
Catalysts are known as" substances that cause a change in the rate of chemical
reaction without it self being changed".
Small quantity of catalyst is required to cause such change in the reaction rate
without affecting the position of equilibrium. Catalysts are used in more than 90%
of the industrial processes such as food and petrochemical industries . Catalysts
are also used in catalytic converters utilized in modern cars to convert the gaseous
combustion products, which cause air pollution , into safe products . Catalysts are
metals, metal oxides or compounds.
Enzymes (high molecular weight proteins produced in living cells) act as catalysts
for many biological and industrial processes .
Catalyst has no effect on equilibrium position because as it increases both
forward and backward reactions by the same rate.
So catalyst attains equilibrium only in short time.
(6) Effect of light :
Some chemical reactions are affected by light , the photosynthesis is an example in
which chlorophyll in plants absorbs light and form carbohydrates in the presence
of carbon dioxide and water .
Photographic films contain silver bromide in a gelatinous layer .When light falls on
such films , silver ions accept electrons from bromide ions and are converted into
silver metal . Bromine is absorbed in the gelatinous layer.
The increase in light intensity is associated with an increase in the amount of silver
formed .
+ -Ag + e Ag
Ionic equilibrium.
To understand the concept of ionic equilibrium you have to know the meaning of
electrolyte.
The electrolytic solution is the solution that conducts electricity by movement of
its ions.
Strong electrolyte: it is completely ionize in the solution, such as strong
acids and strong alkalis and some salts
Weak electrolyte: is the solution not completely ionized as some covalent
compounds
Experiment to show the difference between weak electrolyte and strong electrolyte
Dissolve 0.1 mole of hydrogen chloride gas in one liter of water,
Also dissolve 0.1 mole of pure ethanoic acid in water to make two acid
solutions of equal concentration.
Electrolyte
Strong electrolyte
Weak electrolyte
a solution of HClAcetic acid
Test the electrical conductivity of each solution.
Observation
You can notice that in the case of hydrochloric acid, the lamp gives a strong
illumination, but gives a faint illumination with ethanoic acid.
Conclusion: A solution of HCl in water is strong electrolyte and completely
ionizes 3HCl HOH H O Cl while a solution of acetic acid is weak
electrolyte and partially ionized 3 3 3CH COOH HOH CH COO H O
Ionization: is the process in which all unionized molecules converts into ions
Exp 2Explain experiment to show the relation between ionization
and dilution (Verify Ostwald law)
Dissolve 0.1 mole of hydrogen chloride gas in one liter of water,
also dissolve 0.1 mole of pure ethanoic acid in water to make two acid
solutions of equal concentration.
a solution of HClAcetic acid
Test the electrical conductivity of each solution.
Observation
you can notice that in the case of hydrochloric acid, the lamp gives a strong
illumination, but gives a faint illumination with ethanoic acid
Repeat the above experiment
Dilute each solution to 0.01 molar and test the electrical conductivity, then
further dilute each solution to 0.001 molar and test the conductivity of each
solution again. You will notice that illumination of the lamp is not affected in the
case of dilution of hydrochloric acid, but the illumination increases in the case
of the dilution of acetic acid.
Conclusion.
Electrical conductivity of HCl doesn’t affect by dilution but ethanoic acid
increase, so the degree of ionization of weak electrolyte only increases by
dilution
Ionization constants of some weak acids are given in the following table:
Acid Chemical formula Ionization constant (ka)
Sulphurous H2SO3 1.7 × 10 -2
Hydrofluoric HF 6.7 × 10 -4
Nitrous HNO2 5.1 × 10 -4
Acetic (Ethanoic) CH3COOH 1.8 × 10 -5
Carbonic H2CO3 4.4 × 10 -7
Boric H3BO3 5.8 × 10 -10
In case of weak acid KA is directly proportion to strength of the acid
Hydroxonium ion:
No free hydrogen ion (protons); is present in aqueous of ionized acids .This ion
attract to the lone pair of electrons on the oxygen atom of the water molecule and
connected to a water molecule by a co-ordinate covalent bond. This proton is
called hydrated proton or hydroxonium ion [H3O] +.
