Mercury halides

Mercury forms halides in the two oxidation states, +1 and +2.
1. Mercury(1)Chloride(Hg2Cl2)
Mercury(1) chloride or mercurous chloride is known as calomel.
Mercury(1)chloride is prepared by heating a mixture of mecury(2)chloride and mercury in iron vessel.
HgCl2 + Hg -------> Hg2Cl2
It can also be obtained by reduction of mercury(2)chloride by reducing agents like tin(2)chloride in limited quantity.
2HgCl2 + SnCl2 --------> HgCl2 + SnCl4

When heated, mercury(1)chloride decomposes into mercury(2)chloride and mercury.
Hg2Cl2 -------> HgCl2 + Hg
The action of aqueous ammonia on the solid mercury(1)chloride gives a mixture of black finely divided mercury and white mercury amino chloride. This reaction is an example of disproportion reaction.
Hg2Cl2 + 2Nh3 -------> Hg(NH2)Cl + Hg + NH4Cl

Uses of mercury(1)chloride
Calomel is used in making standard calomel electrodes used as secondary reference electrode. It is also used as a purgative in medicines.

2. Mercury(2)chloride (HgCl2)
Mercury(2)chloride or mercury chloride is known as corrosive sublimate.
Mercury(2)chloride may be prepared by heating the metal in chlorine gas.
Hg + Cl -------> HgCl2
It is also prepared by heating a mixture of mercury(2)sulphate and sodium chloride in the presence of traces of MnO2
HgSO4 + 2NaCl -------> HgCl2 + Na2SO4
Manganese dioxide prevents the formation of mercury(1)chloride.

Mercury(2)chloride is a white crystalline solid, but from aqueous solution it crystallizes into colourless needles. It is a covalent compound sparingly soluble in water. Mercury(2)chloride gives a white precipitate on reduction with stannous chloride, SO2, formaldehyde etc, which changes to grey on standing owing to the formation of metallic mercury.
2HgCl2 + SnCl2 -------> Hg2Cl2 + SnCl4
Hg2Cl2 + SnCl2 -------> 2Hg + SnCl4
Mercury(2)chloride reacts with aqueous ammonia to form infusible white precipitate of mercury amino chloride.
HgCl2 + 2NH3 --------> Hg(NH2)Cl + NH2Cl
Gaseous ammonia or ammonium chloride on reaction with Mercury(2)chloride forms fusible white precipitate of diammine Mercury(2)chloride.
HgCl + 2NH3 -------> Hg(NH3)2Cl2

Uses of mercury(2)chloride
Mercury(2)chloride is used in the preparation of mercuric iodide.

3. Mercury(2)Iodide (HgI2)
Mercury(2)iodide is obtained as a scarlet precipitate on addition of potassium iodide to a solution of mercury(2)chloride.
2KI + HgCl2 --------> HgI2 + 2KCl

Mercury(2)iodide readily dissolves in excess of potassium iodide solution due to the formation of potassium tetra iodo mercurate(2)Complex K2[HgI4].
HgI2 + 2KI -------> K2[HgI4]
This potassium tetraiodo mercurate(2)complex forms light yellow crystals of K2[HgI4].2H2O. The complex dissolves in potassium hydroxide solution to give Nessler's reagent which forms a brown precipitate or colouration with ammonia due to the formation of the iodide of Million's base, Hg2NI.H2O

Uses of mercury iodide
Mercury(2)iodide is used for preparing Nessler's reagent and in the treatment of skin infection.

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