Question 5
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- Draw and label a diagram to illustrate the preparation and collection of dry chlorine gas in the laboratory.
- [9 marks] State two uses of chlorine.
- Describe the preparation of hydrogen from water gas. [8 marks]
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- Name the chief ore of aluminium.
- Why is the ore purified?
- Name the electrode used in the electrolysis.
- Give one reason why cryolite, NaAlF6, is added to the electrolyte. [5 marks]
- Name three products obtained directly from the destructive distillation of coal. [3 marks]
Observation
Majority of the candidates could not satisfactorily answer this question. Their performance was fair.
In part (a), majority of the candidates could not draw and a label a diagram to illustrate the preparation and collection of dry chlorine gas in the laboratory. However, many of them were familiar with the uses of chlorine.
In part (b), majority of the candidates could not describe the preparation of hydrogen from water gas.
In part (c), majority of the candidates could name the chief ore of aluminium, but could not explain why the ore is purified and why cryolite is added to the electrolyte.
In part (d), majority of the candidates could not name the products obtained directly from the destructive distillation of coal.
The expected answers include:
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NOTE:
3 marks maximum if reagents are wrong, score for drying agent, purifying agent and
mode of collection.
If reagents are conc HCl and MnO2, candidate must heat to score for workability.
If reagents are conc. HCl and KMnO4, heat is not required
- used as germicide/disinfectant/sterilization/antiseptic/insecticides
- bleaching of materials/wood pulp/textiles
- commercial production of HCl
- production of polymers/PVC/plastics
- production of chlorinated chemicals e.g. CHCl3, CCl4
- in medicine
- production of dyes
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Water gas is CO + H2 steam is mixed with water gas and passed over heated catalyst of iron (III) oxide or chromium (III) oxide at 450oC
The CO in water gas is converted to CO2 with further yield of hydrogen The CO2 is removed by dissolving the mixture in water / NaOH/KOH/Ca(OH),2 solutions under pressure
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- Bauxite
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- To produce aluminium oxide/alumina.
- To prevent poisoning of the electrode.
- Carbon/ graphite
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- It is added to reduce some of the energy (costs) involved in extracting aluminium
- to increase the electrical conductivity of alumina
- to lower the melting points to of the alumina
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- Coke
- Coal tar
- Coal gas
- ammoniacal liquor