Properties of Potassium dichromate
- It forms orange red crystals which melt at 396°C.
- It is moderately soluble in cold water but freely soluble in hot water.
Action of Heat
On heating, K2Cr2O7 decomposes to give potassium chromate with evolution of O2 gas.
4 K2Cr2O7 → 4K2CrO4 + 2Cr2O3 + 3 O2
When an alkali is added to an orange-red solution of K2Cr2O7, a yellow solution results due to formation of potassium chromate.
K2Cr2O7 (Orange-red) + 2KOH → 2K2CrO4 (Yellow) + H2O
On acidifying the yellow colour of K2CrO4 again changes to orange red due to reformation of K2Cr2O7.
2K2CrO4 + H2SO4 → K2Cr2O7 (Orange red) + K2SO4 + H2O
The inter conversion is explained on the basis of the fact that in K2Cr2O7 solution, orange red Cr2O72- ions are in equilibrium with yellow CrO42- ions.
Cr2O72- (Orange red) + H2O ↔ 2CrO42- (Yellow) + 2H+
K2Cr2O7 is a powerful oxidising agent. In presence of dil.H2SO4, one mole of this compound gives three atoms of oxygen as shown below.
K2Cr2O7 + 4 dil.H2SO4 → K2SO4 + Cr2 (SO4)3 + 4H2O+ 3(O)
Some other examples of oxidizing property of K2Cr2O7 are given below.
- It liberates I2 from KI
K2Cr2O7 + 7H2SO4 + 6KI → 4 K2SO4 + Cr2(SO4)3 + 3 I2 + 7H2O
- It oxidizes ferrous to ferric salt
K2Cr2O7 + 7H2SO4 + 6 FeSO4 → K2SO4 + Cr2 (SO4)3 + 3 Fe2 (SO4)3 + 2H2O
- It oxidizes H2S to sulphur
K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2 (SO4)3 + 7H2O + 3S