[short_answer] Zinc sulphide (ZnS) and zinc carbonate (ZnCO₃) are two common ores of zinc. (i) Name the process used to convert each of these ores into zinc oxide (ZnO). (ii) Why must these ores be converted to the oxide form before the metal can be extracted by reduction with carbon, rather than reducing them directly?
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(i)
- ZnS (zinc sulphide): The process used is roasting — heating strongly in excess air.
$$2\text{ZnS(s)} + 3\text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{ZnO(s)} + 2\text{SO}_2\text{(g)}$$
- ZnCO₃ (zinc carbonate): The process used is calcination — heating strongly in limited air.
$$\text{ZnCO}_3\text{(s)} \xrightarrow{\text{Heat}} \text{ZnO(s)} + \text{CO}_2\text{(g)}$$
(ii) It is easier to obtain a metal from its oxide than from its sulphide or carbonate, because carbon can readily reduce metal oxides. If sulphides or carbonates were reduced directly, the reaction would be less efficient and carbon cannot displace the metal as easily from these compounds.
Source: Chapter 3, Section 3.4.4
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Explanation
- Examiners award 1 mark each for naming roasting and calcination correctly (with equations if space allows), and 1 mark for the reason.
- Key distinction: roasting = excess air; calcination = limited/no air. Don't mix these up.
- The reason must state that metal oxides are easier to reduce (with carbon) compared to sulphides/carbonates — this is the textbook's exact reasoning. Vague answers like "to purify the ore" will not score.