Question

Which one of the following statements is correct when SO₂ is passed through acidified K₂Cr₂O₇ solution?

Options

• The solution turns blue.

• The solution is decolourized.

• SO₂ is reduced.

• Green Cr₂(SO₄)₃ is formed.

Answer 1

Green Cr₂(SO₄)₃ is formed.

Explanation:

When sulphur dioxide (SO₂) is passed through an acidified potassium dichromate (K₂Cr₂O₇) solution, SO₂ acts as a reducing agent. It reduces the orange \[\ce{Cr2O^{2-}_7}\] ions to green Cr³⁺ ions. These Cr³⁺ ions form chromium(III) sulphate, Cr₂(SO₄)₃, which gives the solution a green colour. The balanced equation is: 

\[\ce{Cr2O^{2-}_7 + 3SO2 + 8H+ -> 2Cr^{3+} + 3SO^{2-}_4 + 4H2O}\]