Which one of the following statements is correct when SO2 is passed through acidified K2Cr2O7 solution?
- SO2 is reduced.
- Green Cr2(SO4)3 is formed.
- The solution turns blue.
- The solution is decolourised.
Answer
(2) Green Cr2(SO4)3 is formed.
Solution
Acidified dichromate is a strong oxidising agent; SO2 acts as a reducing agent and is oxidised to sulphate:
K2Cr2O7 + H2SO4 + 3SO2 K2SO4 + Cr2(SO4)3 + H2O
Cr is reduced from +6 to +3 ⇒ the orange solution turns green due to Cr2(SO4)3.
∴ SO2 is oxidised (not reduced), and the solution turns green (not blue or colourless).
Answer: (2)