Potassium Dichromate | d Block Chemistry | Jee Mains, Advance, BITSAT, NEET & AIIMS Video
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Potassium Dichromate, K2Cr2O7
• Preparation of Potassium Dichromate
It is prepared from the ore called chromate or ferrochrome or chrome iron, FeO.Cr2O3. The various steps involved are
(a) Preparation of sodium chromate
4FeO.Cr2O3 + O2 → Fe2O3 + 4Cr2O3
4Na2CO3 + 2Cr2O3 + 3O2 → 4Na2CrO4 + 4CO2
(b) Conversion of sodium chromate into sodium dichromate.
2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
(c) Conversion of sodium dichromate into potassium dichromate.
Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl
• Properties of Potassium Dichromate
It forms orange red crystals. It is moderately soluble in cold water but freely soluble in hot water.
1. Action of heat
When heated, it decomposed to its chromate
4K2Cr2O7 → 4K2CrO4 + 2Cr2O3 + 3O2
2. Action of alkalis
With alkalis it is converted into chromate which on acidifying gives back dichromate.
K2Cr2O7 + 2KOH → 2K2CrO4 + H2O
2K2Cr2O7 + H2SO4 → K2Cr2O7 + K2SO4 + H2O
In dichromate solution the Cr2O72– ions are in equilibrium with Cr2O72– ions at pH = 4.
Cr2O72– + H2O →2CrO42– + 2Hl
orange redyellow
3. Action of conc. H2SO4 solution
• In cold conditions
K2Cr2O7 + 2H2SO4 ———→ 2CrO3 + 2KHSO4 + H2O
• In hot conditions
2K2Cr2O7 + 8H2SO4 ———→ 2K2SO4 + 2Cr2(SO4)3 + 8H2O + 3O2
4. Oxidising properties
It is a powerful oxidising agent. In the presence of dil. H2SO4 it furnishes 3 atoms of available oxygen.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3O
Some of the oxidizing properties of K2Cr2O7 are
1. It liberates I2 from KI
K2Cr2O7 + 7H2SO4 + 6Kl → 4K2SO4 + Cr2(SO4)3 + 3l2 + 7H2O
2. It oxidises ferrous salts to ferric salts
K2Cr2O7 + 7H2SO4 + 6FeSO4 → K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 2H2O
3. It oxidises S-2 to S
K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2(SO4)3 + 7H2O + 3S
4. It oxidises nitrites to nitrates
K2Cr2O7 + 4H2SO4 + 3NaNO2 → K2SO4 + Cr2(SO4)3 + 3NaNO3 + 4H2O
5. It oxidises SO2 to SO42–
K2Cr2O7 + H2SO4 + 3SO2 → K2SO4 + Cr2(SO4)3 + 3H2O
6. It oxidises ethyl alcohol to acetaldehyde and acetic acid.
5. Chromyl Chloride Test
When heated with conc. HCl or with a chloride in the presence of sulphuric acid, reddish brown vapours of chromyl chloride are obtained.
K2Cr2O7 + 4KCl + 6H2SO4 → 2CrO2Cl2 + 6KHSO4 + 3H2O
Thus reaction is used in the detection of chloride ions in qualitative analysis.
We Hope you will enjoy the lecture.
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