Question

Which one of the following statements is correct when SO₂
is passed through acidified K₂Cr₂O₇ solution ? [2016]
(a) The solution turns blue
(b) The solution is decolourized
(c) SO₂ is reduced
(d) Green Cr₂(SO₄)₃ is formed

Answer 1

The color of the solution turns green and a Green Cr₂(SO₄)₃ is formed.

Explanation:

• When SO2 is passed through the acidified K2Cr2O7 solution, the orange color of potassium dichromate solution turns to clear green due to the formation of chromium sulfate.
• In this reaction, the oxidation of Cr changes from +6 to +3.
• K2Cr2O7 + H2SO4 + 3SO2  ---> K2SO4 + Cr2(SO4)3 + H2O
• Oxidation state of Cr = +6                          oxidation state of Cr = +3
• The appearance of green color is due to the reduction of chromium metal.
• Thus the color of the solution turns green and a Green Cr₂(SO₄)₃ is formed.

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