Question 5
(a) (i) State two industrial uses of hydrogen.
(ii) Consider the equation below.
Mg(HCO3)2(aq) --> MgCO3(s) + H2O(l) + CO2(g)
- State the type of hardness of water being removed as shown by the above equation.
- Give two disadvantages of hardness of water. [5 marks]
(b) (i)In the extraction of aluminium by electrolysis, graphite electrode are used. State the disadvantages of using this type of electrode.
(ii) Calcium oxide reacts with water to form slaked lime:
l. Write a balanced equation for this reaction;
II. State one use of slaked lime. [6 marks]
(c) (i) What is meant by saponification?
(ii) List the raw materials needed for the manufacture of soap.
(iii) Name the main by-product obtained from the manufacture of soap. [6 marks]
(d) With the aid of chemical equations, explain briefly how iron is extracted in the blast furnace using iron ore, coke and limestone as raw materials at the:
(i) bottom of the furnace;
(ii) middle of the furnace;
(iii) top of the furnace. [8 marks]
Observation
In part (a), majority of the candidates were not familiar with the industrial uses of hydrogen. They could not name the main by-product obtained from the manufacture of soap. However, majority of them gave two disadvantages of hardness of water.
In part (b), majority of the candidates could not give the disadvantage of using graphite electrode in the extraction of aluminium.
In part (c), majority of the candidates gave the correct definition of saponification but the raw materials needed for the manufacture of soap were not listed.
In part (d), majority of the candidates could not write chemical equations to explain how iron is extracted in the blast furnace using iron ore, coke and lime stone as raw materials.
The expected answers include:
(a) (i) - filling of weather balloons
- manufacture of plastics
- manufacture of ammonia by the Haber process
- manufacture of methanol
- oil refinery process / hydrocracking
- used in fuel cells
- manufacture of margarine
- in oxy-hydrogen flame (for welding and cutting metals}
- as reducing agent in production of metals e.g. Cu and Pb from their oxides
- conversion of coal to crude oil
- (liquid) hydrogen is used as rocket fuel / gaseous fuel
(ii) I. - temporary hardness of water
II. - scales / furring of kettles / pipes cause blockage leading to power wastage
- soap is wasted / not suitable for laundry
- not suitable for use in tanning, textiles and paper production
(b) (i) at high temperatures oxygen gas produced at the anode reacts with the
graphite electrodes to form carbon (IV) oxide
(ii) I. CaO + H2O → Ca(OH)2
II. - used to treat acidic soil
- used to make mortar/plasters/cement/hold bricks
- in manufacture of glass
- purification of water/removing hardness of water
- manufacture of white wash
- to test for CO2
- production of bleaching powder
- to recover / produce ammonia from ammonium chloride/ used in solvay process
(c) (i) Is the alkaline hydrolysis of fats/oils to produce soap ( and propane- 1,2,3 triol) OR
The process in which fat or oil is boiled with alkali to produce soap (and propane -1, 2, 3-triol)
(ii) oil / fat and alkali (sodium hydroxide / potassium hydroxide)
(iii) propane-1,2,3-triol
Bottom of furnace
(d) (i) The O2 in the hot air combines with carbon (Coke) to produce carbon (IV)
Oxide which is reduced to carbon (II) oxide.
C + O2 → CO2
CO2 + C → 2CO
Middle of furnace
(ii) The CaCO3 decomposes under great heat to form CO2 and CaO / CaCO3 decomposes to remove the impurities / SiO2
CaCO3 → CaO + CO2
CaO + SiO2 → CaSiO3
Top of furnace
(iii) Iron (III) oxide is reduced to metallic Iron
Fe2O3 + 3CO → 2Fe + 3CO2
OR
Fe3SO4 + 4C0 --> 3 Fe + 4 CO2