Which of the following methods is commonly used to extract metals low in the activity series, such as mercury

Question

Which of the following methods is commonly used to extract metals low in the activity series, such as mercury and copper, from their ores?

Examoogle · Accepted Answer

The correct answer is Option C: Roasting. Metals low in the reactivity series, such as Mercury ($Hg$) and Copper ($Cu$), exist as sulfide ores like Cinnabar ($HgS$). Roasting is the process of heating sulfide ores strongly in the presence of excess air to convert them into their metallic oxides or directly into the metal.

Step-by-Step Solution & Concepts:
Metals low in the reactivity series, such as Mercury ($Hg$) and Copper ($Cu$), exist as sulfide ores like Cinnabar ($HgS$). Roasting is the process of heating sulfide ores strongly in the presence of excess air to convert them into their metallic oxides or directly into the metal.

Working Principle: In the case of mercury sulfide ($HgS$), heating converts it to mercury oxide ($HgO$), which then thermally decomposes into mercury metal upon further heating. The reaction is self-reduction: $2HgS + 3O_2 \rightarrow 2HgO + 2SO_2$, followed by $2HgO \rightarrow 2Hg + O_2$.

Key Formulas: $2HgS(s) + 3O_2(g) \rightarrow 2HgO(s) + 2SO_2(g)$ — Roasting of Cinnabar; $2HgO(s) \xrightarrow{\Delta} 2Hg(l) + O_2(g)$ — Thermal decomposition of mercuric oxide

Key Points: Roasting is strictly used for sulfide ores.; The process occurs in the presence of excess oxygen.; Metals low in the activity series are easily reduced because they do not have a strong affinity for oxygen compared to highly reactive metals.

Advantages: Energy-efficient for low-reactivity metals.; Sulfur is removed as useful sulfur dioxide gas.

Disadvantages: Produces $SO_2$ gas, which is a major environmental pollutant and causes acid rain if released untreated.

Applications: Extraction of Mercury from Cinnabar ($HgS$).; Extraction of Copper from Copper Glance ($Cu_2S$).

Additional Info: Calcination is used for carbonate ores (absence of air), whereas Roasting is for sulfide ores (presence of air).. Option D (Electrolysis) is used for highly reactive metals like Sodium ($Na$) and Aluminium ($Al$). Option B (Reduction with Aluminum) is the Thermite process used for moderately reactive metals like Iron.

Answer: C is correct — Roasting is the thermal oxidation process used to convert sulfide ores of low-reactivity metals into oxides or pure metals.

Exam Tip: Always remember: Roasting uses air (A for Air), while Calcination does not (C for No Air/Carbonate). This distinction is a favorite for competitive exam setters.

Examoogle · Answer

Option A: Displacement by carbon monoxide gas

Examoogle · Answer

Option B: Reduction with aluminium powder

Examoogle · Answer

Option D: Electrolysis of molten ore

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