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  Properties of Potassium dichromate

Properties of Potassium dichromate:

Physical properties:

1. It made orange red crystals that melt at 396°C.

2. It is fairly soluble in cold water and however freely soluble in hot water.

Chemical properties:

1. Action of Heat:

On heating, K2Cr2O7 decomposes to provide potassium chromate with the evolution of O2 gas.

4K2Cr2O7 → 4K2CrO4 + 2Cr2O3 + 3O2

2. With Alkalies:

Whenever an alkali is added to an orange-red solution of K2Cr2O7, a yellow solution outcomes due to the formation of potassium chromate.

K2Cr2O7 + 2KOH → 2K2CrO4 + H2O
Orange-red                Yellow

On acidifying the yellow color of K2CrO4 again alters to orange red due to the reformation of K2Cr2O7.

2K2CrO4 + H2SO4 → K2Cr2O7 + K2SO4 + H2O
                              Orange red

The inter conversion is demonstrated on the basis of the fact that in K2Cr2O7 solution, orange red Cr2O72- ions are in symmetry with yellow CrO42- ions.

Cr2O72- + H2O ↔ 2CrO42- + 2H+
Orange red           Yellow

3. Oxidising properties:

K2Cr2O7 is a powerful oxidising agent. In the presence of dilute H2SO4, one mole of this compound provides three atoms of oxygen as illustrated below.
K2Cr2O7 + 4 dil.H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3(O)

4. Chromyl chloride test:

Whenever salt having chloride is treated with K2Cr2O7 and concentrated H2SO4 reddish brown vapors of chromyl chloride are acquired.

K2Cr2O7 + 4KCl + 6H2SO4 → 2CrO2Cl2 + 6 KHSO4 + 3H2O
                                           Chromylchloride

This reaction is utilized in the detection of chloride ions in qualitative analysis.

 

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