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)

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

Silver fluoride is prepared by the action of HF on silver (I) oxide.

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

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.

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

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.

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

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

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.

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

(ii) It can also prepared by heating a mixture of mercuric chloride and mercury in an iron vessel.

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

Properties

(i) It is a white power insoluble in water but soluble in chlorine water.

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

(ii) It decomposes on heating to HgCl2

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

(iii) On treatment with ammonia, if turns black due to the formation of finely divided mercury.

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

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.

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

2. It is also obtained by treating HgO with HCl

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

3.  Commercially, it is prepared by heating a mixture of HgSO4 and NaCl in the presence of MnO2

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

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-.

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

2. It is readily soluble in organic solvents suggesting its covalent nature.

3.  When treated with SnCl2 it is reduced to mercury.

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

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

4.  When Cu turnings are placed in its contact a shining grey film of mercury deposits over them.

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

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.

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

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.

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

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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