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2024 O Level Chem P1 Q28 — Reactivity Series Solution

Reactivity Series 2024 O Level Pure Chemistry · Paper 1 · Q28

Question

Mercury sulfide, HgS, and copper(II) sulfide, CuS, are compounds in ores which can be heated to extract the pure metal. HgS only has to be heated to 300°C to produce a pure metal; however, CuS requires 1200°C. Which two deductions can be made about the reactivity of mercury?

Adeduction 1: mercury displaces copper from CuSO₄(aq); deduction 2: mercury reacts with dilute HCl(aq).
Bdeduction 1: mercury displaces copper from CuSO₄(aq); deduction 2: mercury does not react with dilute HCl(aq).
Cdeduction 1: mercury cannot displace copper from CuSO₄(aq); deduction 2: mercury reacts with dilute HCl(aq).
Ddeduction 1: mercury cannot displace copper from CuSO₄(aq); deduction 2: mercury does not react with dilute HCl(aq).

Correct answer: D

Video Solution

Explanation

Answer: D. HgS decomposes at 300 °C; CuS requires 1200 °C. The much lower decomposition temperature for HgS implies the Hg–S bond is far weaker than the Cu–S bond — equivalently, mercury holds onto its sulfide much less strongly than copper. This places mercury below copper in the reactivity series.

Deduction 1 ('Hg displaces Cu from \(\text{CuSO}_4\)') is wrong: only a more-reactive metal displaces a less-reactive one. Hg cannot displace Cu.

Deduction 2 ('Hg reacts with dilute HCl') is also wrong: only metals above hydrogen in the reactivity series react with dilute acids. Mercury is well below hydrogen.

Both correct deductions are negative → row D.

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