Mercury Extraction from Cinnabar Ore

The extraction of mercury from its principal ore, cinnabar (HgS), typically involves a two-step heating process rather than direct reduction. This method is employed for specific chemical reasons related to the ore's properties.

Reasoning for Two-Step Heating

1. Roasting to Form Oxide: Cinnabar (HgS) is first heated in the presence of excess air. This process, called roasting, converts the mercury sulfide into mercury(II) oxide (HgO) and sulfur dioxide ($SO_2$).

   Chemical Equation: $2 \text{HgS(s)} + 3 \text{O}_2\text{(g)} \xrightarrow{\Delta} 2 \text{HgO(s)} + 2 \text{SO}_2\text{(g)}$

2. Decomposition of Oxide: The mercury(II) oxide (HgO) formed in the first step is then heated further, typically to a temperature around 350°C. At this temperature, mercury(II) oxide readily decomposes to produce liquid mercury metal and oxygen gas.

   Chemical Equation: $2 \text{HgO(s)} \xrightarrow{\Delta} 2 \text{Hg(l)} + \text{O}_2\text{(g)}$

Why Direct Reduction is Avoided

• Direct reduction of mercury sulfide (HgS) using typical reducing agents like carbon is less efficient and more complex compared to the two-step process.
• The intermediate formation of mercury(II) oxide (HgO) provides a more easily reducible compound. HgO decomposes upon moderate heating, yielding mercury metal directly.
• Option A correctly identifies the crucial first step: mercury sulfide must first be converted into mercury oxide before the metal can be easily obtained.