using ellingham diagram explain the auto reduction of cinnabar to mercury.
Auto reduction is also known as self reduction in which the anion present with the metal ion helps in the reduction. Hence in this case of exaction no reducing agent is required and the metal is obtained directly from roasting or by reduction of its partly oxidized form.
In case of exaction of mercury from cinnabar, we directly roast HgS (cinnabar) in air.
The balanced equation for the above reaction is as follow:
Step 1. HgS + 3O2 → 2 HgO + 2SO2
Step 2. 2 HgO → 2 Hg + O2
As soon as HgO is formed it decomposes in to Mercury and dioxygen.
This decomposition is quite evident from the ellingham diagram.
In this case, the line of Hg increases as the temperature increase and around 600 degree Celsius it crosses the ΔG = 0 line, which means that HgO become unstable above this temperature and decomposes readily.
This temperature is atainable and hence we can reduce HgS above this temperature without using any reducing agent.