O-LevelChemistryStoichiometry5070/22 · May/June 2018 · Q710 Marks

Sphalerite is an ore containing compounds of zinc. One of the compounds in the ore is ZnS.

(a)Calculate the mass of zinc in 30.0 tonnes of ZnS. mass of zinc ..................................................... tonnes[2]
(b)ZnS is heated in air. Zinc oxide and sulfur dioxide are formed.
(b)(i)Construct the equation for this reaction.[2]
(b)(ii)Suggest one environmental problem involved with heating ZnS in air.[1]
(c)Zinc oxide is reacted with dilute sulfuric acid to form aqueous zinc sulfate. Construct the equation for this reaction.[1]
(d)Pure zinc is made by the electrolysis of aqueous zinc sulfate. Zinc forms at the negative electrode. Hydroxide ions react at the positive electrode. Construct the equations for the reactions at both electrodes. positive electrode (anode) negative electrode (cathode)[2]
(e)Zinc is used to galvanise iron to prevent the iron from rusting. Explain how galvanising prevents iron from rusting.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[2] 7(a) Mr = 97 OR % zinc = 67 % (1) (% zinc = 67 so mass of zinc = % × 30) = 20.1 (1)
[2] 7(b)(i) ZnS + 1½O2 → ZnO + SO2 correct formulae (1) balancing – dependent on correct formulae (1)
[1] 7(b)(ii) (sulfur dioxide) causes acid rain / (sulfur dioxide) irritates the nose / throat / eyes / lungs (1)
[1] 7(c) ZnO + H2SO4 → ZnSO4 + H2O (1)
[2] 7(d) anode: 4OH¯ → O2 + 2H2O + 4e¯ (1) cathode: Zn2+ + 2e¯ → Zn (1)
[2] 7(e) Any two from: • (protective) barrier AND stops water and/or oxygen reaching surface / barrier AND stops water / moisture and / or oxygen reaching iron (1) • zinc is more reactive (than iron) / is a better reducing agent (than iron) / zinc loses electrons more easily (than iron) / zinc is higher in the reactivity series (than iron) (1) • zinc reacts with oxygen and / or water in preference to iron / zinc corrodes instead of iron / zinc loses electrons and gives then to the iron (1)

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.