water and hydrogen - mwalimu ndolo · web viewpure water is a colourless, odourless,...
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A. WATERPure water is a colourless, odourless, tasteless ,neutral liquid. Pure water does not exist in nature but naturally in varying degree of purity. The main sources of water include rain, springs, borehole, lakes, seas and oceans:Water is generally used for the following purposes:
(i)drinking by animals and plants.(ii)washing clothes.(iii)bleaching and dyeing.(iv) generating hydroelectric power.(v)cooling industrial processes.
Water dissolves many substances/solutes. It is therefore called universal solvent.It contains about 35% dissolved Oxygen which support aquatic fauna and flora.Water naturally exist in three phases/states solid ice,liquid water and gaseous water vapour. The three states of water are naturally interconvertible. The natural interconvertion of the three phases/states of water forms the water cycle.
Precipitation
condensation CLOUDS (Water in solid state)
RAIN
SPRING, RIVERS,WELLS.
OCEAN,LAKES,SEAS(water as liquid)
Evaporation(Water in gaseous state)
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Liquid water in land, lakes , seas and oceans use the solar/sun energy to evaporate/vapourize to form water vapour/gas. Solar/sun energy is also used during transpiration by plants and respiration by animals. During evaporation, the water vapour rises up the earths surface. Temperatures decrease with height above the earth surface increase. Water vapour therefore cools as it rises up. At a height where it is cold enough to below 373Kelvin/100oC Water vapour looses enough energy to form tiny droplets of liquid.The process by which a gas/water vapour changes to a liquid is called condensation/liquidification. On further cooling, the liquid looses more energy to form ice/solid. The process by which a liquid/water changes to a ice/solid is called freezing/solidification. Minute/tiny ice/solid particles float in the atmosphere and coalesce/join together to form clouds. When the clouds become too heavy they fall to the earths surface as rain/snow as the temperature increase with the fall.
Interconversion of the three phases/states water
Evaporation Liquidification/ /boiling/vapourization condensation
Melting Freezing liquidification
Solidification
Pure water has :(i) fixed/constant/sharp freezing point/melting point of 273K/0oC(ii) fixed/constant/sharp boiling point of 373K/100oC at sea level/1 atmosphere pressure(iii) fixed density of 1gcm-3 This is the criteria of identifying pure/purity of water.
Gas/water vapour
Liquid/Water
Solid/Ice
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Whether a substance is water can be determined by using the following methods:a)To test for presence of water using anhydrous copper(II)suphate(VI)
Procedure. Put about 2g of anhydrous copper(II)sulphate(VI)crystals into a clean test tube. Add three drops of tap water. Repeat the procedure using distilled water. Observation.Colour changes from white to blueExplanation.Anhydrous copper(II)sulphate(VI)is white. On adding water ,anhydrous copper(II)sulphate(VI) gains/reacts with water to form hydrated copper(II) sulphate(VI). Hydrated copper(II) sulphate(VI) is blue. Hydrated copper(II) sulphate(VI) contain water of crystallization. The change of white anhydrous copper(II)sulphate(VI) to blue hydrated copper(II) sulphate(VI) is a confirmatory test for the presence of water Chemical equation.
Anhydrous Hydratedcopper(II)sulphate(VI) + Water -> copper
(II)sulphate(VI) (white) (blue)
CuSO4(s) + 5H2O(l) -> CuSO4.5H2O(s)
b)To test for presence of water using anhydrous cobalt(II)chloride
Procedure. Put about 5cm3 of water into a clean test tube. Dip a dry anhydrous cobalt(II)chloride paper into the test tube. Repeat the procedure using distilled water.
Observation.Colour changes from blue to pink Explanation.Anhydrous cobalt(II)chloride is blue. On adding water, anhydrous cobalt(II)chloride gains/reacts with water to form hydrated cobalt(II) chloride. Hydrated cobalt(II)chloride is pink. Hydrated cobalt (II)chloride contain water of crystallization.
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The change of blue anhydrous cobalt(II)chloride to pink hydrated cobalt(II)chloride is a confirmatory test for the presence of water Chemical equation.
Anhydrous Hydratedcobalt(II)chloride + Water -> cobalt (II)chloride
(Blue) (pink)CoCl2 (s) + 5H2O(l) -> CoCl2.5H2O(s)
Burning a candle in air
Most organic substances/fuels burn in air to produce water. Carbon(IV)oxide gas is also produced if the air is sufficient/excess.
Procedure Put about 2g of anhydrous copper(II)sulphate(VI)crystals in a boiling tube. Put about 5cm3 of lime water in a boiling tube.Light a small candle stick. Place it below an inverted thistle/filter funnelCollect the products of the burning candle by setting the apparatus as below Set up of apparatus
ObservationThe sunction pump pulls the products of burning into the inverted funnel. Colour of anhydrous copper(II) sulphate(VI)changes from white to blue. A white precipitate is formed in the lime water/calcium hydroxide.
