Potassium Dichromate (K₂Cr₂O₇)

Preparation:- Potassium dichromate is prepared from the ore called chromite ore or ferrochrome or chrome ion (FeO.Cr₂O₃) in following steps.

1. Conversion of chrome iron into sodium chromate by heating with sodium carbonate

2. Conversion of  Sodium Chromate into sodium dichromate by the action of H₂SO₄                             2Na₂CrO₂ + H₂SO₄(con.) → Na₂Cr₂O₇ + Na₂SO₄ + H₂O  
3. Conversion of Sod. Dichromate into potassium dichromate by the action of KCl

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

                                        (pot. Dichromate)

Properties:-

1. Colour and M.Pt:- It forms orange coloured crystals melting at 669K.
2. Solubility:- It is moderately soluble in cold water but freely soluble in hot water.
3. Action of heat:- When heated K₂Cr₂O₇ decomposes to give oxygen.
4. Action of Alkalis:- When alkali is added to orange red colored solution of Pot. Dichromate a yellow solution of Pot. Chromate is obtained and the reaction can be reversed by adding acid to it.
5. Action of Concentrated Sulphuric Acid:-

(a) In Cold:- K₂Cr₂O₇ + 2H₂SO₄(dil.) → 2KHSO₄ + 2CrO₃ + H₂O

(b) In Hot:- 2K₂Cr₂O₇ + 8H₂SO₄ →2K₂SO₄ + 2Cr₂(SO₄)₃ + 8H₂O + 3O₂

•  Oxidizing Properties:- K₂Cr₂O₇ is very powerful oxidizing agent in acidic medium. For example:Oxidation of Kl liberate I₂:

K₂Cr₂O₇ + 4H₂So₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O +3[O]

2Kl + H₂SO₄ + O → K₂SO₄ + I₂ + H₂O] x 3

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K₂Cr₂O₇ + 6KI + 7H₂SO₄ → 4K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3I₂

_{Or           Cr2O7^-2 + 6I^- + 4H⁺ → 2Cr⁺³ + 7H2O + 3I2       (Ionic form).}

• Oxidation of Ferrous salt to ferric salt:

K₂Cr₂O₇ + 4H₂SO → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3[O]

2FeSO₄ + H₂SO₄ + H₂SO₄ + O → Fe₂(SO₄)₃ + H₂O ] x 3

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K₂Cr₂O₇ + 7H₂SO₄ + 6FeSO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 3 Fe₂(SO₄)₃ + 7H₂O

Or Cr₂o₇^-2 + 6 Fe⁺² + 14 H⁺ → 2Cr⁺³ + 6Fe⁺³ + 7H₂O (Ionic form)

• Oxidation of H₂S to sulphur:-

• Oxidation of Sulphites to sulphates:

• Oxidation of SO₂ to sulphuric acid:

7.Chemistry of chromyl chloride test:- It is used as a confirmatory test for chloride radicals. The salt containing chlorine is shook with conc. H2SO4 and then heated – A radish brown vapours of chromyl chloride are obtained.

Uses:-

It is used in volumetric analysis.

It is used in chrome tanning in lather industry.

It is used in Calico printing and dyeing

It is used as an oxidizing agent in organic chemistry.

Structure:-  Chromate ion (CrO₄^-2) and dichromate ion (Cr₂O₇^-2) exist in equilibrium at a pH of 4.

They are interconvertible by change in pH.