JEE Main 2024 · 27 January · evening · Single correct · +4 / −1
Which of the following cannot function as an oxidising agent?
- (a) SO₄²⁻
- (b) MnO₄⁻
- (c) N³⁻
- (d) BrO₃⁻
Answer and solution
Answer (c) N³⁻
Solution
Chemistry ma’am · 3 steps
An oxidising agent must be reduced, so ask which atom is already at its lowest oxidation state.
- What oxidising means. An oxidising agent takes electrons from something else, so it gets reduced itself. Its key atom must be able to move to a lower oxidation state.
- Find each oxidation state. Take oxygen as -2 and match the ion's charge. For example, in SO₄²⁻, x + 4(-2) = -2 gives x = +6.
Species Oxidation state Range for that element Can it go lower? SO₄²⁻ S: +6 -2 to +6 Yes, it is at the top MnO₄⁻ Mn: +7 0 to +7 Yes, it is at the top N³⁻ N: -3 -3 to +5 No, it is already at the bottom BrO₃⁻ Br: +5 -1 to +7 Yes, it can drop to -1 - So. Nitrogen in N³⁻ cannot gain any more electrons, so it cannot be reduced. It can only lose electrons, so it acts as a reducing agent and never as an oxidising agent. The answer is (c).
The idea to keep. An element at its lowest oxidation state can only be a reducing agent. One at its highest can only be an oxidising agent. One in between can act either way.
Answer from the NTA final answer key. Solution by PrepWiser.