CBSE Class 12 Chemistry Notes: d and f Block Elements – Halide

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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