Question

5. (a) Describe how iron and aluminium react with each of the following substances:
 (i) dilute H2SO4;
 (ii) dilute HNO3.
(b) (i) Write an equation for the burning of sulphur in air.
 (ii) Name the catalyst used in the contact process.
 (iii) In the contact process, why is an excess of air used?
 (iv) Why is it necessary to cool the catalyst used in 5(b)(ii)?
 (v) Give a reason why the air used in the contact process needs to be as clean as possible.
 (vi) State two reasons why SO2 should not be discharged into the atmosphere.
(c) (i) State the reagent and condition used in the laboratory preparation of chlorine.
 (ii) State two uses of chlorine.
(d) (i) Name the drying agent for each of the following gases:
  I. hydrogen;
  II. sulphur (IV) oxide;
  III. ammonia.
 (ii) State the components of the following alloys:
  I. Bronze;
  II. Brass.

Accepted Answer

(a)(i) Iron dissolves readily in dilute H2SO4, liberating hydrogen and forming FeSO4. Aluminium reacts with dilute H2SO4 to form aluminium sulphate, Al2(SO4)3, also releasing hydrogen gas.
(ii) With aluminium, there is essentially no reaction (a protective oxide layer prevents it), while iron dissolves in dilute HNO3, liberating hydrogen and forming Fe(NO3)2 (dilute conditions).

(b)(i) S + O2 → SO2
(ii) Vanadium (V) oxide (V2O5) / platinized asbestos
(iii) An excess of air is used to favour the formation of more product (SO3) by shifting the equilibrium.
(iv) The reaction (SO2 + ½O2 ⇌ SO3) is exothermic; cooling favours the forward reaction since high temperature would favour the backward (endothermic) reaction.
(v) Catalysts are easily poisoned/damaged by dirt/impurities in the air.
(vi) SO2 should not be discharged into the atmosphere because: it damages buildings; it damages living things/causes acid rain; it causes irritation to the eyes, nose and throat; it causes lung and respiratory diseases (e.g. coughing, bronchitis). (Any two.)

(c)(i) Reagent: manganese (IV) oxide (MnO2) with concentrated HCl, under heating; OR bleaching powder/CaOCl2/KMnO4 crystals with concentrated HCl at room temperature (no heat required).
(ii) Uses of chlorine: bleaching wood pulp and textiles; production of plastics (e.g. PVC); extraction/recovery of tin from its ore; as a germicide/disinfectant/insecticide (e.g. in swimming pools); purification/sterilization of water supply; commercial manufacture of HCl; production of dyes and drugs; manufacture of solvents (e.g. CCl4, CHCl3). (Any two.)

(d)(i) I. Hydrogen — fused calcium chloride / concentrated tetraoxosulphate (VI) acid.
II. Sulphur (IV) oxide — concentrated tetraoxosulphate (VI) acid / anhydrous calcium chloride.
III. Ammonia — calcium oxide (quicklime) / silica gel.
(ii) I. Bronze — copper + tin. II. Brass — copper + zinc.

WAEC Chemistry 2023 Theory — Question 6

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