Question

An example of a disproportionation reaction is :

• A. \(\mathrm{2NaBr + Cl_2 \to 2NaCl + Br_2}\)
• B. \(\mathrm{2KMnO_4 \to K_2MnO_4 + MnO_2 + O_2}\)
• C. \(\mathrm{2CuBr \to CuBr_2 + Cu}\)
• D. \(\mathrm{2MnO_4^- + 10I^- + 16H^+ \to 2Mn^{2+} + 5I_2 + 8H_2O}\)

Hint:

In a disproportionation reaction the same element is simultaneously oxidised and reduced. Check which reaction has one element going both up and down in oxidation state.

Answer 1

The Correct Option is C

(1) Br goes -1 to 0 (oxidised) and Cl goes 0 to -1 (reduced): two different elements, a simple displacement redox.
(2) In the decomposition of \(\mathrm{KMnO_4}\), Mn goes +7 to +6 and +4 (only reduction) while oxygen goes -2 to 0 (oxidation): Mn is only reduced, so it is not a disproportionation of one element.
(3) \(\mathrm{2CuBr \to CuBr_2 + Cu}\): Cu goes +1 to +2 (oxidised) and +1 to 0 (reduced). The same element is both oxidised and reduced, so this is disproportionation.
(4) Mn goes +7 to +2 and I goes -1 to 0: two different elements.
Answer: (3) \(\mathrm{2CuBr \to CuBr_2 + Cu}\)