Write two methods of preparation of corrosive sublimate. [2] Give its action with a) excess of KI [2] b) NH3 [1]

Preparation of Corrosive Sublimate (Mercuric Chloride, $\mathrm{HgCl}{}_2$)

Corrosive sublimate, also known as mercuric chloride ($\mathrm{HgCl}{}_2$), is a white crystalline solid that is highly toxic. It can be prepared by the following two methods:

1. By the Action of Chlorine on Mercury

When chlorine gas is passed over mercury, mercuric chloride is formed: $$\mathrm{Hg}+\mathrm{Cl}{}_2\stackrel{\mathrm{heat}}{\to }\mathrm{HgCl}{}_2$$

• Procedure:
  • Mercury is taken in a glass vessel.
  • Dry chlorine gas is passed over it.
  • The reaction is carried out in the presence of heat.
  • The product, $\mathrm{HgCl}{}_2$, sublimes and can be collected as white crystals.

Hg+Cl₂Room temperatureHgCl₂

2. By the Action of Hydrochloric Acid on Mercurous Chloride ($\mathrm{Hg}{}_2\mathrm{Cl}{}_2$)

When mercurous chloride (calomel) is treated with hydrochloric acid, it forms mercuric chloride: $$\mathrm{Hg}{}_2\mathrm{Cl}{}_2+2\mathrm{HCl}\to \mathrm{HgCl}{}_2+\mathrm{Hg}+\mathrm{H}{}_2$$

• Procedure:
  • Mercurous chloride ($\mathrm{Hg}{}_2\mathrm{Cl}{}_2$) is taken in a flask.
  • Concentrated hydrochloric acid is added.
  • The reaction produces mercuric chloride ($\mathrm{HgCl}{}_2$), mercury metal, and hydrogen gas.
  • The $\mathrm{HgCl}{}_2$ can be purified by sublimation.

Hg₂Cl₂+2HClRoom temperatureHgCl₂+Hg+H₂

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Action of Corrosive Sublimate ($\mathrm{HgCl}{}_2$) with Different Reagents

a) Action with Excess Potassium Iodide (KI)

When $\mathrm{HgCl}{}_2$ reacts with excess potassium iodide, it forms a white precipitate of mercurous iodide ($\mathrm{Hg}{}_2\mathrm{I}{}_2$), which then decomposes into yellow mercuric iodide ($\mathrm{HgI}{}_2$), and potassium chloride ($\mathrm{KCl}$) is also formed: $$\mathrm{HgCl}{}_2+2\mathrm{KI}\to \mathrm{HgI}{}_2+2\mathrm{KCl}$$

• Observation:
  • Initially, a white precipitate of $\mathrm{Hg}{}_2\mathrm{I}{}_2$ may form, but it quickly turns yellow due to the formation of $\mathrm{HgI}{}_2$.
  • The reaction is used as a test for $\mathrm{HgCl}{}_2$ because the yellow color of $\mathrm{HgI}{}_2$ is characteristic.

b) Action with Ammonia ($\mathrm{NH}{}_3$)

When $\mathrm{HgCl}{}_2$ reacts with ammonia, it forms amido-mercuric chloride ($\mathrm{HgNH}{}_2\mathrm{Cl}$), a white precipitate: $$\mathrm{HgCl}{}_2+2\mathrm{NH}{}_3\to \mathrm{HgNH}{}_2\mathrm{Cl}+\mathrm{NH}{}_4\mathrm{Cl}$$

• Observation:
  • A white precipitate of $\mathrm{HgNH}{}_2\mathrm{Cl}$ is formed.
  • This reaction is used as a qualitative test for $\mathrm{HgCl}{}_2$ in analytical chemistry.

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HgCl₂+2NH₃Room temperatureHgNH₂Cl+HCl