Write the chemistry of corrosive sublimate. [5]

Chemistry of Corrosive Sublimate (HgCl₂)

1. Definition and Physical Properties

Corrosive sublimate is the common name for mercury(II) chloride (HgCl₂), a white crystalline solid that sublimes easily upon heating. It is highly toxic and corrosive to tissues, hence the name. Key physical properties include:

• Melting point: 276°C
• Boiling point: 302°C (sublimes without melting)
• Solubility: Soluble in water, ethanol, and ether but insoluble in concentrated hydrochloric acid.

mercury(II) chloridemercury(II) chloride

Ethanol (solvent)Ethanol (solvent)Diethyl ether (solvent)Diethyl ether (solvent)Corrosive sublimate (HgCl₂)Corrosive sublimate (HgCl₂)

2. Chemical Properties

HgCl₂+ZnReduction reactionHg+ZnCl₂

a) Preparation

Corrosive sublimate is prepared by reacting mercury with chlorine gas: $$\mathrm{Hg}+\mathrm{Cl}{}_2\to \mathrm{HgCl}{}_2$$ Alternatively, it can be obtained by treating mercury(II) oxide with hydrochloric acid: $$\mathrm{HgO}+2\mathrm{HCl}\to \mathrm{HgCl}{}_2+\mathrm{H}{}_2\mathrm{O}$$

b) Reaction with Ammonia (Formation of Amido Complex)

When HgCl₂ reacts with excess ammonia, it forms a soluble complex: $$\mathrm{HgCl}{}_2+2\mathrm{NH}{}_3\to \mathrm{Hg}(\mathrm{NH}{}_3){}_2\mathrm{Cl}{}_2(\mathrm{white}\mathrm{ppt})$$ Further reaction with excess ammonia yields tetraammine mercury(II) chloride: $$\mathrm{Hg}(\mathrm{NH}{}_3){}_2\mathrm{Cl}{}_2+2\mathrm{NH}{}_3\to [\mathrm{Hg}(\mathrm{NH}{}_3){}_4]\mathrm{Cl}{}_2$$ This reaction is used as a test for Hg²⁺ ions (white precipitate dissolving in excess ammonia).

c) Reaction with Potassium Iodide (Reduction to Mercury)

HgCl₂ reacts with potassium iodide (KI) to form a red precipitate of mercury(II) iodide (HgI₂), which further reduces to grey mercury (Hg): $$\mathrm{HgCl}{}_2+2\mathrm{KI}\to \mathrm{HgI}{}_2(\mathrm{red})+2\mathrm{KCl}$$ $$\mathrm{HgI}{}_2+2\mathrm{KI}\to \mathrm{K}{}_2[\mathrm{HgI}{}_4](\mathrm{soluble}\mathrm{complex})$$ If the reaction is heated, grey mercury metal is obtained: $$\mathrm{HgCl}{}_2+2\mathrm{KI}\to \mathrm{Hg}(\mathrm{grey})+2\mathrm{KCl}+\mathrm{I}{}_2$$

d) Hydrolysis in Water

HgCl₂ undergoes partial hydrolysis in water to form oxychloride (Hg₂Cl₂) and hydrochloric acid: $$\mathrm{HgCl}{}_2+\mathrm{H}{}_2\mathrm{O}\to \mathrm{Hg}{}_2\mathrm{Cl}{}_2(\mathrm{calomel})+2\mathrm{HCl}$$ This reaction explains its corrosive nature due to the release of HCl.

e) Reaction with Sodium Hydroxide (Formation of Mercury(II) Oxide)

When treated with NaOH, HgCl₂ forms yellow mercury(II) oxide (HgO): $$\mathrm{HgCl}{}_2+2\mathrm{NaOH}\to \mathrm{HgO}(\mathrm{yellow})+2\mathrm{NaCl}+\mathrm{H}{}_2\mathrm{O}$$

3. Uses

• Used in medicine (disinfectant, antiseptic) (though now largely obsolete due to toxicity).
• Employed in laboratories for detecting ammonia and other gases.
• Used in electroplating and pesticides (restricted due to environmental hazards).

4. Toxicity and Safety

• Highly toxic when ingested, inhaled, or absorbed through skin.
• Causes severe burns on contact with skin/mucous membranes.
• Environmental hazard: Accumulates in food chains, affecting kidneys and nervous system.

5. Environmental Impact

• Non-biodegradable: Persists in soil and water for long periods.
• Bioaccumulation: Accumulates in aquatic organisms, posing risks to ecosystems.

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Note: Due to its extreme toxicity, handling requires strict safety measures (gloves, fume hood, proper disposal). Modern chemistry has largely replaced HgCl₂ with safer alternatives.