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ExplanationWhen a candle burn it forms a water and carbon(IV)oxide. Water turns anhydrous copper(II) sulphate(VI)changes from white to blue .Carbon(IV)oxide gas forms white precipitate when bubbled in lime water/calcium hydroxide. Since: (i)hydrogen in the wax burn to form water
Hydrogen + Oxygen -> Water(from candle) (from the air) 2H2(g) + O2(g) -> 2H2O (g/l)
(ii) carbon in the wax burn to form carbon(IV)oxide Hydrogen + Oxygen -> Water
(from candle) (from the air) C(s) + O2(g) -> CO2 (g)
The candle before burning therefore contained only Carbon and Hydrogen only. A compound made up of hydrogen and carbon is called Hydro carbon . A candle is a hydrocarbon.Other hydrocarbons include: Petrol, diesel, Kerosene, and Laboratory gas. Hydrocarbons burn in air to form water and carbon(IV)oxide gas.
Hydrocarbons + Oxygen -> Water + Oxygen
Water pollutionWater pollution take place when undesirable substances are added into the water. Sources of water pollution include:(i)Industrial chemicals being disposed into water bodies like rivers, lakes and oceans.(ii)Dicharging untreated /raw sewage into water bodies.(iii)Leaching of insecticides/herbicides form agricultural activities into water bodies.(iv)Discharging non-biodegradable detergents after domestic and industrial use into water bodies.(v)Petroleum oil spilling by ships and oil refineries(vi)Toxic/poisonous gases from industries dissolving in rain .(vii) Acidic gases from industries dissolving in rain to form “acid rain”(viii)Discharging hot water into water bodies.This reduces the quantity of dissolved Oxygen in the water killing the aquatic fauna and flora.
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Water pollution can be reduced by:(i)reducing the use of agricultural fertilizers and chemicals in agricultural activities.(ii)use of biological control method instead of insecticides and herbicides(iii)using biodegradable detergents
Reaction of metals with waterSome metals react with water while others do not. The reaction of metals with water depend on the reativity series. The higher the metal in the reactivity series the more reactive the metal with water .The following experiments shows the reaction of metals with cold water and water vapour/steam.
(a)Reaction of sodium/ potassium with cold water:ProcedurePut about 500cm3 of water in a beaker. Add three drops of phenolphthalein indicator/litmus solution/universal indicator solution/methyl orange indicator into the water.Cut a very small piece of sodium .Using a pair of forceps, put the metal into the water.
ObservationSodium melts to a silvery ball that floats and darts on the surface decreasing in size.Effervescence/fizzing/ bubbles of colourless gas produced.Colour of phenolphthalein turns pinkColour of litmus solution turns blueColour of methy orange solution turns OrangeColour of universal indicator solution turns blue
ExplanationSodium is less dense than water. Sodium floats on water and vigorously react to form an alkaline solution of sodium hydroxide and producing hydrogen gas. Sodium is thus stored in paraffin to prevent contact with water.
Chemical equationSodium + Water -> Sodium hydroxide + Hydrogen gas 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)
To collect hydrogen gas , Sodium metal is forced to sink to the bottom of the trough/beaker by wrapping it in wire gauze/mesh.
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Potassium is more reactive than Sodium. On contact with water it explodes/burst into flames. An alkaline solution of potassium hydroxide is formed and hydrogen gasChemical equationPotassium + Water -> Potassium hydroxide + Hydrogen gas 2K(s) + 2H2O(l) -> 2KOH(aq) + H2(g)
Caution: Reaction of Potassium with water is very risky to try in a school laboratory.
(b)Reaction of Lithium/ Calcium with cold water:ProcedurePut about 200cm3 of water in a beaker. Add three drops of phenolphthalein indicator/litmus solution/universal indicator solution/methyl orange indicator into the water.Cut a small piece of Lithium .Using a pair of forceps, put the metal into the water.Repeat with a piece Calcium metal
ObservationLithium sinksto the bottom of the water.Rapid effervescence/fizzing/ bubbles of colourless gas produced.Colour of phenolphthalein turns pink
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Colour of litmus solution turns blueColour of methy orange solution turns OrangeColour of universal indicator solution turns blue
ExplanationLithium and calcium are denser than water. Both sink in water and vigorously react to form an alkaline solution of Lithium hydroxide / calcium hydroxide and producing hydrogen gas. Lithium is more reactive than calcium. It is also stored in paraffin like Sodium to prevent contact with water.
Chemical equationLithium + Water -> Lithium hydroxide + Hydrogen gas 2Li(s) + 2H2O(l) -> 2LiOH(aq) + H2(g)Calcium + Water -> Calcium hydroxide + Hydrogen gas Ca(s) + 2H2O(l) -> Ca(OH)2(aq) + H2(g)
(c) Reaction of Magnesium/Zinc/ Iron with Steam/water vapour:
Procedure method1Place some wet sand or cotton/glass wool soaked in water at the bottom of an ignition/hard glass boiling tube.Polish magnesium ribbon using sand paper. Coil it at the centre of the ignition/hard glass boiling tube.Set up the apparatus as below. Heat the wet sand or cotton/glass wool soaked in water gently to: (i)drive away air in the ignition/hard glass boiling tube.
