JEE Main 2021 · 20 July · morning · Single correct · +4 / −1

The species given below that does NOT show disproportionation reaction is :

  1. (a) BrO₄ -
  2. (b) BrO -
  3. (c) BrO₂ -
  4. (d) BrO₃ -
Answer and solution

Answer (a) BrO₄ -

Solution

Chemistry ma’am · 3 steps

Find bromine's oxidation state in each ion. The one already at its highest state cannot disproportionate.

  1. What disproportionation needs. In disproportionation, one element is oxidised and reduced at the same time. Its starting oxidation state must have room to go both up and down.
  2. Range for bromine. Bromine has 7 valence electrons, so its oxidation states run from -1 (as in Br⁻) to +7. Any state strictly between these two limits can disproportionate.
    IonOxidation state of BrCan go up?Can go down?
    BrO₄⁻x - 8 = -1, so +7No, already the maximumYes
    BrO⁻x - 2 = -1, so +1YesYes
    BrO₂⁻x - 4 = -1, so +3YesYes
    BrO₃⁻x - 6 = -1, so +5Yes, to +7Yes
  3. So. In BrO₄⁻, bromine is at +7 and cannot be oxidised any further, so it can only be reduced. It does not show disproportionation, and the answer is (a).

The idea to keep. An element at its highest or lowest oxidation state can never disproportionate. Only an intermediate state can go both ways, so check where the element sits in its range.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Redox Reactions

Asked about 90 times in JEE Main since 2020. This question is from 2021.