+ -
2 3HCl +H O H O +Cl
Ionic equilibrium:-
It is the equilibrium arising between molecules of a weak electrolyte
and ions resulting from it.
The law of mass action can be applied only in the case of weak electrolyte
solutions. Strong electrolyte solutions do not contain un-dissociated molecules,
since they are completely ionized.
In 1888 Ostwald described the relation between the extent
of ionization - alpha ( α ) and concentration ( C ) per mole/litre.
Ostwald law
At constant temperature, the degree of ionization ( ) increases
by dilution
To understand Ostwald law assume that a mono-protonic weak acid (HA)
dissociates in water according to the equation :
+ -HA H +A
By applying the law of mass action on this equilibrium system , the following
relation is obtained
[ ][ ]
[ ]
H AKa
HA
Then at equilibrium :
Degree of dissociation = Number of dissociated moles
Total number of moles before dissociation
If the number of dissociated moles is ( ) mole then, the number of un
dissociated moles from HA= (1- ) and the number of moles of H- and A-produced
equals mole.
The concentration (c) = Number of moles
Volume by liter (V)
The concentration of the substance (mole/ L) at equilibrium is
+ - HA H +A
(1 )
V
V
V
Substitution in the equation of mass action gives :
2
1
[ ][ ]
[ ] (1 )
v v
v
Kav
This relation is known as Ostwald Law for dilution which illustrate the quantitative
relationship between the degree of ionization ( ) and dilution . it can be seen that
At constant temperature, the degree of ionization ( ) increases
by dilution (so that Ka value remains constant ) . In case of weak electrolytes , the
degree of ionization ( ) is small enough and can be neglected. Consequently the
value (1- ) is considered approximately one and the relation becomes :
2
Kav
Since the concentration of the weak acid (c) =1
vmale /liter , the above equation
becomes: Ka= 2 x c
This means that increasing dilution (decrease in concentration )
causes an increase in dissociation degree and vice versa .
Example : calculate degree of ionization of 0.1 molar hydrocyanic (HCN) acid
solution at 25°C , providing that the equilibrium constant of the acid Ka= 7.2x10-10
Solution :
+ -
2 3HCN + H O H O +CN
Applying Ostwald Law : Ka= 2 x c
7.2x 1010 = 2 x 0.1
-102 -107.2x10= =72x10
0.1
= -1072x10 = 8.5x10-5
Calculation of hydroxonium ion concentration of weak acids :
+
3
+
3 A
[H O ] = C x Ka
[H O ]= .C
Example :
Calculate the hydrogen ion concentration in 0.1 molar acetic acid solution at
25oC , provided that the equilibrium constant of the acid is 1.8 x 10-5
Solution :
- +
3 3
3
[CH OO ] [H O ]Ka =
[CH COOH]
+ -5 -3
3[H O ] = 1.8 x 10 x 0.1 = 1.342 x10 molar
Calculation of the hydroxyl ion of weak bases :
Partially dissociated bases in aqueous solutions are know as weak bases . The
concentration of hydroxyl ion can be calculated as following -
b b [OH ]= C xK
2. Ionization of water:
Pure water is a very poor conductor of electricity, but some conductivity is
detected, so some ionization of water must take place as shown in the following
equation :
+ -
2 3H O H O +OH
For simplification , this equation can be written :
+ -
2H O H +OH
The equilibrium constant is expressed by the relation
+ --14
2
[H ] [OH ]Kc = =10
[H O]
From the value of the equilibrium constant, it is clear that a very small number of
water molecules ionizes.
The number of unionized water molecules can be considered as a constant value.
Therefore, the above relation can be rewritten as follows ;
Kw = [H+][OH
-] = 10
-14
Since water is neutral ( no action on litmus ), the concentration of H+ ( responsible
for the acidic properties ) equals the concentration of OH- ( responsible for the
basic properties ).
Then Kw = [10-7
][ 10-7
] = 10-14
K w( ionic product of water ):
The ionic product of water is the resultant of multiplying the concentration of the
hydrogen ion [H+] times that of hydroxide ion [OH-] .It is a constant value equals
10-14 mole/liter.
pH value
It is the negative logarithm of hydrogen ion concentration:
pH = -log [H3O+]
The symbol (p) means (-log) .