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(ii)generate steam Heat the coiled ribbon strongly using another burner.Repeat the experiment using Zinc powder and fresh Iron filings.
Set up of apparatus
Observations(i)With Magnesium ribbon: The Magnesium glow with a bright flame (and continues to burn even if heating is stopped)White solid /ash formedWhite solid /ash formed dissolve in water to form a colourless solutionColourless gas produced/collected that extinguish burning splint with “pop sound” (ii)With Zinc powder: The Zinc powder turns red hot on strong heating Yellow solid formed that turn white on coolingWhite solid formed on cooling does not dissolve in water.(iii)With Iron fillings: The Iron fillings turns red hot on strong heating Dark blue solid formed Dark blue solid formed does not dissolve in water.
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Procedure method 2Put some water in a round bottomed flask Polish magnesium ribbon using sand paper. Coil it at the centre of a hard glass tubeSet up the apparatus as below. Heat water strongly to boil so as to: (i)drive away air in the glass tube.(ii)generate steam Heat the coiled ribbon strongly using another burner. Repeat the experiment using Zinc powder and fresh Iron filings.
Observations(i)With Magnesium ribbon: The Magnesium glow with a bright flame (and continues to burn even if heating is stopped)White solid /ash formedWhite solid /ash formed dissolve in water to form a colourless solutionColourless gas produced/collected that extinguish burning splint with “pop sound” (ii)With Zinc powder: The Zinc powder turns red hot on strong heating Yellow solid formed that turn white on coolingWhite solid formed on cooling does not dissolve in water.
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(iii)With Iron fillings: The Iron fillings turns red hot on strong heating Dark blue solid formed Dark blue solid formed does not dissolve in water.
Explanations
(a)Hot magnesium burn vigorously in steam. The reaction is highly exothermic generating enough heat/energy to proceed without further heating.White Magnesium oxide solid/ash is left as residue. Hydrogen gas is produced .It extinguishes a burning splint with a “pop sound”.Chemical Equation
Magnesium + Steam -> Magnesium oxide + HydrogenMg(s) + H2O(g) -> MgO(s) + H2(g)
Magnesium oxide reacts /dissolves in water to form an alkaline solution of Magnesium oxide Chemical Equation
Magnesium oxide + Water -> Magnesium hydroxide MgO(s) + H2O(l) -> Mg(OH) 2 (aq)
(b)Hot Zinc react vigorously in steam forming yellow Zinc oxide solid/ash as residue which cools to white. Hydrogen gas is produced .It extinguishes a burning splint with a “pop sound”.Chemical Equation
Zinc + Steam -> Zinc oxide + HydrogenZn(s) + H2O(g) -> ZnO(s) + H2(g)
Zinc oxide does not dissolve in water.
(c)Hot Iron react with steam forming dark blue tri iron tetra oxide solid/ash as residue. Hydrogen gas is produced .It extinguishes a burning splint with a “pop sound”.Chemical Equation
Iron + Steam -> Tri iron tetra oxide + Hydrogen2Fe(s) + 4H2O(g) -> Fe2O4(s) + 4H2(g)
Tri iron tetra oxide does not dissolve in water.
(d)Aluminium reacts with steam forming an insoluble coat/cover of impervious layer of aluminium oxide on the surface preventing further reaction.
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(e) Lead, Copper, Mercury, Silver, Gold and Platinum do not react with either water or steam.
HYDROGENOccurrenceHydrogen does not occur free in nature. It occurs as Water and in Petroleum.
School laboratory PreparationProcedurePut Zinc granules in a round/flat/conical flask. Add dilute sulphuric(VI) /Hydrochloric acid. Add about 3cm3 of copper(II)sulphate(VI) solution.Collect the gas produced over water as in the set up below. Discard the first gas jar. Collect several gas jar.
Observation/ExplanationZinc reacts with dilute sulphuric(VI)/hydrochloric acid to form a salt and produce hydrogen gas.When the acid comes into contact with the metal, there is rapid effervescence/ bubbles /fizzing are produced and a colourless gas is produced that is collected:
(i) over water because it is insoluble in water(ii)through downward displacement of air/upward delivery because it is less dense than air.
The first gas jar is impure. It contains air that was present in the apparatus.Copper(II)sulphate(VI)solution act as catalyst.
