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2. Mercury halides (a) Mercury (I) chloride or mercurous chloride or calomel, Hg2Cl2 Preparation
(i) It can be prepared by mixing a chloride solution with a mercury (I) salt solution.
Hg2(NO3)2 + 2NaCl ———→ Hg2Cl2 ↓ + 2NaNO3
(ii) It can also prepared by heating a mixture of mercuric chloride and mercury in an iron vessel.
Δ
HgCl2 + Hg ———→ Hg2Cl2
Properties
(i)It is a white power insoluble in water but soluble in chlorine water.
Hg2Cl2 + Cl2———→ 2HgCl2
(ii)It decomposes on heating to HgCl2
Hg2Cl2———→ HgCl2 + Hg
(iii)On treatment with ammonia, if turns black due to the formation of finely divided mercury.
Hg2Cl2 + 2NH3———→ Hg + Hg(NH2)Cl + NH4Cl
Uses
(i)In making standard calomel electrode and
(ii)As a purgative in medicine.
(b) Mercury (II) chloride HgCl2
1. It is prepared by passing dry chlorine over heated mercury.
Δ
HgCl2 + Hg ———→ Hg2Cl2
2. It is also obtained by treating HgO with HCl
HgO + 2HCl ———→ HgCl2 + H2O
3. Commercially, it is prepared by heating a mixture of HgSO4 and NaCl in the presence of MnO2
MnO2
HgSO4 + 2NaCl ————→HgCl2 + Na2SO4
Δ
Properties
1. It is a white crystalline solid sparingly soluble in cold water but soluble in hot water. Its solubility can be increased by adding Cl-.
HgCl2 + 2Cl–———→ [HgCl4]2–
2. It is readily soluble in organic solvents suggesting its covalent nature.
3. When treated with SnCl2 it is reduced to mercury.
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