This is a way for expressing the degree of acidity or alkalinity for aqueous solution.
Referring to the ionic product of water and using the negative logarithm of this
equation, the above relation becomes:
-log Kw=-log [H+] –log [OH-] =-log 10-14
Substituting the value (-log ) by the symbol (p) , the above equation becomes :
pH + pOH= 14
In case of a neutral solution , the pH=7 and pOH =7. For solution of pH 5 , the
pOH=9 and for solution has a pH 8 , the pOH =6. Consequently , the pH value of
the acidic solutions is < 7 and the POH is >7 and for basic solutions the pH is >7 and
pOH is<7.
Hydrolysis of some salts
Hydrolysis is opposite to neutralization and defined as decomposition of salt using
water to form acid and base one of them at least is weak.
Acid + base salt + water
(neutralization) (hydrolysis)
Hydrolysis of sodium chloride
Sodium chloride solution has neutral effect to litmus solution, as it consists of
strong acid HCl and strong alkali NaOH, and hydrolysis can be explained as follow:
HOH H + OH
NaCl Na+ + Cl
---------------------------------------------------------------------
By adding:NaCl + HOH Na+ + OH + H + Cl
salts
Acidic if it consist of strong acid and weak base
Basic if the base is strong and the acid is weak
Neutral if both acid and alkali in the same strength
We found that neither proton H+ nor hydroxide ions disappear from the solution
so that sodium chloride has neutral effect.
Hydrolysis of ammonium chloride NH4Cl
Ammonium chloride has acidic effect to litmus solution as it consists of strong acid
and weak base and hydrolysis can be explained as following:
HOH H + OH
4 NH Cl 4
NH + Cl by adding the two equation we find that
NH4Cl + H2O H + Cl + NH4OH.
Hydrolysis of sodium carbonate.
Sodium carbonate has alkaline effect to litmus solution since it hydrolyzes to form
strong alkali (NaOH) and weak acid (H2CO3) as following:
2HOH 2 H + 2 OH
Na2CO3 2Na+ + CO3-2
By adding the two equations
Na2CO3 + 2H2O H2CO3 + 2Na+ + 2 OH
According to Le Chatileir principle hydrogen ions disappear so water ionizes more
to produce more hydroxide ions which is responsible for alkaline effect
Hydrolysis of ammonium acetate
Ammonium acetate has neutral effect to litmus solution since it consists of weak
acid and weak base, the hydrolysis process takes place as following:
HOH H + OH
CHCOONH4 CH3COO +
4NH
By adding:
CH3COONH4 + HOH CH3COOH(weak acid) + NH4OH (weak base)
We found that both hydrogen ion and hydroxide ion disappear from the solution
so that ammonium acetate has neutral effect to litmus solution
Note that the strong alkalis are from group 1A and the strong acid HCl, H2SO4
and nitric acid HNO3
Solubility product
The dissolution of the sparingly soluble lead bromide in water is given by relation :
2+ -
2Pb Br Pb +2Br
The equilibrium constant is given by the relation:
2+ - 2
2
[Pb ][Br ]Kc =
[ Pb Br ]
The solid Pb Br2 concentration remains almost constant :
Ksp =[Pb 2+] [ Br -]2
Ksp is known as solubility product .
Solubility product for any sparingly soluble ionic compound is the
product of multiplication of the concentration (expressed by male /
liter )of its ions raised to power number of ions which exist in
equilibrium with its saturated solution
Example :If the degree of solubility of silver chloride (AgCl)is 10-5 mole / liter ,
calculate the solubility product of Agc1 . AgCl
Solution :
AgCl Ag C l
Ksp = [A g +] [C l-]
Ksp = 10-5x 10-5=10-10
Example
Calculate KSP of calcium phosphate Ca3(PO4)2 given that concentration of calcium ions is ( 81 10 ) mole
/liter and concentration of phosphate ions is 30.5 10 mole /liter
2 3
3 4 2 4( ) 3 2Ca PO Ca PO
KSP= 2 3 3 2
4[ ] [ ]Ca PO = 8 3 3 2 31[10 ] [0.5 10 ] 2.5 10