Halides
1. Silver halides (AgF, AgCl, AgBr & AgI)
Preparation
Silver halides are prepared by the action of sodium or potassium halide on silver nitrate solution (except for AgF)
Silver fluoride is prepared by the action of HF on silver (I) oxide.
Properties
(i) AgCl is white solid, AgBr is a pale yellow solid and AgI is a yellow solid.
(ii) AgF is soluble in water whereas other halides are insoluble in water. AgCl dissolves in ammonia to form a complex.
AbBr is partially soluble and AgI is insoluble in NH4OH.
(iii) All the silver halides dissolve in potassium cyanide and Na2S2O3 solution to form complexes.
Uses
All silver halides (particularly AgBr) are photosensitive and hence are widely used in photography.
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.
(ii) It can also prepared by heating a mixture of mercuric chloride and mercury in an iron vessel.
Properties
(i) It is a white power insoluble in water but soluble in chlorine water.
(ii) It decomposes on heating to HgCl2
(iii) On treatment with ammonia, if turns black due to the formation of finely divided mercury.
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.
2. It is also obtained by treating HgO with HCl
3. Commercially, it is prepared by heating a mixture of HgSO4 and NaCl in the presence of MnO2
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–.
2. It is readily soluble in organic solvents suggesting its covalent nature.
3. When treated with SnCl2 it is reduced to mercury.
4. When Cu turnings are placed in its contact a shining grey film of mercury deposits over them.
Uses
It is used for preserving wood and hides and for making fungicides.
(c) Mercury (II) Iodide
Preparation
It is prepared by treating HgCl2 with KI.
Properties
1. Mercuric iodide exists in two forms, i.e. red and yellow. The yellow form is stable above 400 K white the red form is stable below this temperature.
2. It readily dissolves in KI forming a complex
HgI2 + 2KI ->K2[HgI4]
An alkaline solution of K2HgI4 is called Nessler’s reagent and is used to detect the presence of NH4+ with which it gives a brown precipitate due to the formation of iodide of Million’s base.
Uses
It is used to prepare Nessler’s reagent and for making ointments for treating skin infections.
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