Trace the complete journey of zinc from its naturally occurring sulphide ore to a pure, usable metal. Your answer must include: the reason its ore form must be converted before reduction, the two processes used for this conversion and what they involve, the reduction step, and how the final impure metal is purified.
Generated by claude-sonnet-4-6 · 2026-06-26 01:07 · grounding rag
Model Answer
Zinc is a moderately reactive metal found mainly as zinc sulphide (ZnS) or zinc carbonate (ZnCO₃) in nature.
Why conversion is needed: It is easier to reduce a metal oxide than a sulphide or carbonate. So the ore must first be converted to zinc oxide (ZnO).
Two conversion processes:
- Roasting – ZnS is heated strongly in excess air, converting it to ZnO:
$2\text{ZnS} + 3\text{O}_2 \xrightarrow{\Delta} 2\text{ZnO} + 2\text{SO}_2$
- Calcination – ZnCO₃ is heated strongly in limited air, converting it to ZnO:
$\text{ZnCO}_3 \xrightarrow{\Delta} \text{ZnO} + \text{CO}_2$
Reduction: ZnO is heated with carbon (coke), which acts as the reducing agent:
$\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$
Purification: The impure zinc is refined by electrolytic refining. Impure zinc is made the anode, pure zinc the cathode, and zinc salt solution the electrolyte. On passing current, pure zinc deposits on the cathode; insoluble impurities collect as anode mud.
Source: Metals and Non-metals, Section 3.4.4 and 3.4.6
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Explanation
- The question has four clearly labelled parts — examiners will award marks for each, so don't skip any.
- The key distinction between roasting (excess air) and calcination (limited air) is a favourite examiner point — get those details right.
- Writing the chemical equations earns marks and shows precision; keep them balanced.
- For electrolytic refining, remember the three essentials: impure metal = anode, pure metal = cathode, metal salt solution = electrolyte. Mentioning "anode mud" shows complete knowledge.
- Don't confuse calcination (for carbonates) with roasting (for sulphides) — a common error.