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Chemical equation(a) Zinc + Hydrochloric acid -> Zinc chloride + Hydrogen Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)Ionic equation
Zn (s) + 2H+ (aq) -> Zn2+ (aq) + H2 (g)
Zinc + Sulphuric(VI)acid -> Zinc Sulphate(VI) + Hydrogen Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)Ionic equation
Zn (s) + 2H+ (aq) -> Zn2+ (aq) + H2 (g)
(b) Chemical equation Magnesium + Hydrochloric acid -> Magnesium chloride + Hydrogen Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)Ionic equation
Mg (s) + 2H+ (aq) -> Mg2+ (aq) + H2 (g)
Magnesium + Sulphuric(VI)acid -> Magnesium Sulphate(VI) + Hydrogen Mg(s) + H2SO4(aq) -> MgSO4(aq) + H2(g)Ionic equation Mg (s) + 2H+ (aq) -> Mg2+ (aq) + H2 (g)
(c) Chemical equation Iron + Hydrochloric acid -> Iron(II)chloride + Hydrogen Fe(s) + 2HCl(aq) -> FeCl2(aq) + H2(g)Ionic equation
Fe (s) + 2H+ (aq) -> Fe2+ (aq) + H2 (g)
Iron + Sulphuric(VI)acid -> Iron(II) Sulphate(VI) + Hydrogen Fe(s) + H2SO4(aq) -> FeSO4(aq) + H2(g)Ionic equation Fe (s) + 2H+ (aq) -> Fe2+ (aq) + H2 (g)Note
1.Hydrogen cannot be prepared from reaction of: (i)Nitric(V)acid and a metal. Nitric(V)acid is a strong oxidizing agent. It oxidizes hydrogen gas to water. (ii)dilute sulphuric(VI)acid with calcium/Barium/Lead because Calcium sulphate(VI),Barium sulphate(VI) and Lead(II)sulphate(VI) salts formed are insoluble. Once formed, they cover/coat the unreacted calcium/Barium/Lead
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stopping further reaction and producing very small amount/volume of hydrogen gas. (iii)dilute acid with sodium/potassium. The reaction is explosive.
Properties of Hydrogen gas
(a)Physical properties1. Hydrogen is a neutral ,colourless and odourless gas. When mixed with air it has a characteristic pungent choking smell2. It is insoluble in water thus can be collected over water.3. It is the lightest known gas. It can be transferred by inverting one gas jar over another. (b)Chemical properties.
(i)BurningI. Hydrogen does not support burning/combustion. When a burning splint is inserted into a gas jar containing Hydrogen, the flame is extinguished /put off.
II. Pure dry hydrogen burn with a blue quiet flame to form water. When a stream of pure dry hydrogen is ignited, it catches fire and continues to burn with a blue flame.
III. Impure (air mixed with) hydrogen burns with an explosion. Small amount/ volume of air mixed with hydrogen in a test tube produce a small explosion as a “pop” sound. This is the confirmatory test for the presence of Hydrogen gas. A gas that burns with a “pop” sound is confirmed to be Hydrogen.
(ii)Redox in terms of Hydrogen transfer
Redox can also be defined in terms of Hydrogen transfer.(i)Oxidation is removal of Hydrogen(ii)Reduction is addition of Hydrogen(iii)Redox is simultaneous addition and removal of Hydrogen
ExampleWhen a stream of dry hydrogen gas is passed through black copper (II) oxide, hydrogen gas gains the oxygen from copper(II)oxide. Black copper (II) oxide is reduced to brown copper metal. Black copper(II)oxide os thus the Oxidizing agent. Hydrogen gas is oxidized to Water. Hydrogen is the Reducing agent.
Set up of apparatus
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(a)Chemical equation (i) In glass tube
Copper(II)Oxide + Hydrogen -> Copper + Hydrogen gas(oxidizing agent) (reducing agent) (black) (brown) CuO (s) + H2(g) -> Cu(s) + H2O(l)
(ii)when excess Hydrogen is burning.Oxygen + Hydrogen -> WaterO2(g) + 2H2(g) -> 2H2O(l)
(b)Chemical equation (i) In glass tube
Lead(II)Oxide + Hydrogen -> Lead + Hydrogen gas(oxidizing agent) (reducing agent)
(brown when hot/ (grey) yellow when cool)
PbO (s) + H2(g) -> Pb(s) + H2O(l)
(ii)when excess Hydrogen is burning.Oxygen + Hydrogen -> WaterO2(g) + 2H2(g) -> 2H2O(l)
(c)Chemical equation (i) In glass tube
Iron(III)Oxide + Hydrogen -> Iron + Hydrogen gas(oxidizing agent) (reducing agent)
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(Dark grey) (grey) Fe2O3 (s) + 3H2(g) -> Fe(s) + 3H2O(l)
(ii)when excess Hydrogen is burning.Oxygen + Hydrogen -> WaterO2(g) + 2H2(g) -> 2H2O(l)
(iii) Water as an Oxide as HydrogenBurning is a reaction of an element with Oxygen. The substance formed when an element burn in air is the oxide of the element. When hydrogen burns, it reacts/ combines with Oxygen to form the oxide of Hydrogen.The oxide of Hydrogen is called water. Hydrogen is first dried because a mixture of Hydrogen and air explode. The gas is then ignited .The products condense on a cold surface/flask containing a freezing mixture. A freezing mixture is a mixture of water and ice.
The condensed products are collected in a receiver as a colourless liquid.Tests(a) When about 1g of white anhydrous copper (II)sulphate(VI)is added to a sample of the liquid ,it turns to blue. This confirms the liquid formed is water.
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(b) When blue anhydrous cobalt (II)chloride paper is dipped in a sample of the liquid ,it turns to pink. This confirms the liquid formed is water.(c)When the liquid is heated to boil, its boiling point is 100oC at sea level/one atmosphere pressure. This confirms the liquid is pure water.
