Question

5(a)(i) Describe the observation that would be made when: (I) sulphur is heated from room temperature and rapidly evaporates on exposure to air till 119°C; (II) 50% trioxonitrate (V) acid acts on copper turnings.
(b)(i) State two gaseous pollutants that can be generated by burning coal.
(ii) What gas is responsible for most of the explosions in coal mines?
(iii) The mining of coal leads to environmental pollution. State two environmental effects of the mining activity.
(iv) Explain briefly why coal burns more easily when it is in pieces than in lump form.
(v) Name the non-volatile residue after the destructive distillation of coal.
(c)(i) Describe a chemical test for water.
(ii) State the effect of: (I) boiling a temporary hard water; (II) adding sodium trioxocarbonate (IV) crystals to permanent hard water.
(iii) Write an equation for the process in 5(c)(ii)(I).
(d) With the aid of a labelled diagram, describe briefly the laboratory preparation of oxygen gas.

Accepted Answer

(a)(i)(I) When sulphur is heated: its colour darkens from pale yellow to amber, its crystals become needle-like, and at 119°C it begins to melt.
(II) When 50% trioxonitrate (V) acid acts on copper turnings: the solution turns green, bubbles of a colourless gas rise from the mixture and form brown fumes (NO2 gas) with an irritating odour; the solution later turns blue.

(b)(i) Gaseous pollutants from burning coal: SO2, NO2, CO, CO2, H2S.
(ii) Methane (CH4) is responsible for most explosions in coal mines.
(iii) Environmental effects of coal mining: contamination of soil; contamination of ground water; soil erosion; loss of biodiversity; formation of sinkholes.
(iv) Coal in pieces burns more easily than in lump form because pieces have a larger surface area; the larger the surface area, the faster the rate of reaction (combustion).
(v) The non-volatile residue after destructive distillation of coal is coke.

(c)(i) Chemical test for water: Add the sample to anhydrous copper (II) tetraoxosulphate (VI) [white powder]. If it turns from white to blue, this confirms the presence of water. (Alternative: use of anhydrous cobalt chloride paper — turns from blue to pink in the presence of water.)
(ii)(I) Effect of boiling temporary hard water: it softens/removes the temporary hardness.
(II) Effect of adding Na2CO3 to permanent hard water: it removes the permanent hardness.
(iii) Ca(HCO3)2 --heat--> CaCO3 + CO2 + H2O (or Mg(HCO3)2 --heat--> MgCO3 + CO2 + H2O)

(d) Laboratory preparation of oxygen gas: [Diagram: powdered KClO3 and MnO2 (catalyst) placed in a conical/round flask, gently heated; the gas produced is collected over water in an inverted gas jar sitting on a beehive shelf in a trough of water. Alternative setup: hydrogen peroxide poured into a flask containing manganese (IV) oxide, and the gas produced is collected in an upside-down water-filled gas jar over a trough.]
Equation: 2KClO3(s) --MnO2, heat--> 2KCl(s) + 3O2(g)
(Alternative: 2H2O2(aq) --MnO2 catalyst--> 2H2O(l) + O2(g))

WAEC Chemistry 2022 Theory — Question 7

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