(a) What is the reactivity series of metals? On the basis of the reactivity series, explain how metals are grouped for the purpose of extraction from their ores.
(b) Write the steps involved in the extraction of a metal of medium reactivity from its sulphide ore, giving an example with relevant equations.
(c) Why can carbon not be used to reduce the oxides of metals like sodium and magnesium? How are these metals extracted?
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(a) Reactivity Series and Grouping of Metals:
The reactivity series is a list of metals arranged in decreasing order of their reactivity (K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Hg, Ag, Au).
Based on this, metals are grouped into three categories for extraction:
- Low reactivity (Ag, Au, Hg) – found free in nature; extracted by heating alone.
- Medium reactivity (Zn, Fe, Pb, Cu) – extracted by reduction using carbon or displacement.
- High reactivity (K, Na, Ca, Mg, Al) – extracted by electrolytic reduction.
(b) Extraction of Medium Reactivity Metal from Sulphide Ore (e.g., Zinc from ZnS):
Step 1 – Roasting: Sulphide ore is heated strongly in excess air, converting it to the oxide:
$$2\text{ZnS(s)} + 3\text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{ZnO(s)} + 2\text{SO}_2\text{(g)}$$
Step 2 – Reduction: The oxide is reduced with carbon (coke):
$$\text{ZnO(s)} + \text{C(s)} \rightarrow \text{Zn(s)} + \text{CO(g)}$$
(c) Carbon cannot reduce oxides of Na or Mg because these metals have a greater affinity for oxygen than carbon does, making carbon ineffective as a reducing agent here.
These metals are extracted by electrolytic reduction of their molten chlorides. For example, sodium is obtained by electrolysis of molten NaCl:
- At cathode: $\text{Na}^+ + e^- \rightarrow \text{Na}$
- At anode: $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^-$
Source: Chapter 3, Sections 3.4.1, 3.4.4, 3.4.5
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Explanation
- (a) Always define the reactivity series first, then list the three groups with examples — examiners look for both the definition and the classification.
- (b) The key steps are roasting (sulphide → oxide) then reduction with carbon. Write both equations; missing either loses marks.
- (c) The reason must be stated as "greater affinity for oxygen than carbon." Then name electrolytic reduction and give the electrode reactions — these are frequently asked and carry easy marks.