Uses of Hydrogen gas
1. Hydrogenation/Hardening of unsaturated vegetable oils to saturated fats/margarine. When Hydrogen is passed through unsaturated compounds in presence of Nickel catalyst and about 150oC, they become saturated. Most vegetable oil are unsaturated liquids at room temperature. They become saturated and hard through hydrogenation.
2. In weather forecast balloons.Hydrogen is the lightest known gas. Meteorological data is collected for analysis by sending hydrogen filled weather balloons to the atmosphere. The data collected is then used to forecast weather conditions. 3.In the Haber process for the manufacture of AmmoniaHydrogen is mixed with Nitrogen in presence of Iron catalyst to form Ammonia gas. Ammonia gas is a very important raw material for manufacture of agricultural fertilizers.
4.In the manufacture of Hydrochloric acid.Limited volume/amount of Hydrogen is burnt in excess chlorine gas to form Hydrogen chloride gas. Hydrogen chloride gas is dissolved in water to form Hydrochloric acid. Hydrochloric acid is used in pickling/washing metal surfaces. 5. As rocket fuel.Fixed proportions of Hydrogen and Oxygen when ignited explode violently producing a lot of energy/heat.This energy is used to power/propel a rocket to space.
6. In oxy-hydrogen flame for welding.A cylinder containing Hydrogen when ignited in pure Oxygen from a second cylinder produces a flame that is very hot. It is used to cut metals and welding.
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Sample revision questions
1.A colourless liquid was added anhydrous copper(II)sulphate(VI) which turned blue.(a)Why is it wrong to conclude the liquid was pure water?Anhydrous copper(II)sulphate(VI) test for presence of water. Purity of water is determined from freezing/melting/boiling point.
(b)Write an equation for the reaction that take place with anhydrous copper(II)sulphate(VI)Anhydrous copper(II)sulphate(VI) + Water -> hydrated copper(II)sulphate(VI)
CuSO4(s) + 5H2O(l) -> CuSO4.5H2O(s)
(c)(i)Which other compound would achieve the same results as anhydrous copper(II)sulphate(VI) Anhydrous cobalt (II)chloride/CoCl2.6H2O
(ii)Write the equation for the reactionAnhydrous cobalt (II)chloride + Water -> hydrated cobalt (II)chloride
CoCl2 (s) + 6H2O(l) -> CoCl2.6H2O (s)
(d)Complete the equation (i) Sulphur(VI)oxide + Water -> Sulphuric(VI)acid
(ii) Sulphur(IV)oxide + Water -> Sulphuric(IV)acid(iii) Carbon(IV)oxide + Water -> Carbonic(IV)acid
(iv) Nitrogen(IV)oxide + Water -> Nitric(V)acid(v) Phosphorus(V)oxide + Water -> Phosphoric(V)acid
(vi) Sodium oxide + Water -> Sodium hydroxide(vi) Sodium peroxide + Water -> Sodium hydroxide
2. Metal B reacts with steam. Metal C reacts with cold water. Metal A does not react with water.(a)Arrange the metals as they should appear in the reactivity series. B
C A
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(b)A product residue in D which was brown when hot but turned yellow on cooling during the reaction of metal B was formed. Gas E was also evolved. Identify
(i)Metal B Lead/Pb (ii)Residue D Lead(II)oxide/PbO(iii)Gas E Hydrogen/H2
(c)A portion of product residue in D was added dilute nitric(V)acid. Another portion of product residue in D was added dilute sulphuric(VI)acid. State and explain the observations made.When added dilute nitric(V)acid, D dissolves to form a colourless solution. Lead(II)Oxide + dilute nitric(V)acid -> Lead(II) nitrate(V) + Water
PbO (s) + 2HNO3(aq) -> Pb(NO3)2 (aq) + H2O(l)
When added dilute sulphuric(VI)acid, D does not dissolve. A white suspension/precipitate was formed. Lead(II)Oxide reacts with sulphuric(VI)acid to form insoluble Lead(II)sulphate(VI) that cover/coat unreacted Lead(II)Oxide, stopping further reaction. Lead(II)Oxide + dilute sulphuric(VI)acid -> Lead(II) sulphate(VI) + Water PbO (s) + H2SO4(aq) -> PbSO4 (s) + H2O(l)
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TOPICAL QUESTIONS ON THE TOPIC BASED ON PAST KCSE PAPERS
1. Use the information shown in the diagram below to answer the questions that follows.
i) Explain why it is important to pass the hydrogen gas for some time before lighting it at point Z. (1mk)
ii) Write an equation for the reaction that takes place when hydrogen burns at point Z. (1mk)
2. The order of reactivity of metal p, R and T starting with the most reactive is R.T.P. By using a tick () to indicate no reaction, complete the table below to show what happens when the metals of each are added to solutions containing ions of metal P, R and T.
Aqueous solution containing ions of metalMetal P R TPRT
3. In an experiment, soap solution was added to three separate samples of water. The table below shows the volumes of soap solution required to form lather with 100cm3 of each sample of water before and after boiling.
