Chapter 3 · Question 12

Describe the extraction of metals low in the activity series, taking mercury (from cinnabar) and copper (from copper glance) as examples. Write all the chemical equations involved.

Q12

Describe the extraction of metals low in the activity series, taking mercury (from cinnabar) and copper (from copper glance) as examples. Write all the chemical equations involved.

Answer Revealed

Direct Answer:

Metals low in the activity series (below Cu) are very unreactive, and their oxides can be reduced to metals simply by heating. Mercury is extracted from its sulphide ore cinnabar (HgS) in two steps: (1)

2\text{HgS(s)} + 3\text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{HgO(s)} + 2\text{SO}_2\text{(g)}

— roasting in air converts cinnabar to mercuric oxide. (2)

2\text{HgO(s)} \xrightarrow{\text{Heat}} 2\text{Hg(l)} + \text{O}_2\text{(g)}

— mercuric oxide decomposes on further heating to give mercury. Copper from copper glance (Cu₂S): (1)

2\text{Cu}_2\text{S} + 3\text{O}_2 \xrightarrow{\text{Heat}} 2\text{Cu}_2\text{O} + 2\text{SO}_2

— roasting. (2)

2\text{Cu}_2\text{O} + \text{Cu}_2\text{S} \xrightarrow{\text{Heat}} 6\text{Cu} + \text{SO}_2

— copper(I) oxide reacts with remaining sulphide to produce copper metal.

Simple Explanation

Metals like mercury and copper sit near the bottom of the reactivity series — they are lazy and easy to extract. For mercury from cinnabar (HgS): just roast it in air and it first becomes HgO, then on more heating, HgO breaks down by itself to give liquid mercury — no carbon needed. For copper from copper glance (Cu₂S): roast it to get Cu₂O, then heat Cu₂O with more Cu₂S — they react with each other to produce pure copper metal and SO₂. Notice no external reducing agent is required because these metals are barely holding onto their oxygen.

Exam-Ready Structure

The extraction of metals low in the reactivity series (Section 3.4.3) is the simplest metallurgical path because these metals are the least reactive and their compounds are easily decomposed. 1. General principle: Metals low in the activity series are highly unreactive. Their oxides can be reduced to the corresponding metals by heating alone — no external reducing agent such as carbon is needed. 2. Extraction of mercury from cinnabar (HgS): (a) Step I — Roasting: Cinnabar (mercury(II) sulphide) is heated in air. The sulphide is converted to mercuric oxide:

2\text{HgS(s)} + 3\text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{HgO(s)} + 2\text{SO}_2\text{(g)}

. (b) Step II — Thermal decomposition: Mercuric oxide (HgO) is further heated, and it decomposes on its own to produce mercury metal in the liquid state:

2\text{HgO(s)} \xrightarrow{\text{Heat}} 2\text{Hg(l)} + \text{O}_2\text{(g)}

. Mercury distills over and is collected. This decomposition is possible because HgO is unstable at high temperatures. 3. Extraction of copper from copper glance (Cu₂S): (a) Step I — Roasting in air:

2\text{Cu}_2\text{S(s)} + 3\text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{Cu}_2\text{O(s)} + 2\text{SO}_2\text{(g)}

. Copper(I) sulphide is converted to copper(I) oxide. (b) Step II — Self-reduction: The copper(I) oxide formed in Step I reacts with the remaining unreacted copper(I) sulphide upon further heating:

2\text{Cu}_2\text{O(s)} + \text{Cu}_2\text{S(s)} \xrightarrow{\text{Heat}} 6\text{Cu(s)} + \text{SO}_2\text{(g)}

. The Cu₂O and Cu₂S act as mutual reagents — the oxide is reduced and the sulphide is oxidised to SO₂, leaving pure copper metal. This is called self-reduction (or auto-reduction). 4. Contrast: Unlike medium-reactivity metals, neither mercury nor copper extraction requires a carbon-based reducing agent (coke). The low stability of their oxides at high temperatures permits direct thermal decomposition.

Key Points

Low reactivity metals: oxides can be reduced by heating alone — no carbon needed
Mercury from cinnabar: (1) roast HgS → HgO + SO₂; (2) heat HgO → Hg(l) + O₂
Copper from copper glance: (1) roast Cu₂S → Cu₂O + SO₂; (2) Cu₂O + Cu₂S → 6Cu + SO₂ (self-reduction)
Self-reduction: the oxide and remaining sulphide react with each other to give the metal
Low stability of HgO and Cu₂O at high temperature makes thermal decomposition possible

Describe the extraction of metals low in the activity series, taking mercury (from cinnabar) and copper (from copper glance) as examples. Write all the chemical equations involved.