Sample 1 Sample 2 Sample 3Volume of soap before water is boiled in cm3 27.0 3.0 10.6Volume of soap after water is boiled in cm3 27.0 3.0 3.0
a) Which water sample is likely to be soft? Explain. (2mks)b) Explain the change in volume of soap solution used in sample III (1mk)
4. Study the diagram below and answer questions that follow.
Write an equation for each of the two reactions that take place in the experiment represented by the diagram above (2mks)
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5. Zinc metal and hydrochloric acid react according to the following:Zn(s) + 2HC (aq) → ZnCl2 (aq) + H2 (g)
1.9 g of zinc metal was reacted with 100cm3 of 0.2m Hydrochloric acid.a) Determine the reagent that was in excess. (2mks)b) Calculate the total volume of hydrogen gas that was liberated at S.T.P. (Zn= 65).
Molar gas volume= 22.4dm3 at STP. (1mk)6. The diagram below represents set-up that was used to react lithium with water vapour.
Study it and answer the questions that follow.
a) Write an equation for the reaction that takes place given that the atomic number of Lithium is 3. (2mks)
b) Why would it not be advisable to use potassium in place of Lithium in the above set up? (2mks)
7. A student set up the experiment below to collect gas K. The glass wool was heated before heating the zinc powder.
a) Why was it necessary to heat the moist glass wool before heating zinc powder?
(1mk)b) What would happen if the zinc powder was heated before heating the glass wool?
(1mk)c) What property of gas K made it possible for it to be collected as shown in the
diagram? (1mk)8. A sample of water drawn from a river passing through an agricultural district was divided
into two portions. The first portion gave a white precipitate when acidified barium
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chloride was added. The second portion when warmed with aqueous sodium hydroxide gave a colourless gas which turned a moist red litmus paper to blue.a) Identify the ions present in the river water. (2mks)b) Suggest the possible sources of the ions identified in (a) above (2mks)
9. The column below was used to soften hard water.
a) Explain how the hard water was softened as it passed through column. (1mk)b) After sometime the material in the column is not able to soften hard water. How
can the material be reactivated? (1mk)c) Give one advantage of using hard water for domestic purposes. (1mk)
10. The table below shows the test carried out on separate samples of water drawn from a well and results obtained.Tests ResultsI) Addition of excess ammonia solution White precipitateII) Addition of two drops of dilute sulphuric acid No precipitateIII) Addition of dilute hydrochloric acid followed by
few drops of Barium chlorideWhite precipitate
a) Identify the cation and anion present in the water. (2mks)b) Write an ionic equation for the reaction which takes place in test (III) (1mk)
11. Study the set up below and answer the question that follows.
a) Write an equation for the reaction which takes place in the combustion tube.
(1mk)b) What property of gas z allows it to be collected as shown in the diagram? (2mks)
12. 10g of sodium hydrogen carbonate were dissolved in 20cm3 of water in a boiling tube. Lemon juice was then added dropwise with shaking until there was no further observable change.
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a) Explain the observation which was made in the boiling tube when the reaction was in progress. (2mks)
b) What observation would have been made if the lemon juice had been added to copper turnings in a boiling tube? Give a reason. (1mk)
13. a) State one cause of temporary hardness in water. (1mk)b) How does distillation remove hardness in water? (3mks)
14. Explain why hydrogen forms compounds in which its oxidation state is either + 1 or -1 (atomic number of H =1) (3mks)
15. An atom of hydrogen can form two ions. Write two equations to show how a neutral atom of hydrogen can form the two ions. In each case show the sigh of the energy changes. (2mks)
16. When steam was passed over heated charcoal as shown in the diagram below hydrogen and carbon (II) oxide were formed.
a) Write the equation for the reaction which takes place. (1mk)b) Name two uses of carbon (III) Oxide gas which are also the uses of Hydrogen gas.
(2mks)17. The table below shows the test carried out on a sample of water and the results obtained.
Tests Resultsi) Addition of sodium Hydroxide White precipitate which dissolves in excessii) Addition of excess ammonia solution Colourless solution obtained.iii) Addition of dilute Hydrochloric acid and
barium chlorideWhite precipitate
a) Identify the anions present in water. (1mk)b) Write an ionic equation for the reaction in (iii) (1mk)c) Write the formula of the complex ion formed in (ii) (1mk)
18. The set up below used to demonstrate the effect of heat on hard water.
Water Filtration unit I Process Y
Filtration unit II
Addition of sodium hypochlorideTap water
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a) Name substance, A (1mk)b) Explain why the heating of hard water produces substance A. (2mks)
19. The diagram below shows a student’s set up for the preparation and collection of hydrogen gas.
a) How would the final volume of hydrogen gas produced be affected if 80cm3 of o.7M hydrochloric acid was used? (1mk)
b) Give a reason why helium is increasingly being preferred to hydrogen in weather balloons. (1mk)
20. In a laboratory experiment hydrogen gas was passed over heated copper (II) oxide as shown in the diagram below.
Describe a chemical test that can be used to identify the product E. (2mks)21. a) A student was supplied with a colourless liquid suspected to be water.
i) Describe one chemical test that could have been used to show that the liquid was pure water. (1mk)
ii) How it could have been shown that the liquid was pure water. (1mk)b) The flow chart below shows the various stages of water treatment. Study it and
answer the question that follows.
i) Which substances are likely to be removed in filtration unit I? (1mk)
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ii) What is the name of process Y? (1mk)iii) What is the purpose of;
a) Process Yb) Addition of sodium hypochlorite? (1mk)
iv) It was confirmed that magnesium sulphate was present in the tap water.a) What type of hardness was present in the tap water? (1mk)b) Explain how this hardness can be removed. (2mks)
22. The set up below was used to prepare hydrogen gas.
a) Complete the diagram to show how a dry sample of hydrogen gas can be collected. (3mks)
b) Write an equation which takes place when hydrogen gas burns in air. (1mk)c) 1.2 litres of hydrogen gas was produced at room temperature and pressure when
3.27g of zinc were used. Determine the relative atomic mass of zinc (molar gas volume is 24 litres). (4mks)
d) State two industrial used of hydrogen gas. (2mks)23. The set up was used to collect gas F, produced by the reaction between water and calcium
metal.
i) Name gas F. (1mk)ii) At the end of the experiment, the solution in the solution is a weak box. (2mks)iii) Give one laboratory use of the solution formed in the beaker. (1mk)
24. A piece of sodium was put into a beaker containing water.a) Write the equation for this reaction. (1mk)b) State the observations made in the above reaction. (2mks)
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25. When Na2CO3: XH2O is strongly heated, it loses 63.2% of its mass. Find the value of X.
(2mks)26. Study the diagram below and answer the questions that follows:
a) State two observations that may be made in the combustion tube. (1mk)b) Write an equation for the reaction of hydrogen with Lead (II) Oxide. (1mk)
27. The table below gives information on reactions of metals B, C, D and E.Metal Reaction with acid Action of heat on its nitrate Reaction with waterB Hydrogen evolved Oxide formed No reactionC No reaction Metal formed NO reactionD Hydrogen evolved Oxide formed Hydrogen evolvedE NO reaction Oxide formed NO reaction
Arrange the metals in the order of decreasing reactivity starting with the least reactive.28. The diagram below shows how lithium reacts with steam.
i) Write an equation for the reaction. (1mk)ii) Why is it not advisable to use potassium in place of lithium? (1mk)
29. Steam reacts with iron fillings to form tri-iron tetra oxide.3Fe(s) + 4H2O (g) → 3H2 (g) + 4H2 (g)
a) State one experimental condition that will make the reaction reversible. (1mk)b) Give two commercial uses of Hydrogen gas. (2mks)
30. When a metal oxide of element “W” reacts with hydrogen, the equation for the reaction is:
WO3(s) + 3H2 (g) → W(s) + 3H2O (2)
Comment on the reactivity of element “W” with hydrogen gas. (1mk)31. The following observations were made during the investigation of the reaction of metal
with water.- When a piece of sodium metal was dropped in a bowl; of water, it reacted vigorously,
darting over the surface of water. Hydrogen gas was liberated.
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- Iron metal did not react with cold water but red hot iron reacted with steam liberating hydrogen an tri- iron tetra oxide.
- Copper did not react with cold water but red hot iron reacted with steam liberating hydrogen a tri-iron tetra oxide.
- Copper did not react with water or steam.Answer the following questions
a) Which metal is;i) The most reactive? (1mk)ii) The least reactive? (1mk)
b) i) What other product apart from hydrogen is formed in the reaction between sodium and water? (1mk)
ii) Write a chemical equation for the reaction in (b) above (1mk)c) Comment on the PH of the resulting solution in (b) above. (1mk)d) Name any other two elements which react in similar way to sodium (2mks)e) Give the test for hydrogen gas. (1mk)
32. What is the differences between a deliquescent and hygroscopic substance? (2mks)33. When trying to put off an oil fire, water is not used. Explain. (2mks)
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ANSWERS1. i) If ignited immediately explosion would occur because it would still be
mixed with air.ii) 2H2(g) + O2(g) 2H2O (g)
2.Metals Aqueous solution containing ions of metals
P R TP X X XR X
T X X3. a) Sample II: because the volume of soap used is less i.e. 3.0 cm3 and
remains the same after boiling.b) Sample II is temporary had water because after boiling it became soft. Volume of
soap change from 10.6 to 3.0 cm3
4. 2HCl(aq) + Zn(s) ZnCl2(aq) + H2
2H2(g) + O2(g) 2H2O (g)
5. a) Moles of Zn = 1.96 = 0.03 moles65.4
Moles of HCL: 100 x 0.2 = 0.02 moles 1000
Expected moles ratio of Zn: HCl 1:2
Moles reacting 0.01: 0.02Moles of Zn were in excess by 0.03-0.01 = 0.02 moles
b) Moles of H2 produced = 0.01 molesVolume = 22.4 x 0.01 = 0.224dm3
OR 0.224cm3
6. a) 2Li(s) + H2O(g) Li2O(s) +H2(g)
b) Potassium is very reactive and the reaction is likely to be explosive/violent.7. a) to generate steam which will push air out.
b) Oxygen in air would oxidize zinc to zinc Oxide and no gas/Hydrogen would be produced.
c) It is less dense than air,8. a) SO4
-2 and NH4
b) From ammonium and sulphates based ferterlizers.NH4 can also be from humus- when they decay.
9 a) The Ca2+ and Mg2+ ions in the permutit b) By passing a solution of concentrated sodium chloride/ brine through the
permutit.
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c) Provide Ca+2 ions necessary for bone and teeth formation.-When passed through lead pipe the lead sulphate coat the inside as it is insoluble. This prevents chances of lead poisoning.
10. a) Cations: Al3+
b) Anions: SO4-2
Ba2+(aq) + SO4
-2 BaSo4(s)
C) a) H2O(g) + mg(s) MgO(s) + H2(g)
11. a) H2O(g) + Mg(s) MgO(s) + H2(g)
b) It is insoluble in water.12 a) Effervescence and bubbles of colourless gas were liberated.
b) Copper turnings will settle at the bottom. There will be no reaction since copper does not react with an acid unless the acid is an oxidizing agent.
13. a) Presence of Ca(HCO3)2 and Mg(HCO3)2 salts which aresoluble.b) During distillation pure water is evaporated and then condesed leaving behind
solids CaCO3(s) and MgCO3(s) as their hydrogen carbonates decompose during the process.
14. It has one electron in its outermost energy level which it can lose to form H+ showing oxidation state of +1 or gain one electron to form H- showing oxidation state of -1.
15. H(g) + e- H(g)
∆H = -veH(g) → e-
+ H+(g) ∆H = +ve
16. a) H2O(g) + C(s) H2(g) + CO(s)
17. a) SO4-2 ions
b) Ba2+(aq) + SO4 Tetraamine zinc (II) ions
c) Zn (NH3)2 and Ca (HCO3)2 decomposes producing CO2 when heated.18. a) Carbon (IV) Oxide gas
b) Mg(HCO3) and Ca(HCO3)2 decomposes producing CO2 when heated.19. a) No change in volume since the number of moles of acid is equal in both
cases.b) It is less dense and does not burn like hydrogen.
20. Changes anhydrous copper (II) sulphate from whit to blue. Or changes cobalt chloride paper from blue to pink.
21. a) i) Add one drop of liquid to anhydrous copper (II) sulphate it will turn blue from white.ORAdd one drop to anhydrous cobalt chloride; it will turn pink from blue.
b) i) Large suspended particles e.g leaves, stones, sand, gravel/grit.ii) Sedimentation or precipitationiii) (a) Causes the small suspended particles to settle/precipitate. (b)
Destroy micro-organism
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iv) a) Permanent hardnessc) Addition of washing soda Na2CO3 which precipitate g2+
(aq) as gCO3(s).
22. a)
b) 2H2(g) + O2(g) 2H2O(g)
c) Moles of H2 produced: 1.2 = 0.05 moles 24
Moles of Zn = 0.05 molesRam of Zn; 3.27 = 65.4
0.05 d) -Hydrogenation of fat
-Weather balloons-Welding when mixed with other gases e.g oxygen to give oxyhydrogen.
23. i) Hydrogenii) Calcium hydroxide produced ionizes partially producing few (OH-) ionsiii) Test the presence of carbon (IV) Oxide.
24. a) 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)b) Sodium melts to form a silvery ball. Float on the surface and dart about. Hissing
sound produced.25. Na2CO3 X H2O
% mass 36.8 63.2RMM 10.6 18Moles 0.347 3.5Ratio 1 10
X = 10: Na2CO3: 10H2O26. a) -Lead oxide changes from yellow to brown when heated and finally grey
shiny solid is formed.-Anhydrous cobalt chloride changes from blue to pink.
b) H2(g) + PbO(s) Pb(s) + H2O(g)
27. C, E, B, D28. i) 2Li(s) + H2O(g) LiO(s) + H2(g)
ii) Potassium is very reactive and the reaction may be violent/explosive.29. (i) If the hydrogen gas is not removed from the system it will reduce the
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oxide of iron.(ii) Weather balloon
WeldingRocket fuel together with oxygen.
30. Hydrogen is more reactive than metal W since it is able to displace “W” from its oxide.31. a) i) Sodium
ii) Copperb) i) sodium hydroxide/alkaline solution
ii) 2Na (s) + 2H2O (I) 2NaOH (aq) + H2 (g)
c) Sodium hydroxide is a strong alkali with a pH of 14.This is because it ionizes completely in solution of water producing more hydroxyl (OH-) ions.
d) Potassium and Rubidiume) Burns with a “pop” sound.
32. Deliquescent absorbs water from the atmosphere and dissolves.Hygroscopic absorbs water from the atmosphere and becomes fizzed i.e. it will float and helps in spreading of fire.