JEE Main 2020 · 9 January · morning · Single correct · +4 / −1
The compound that cannot act both as oxidising and reducing agent is :
- (a) H₃PO₄
- (b) H₂SO₃
- (c) H₂O₂
- (d) HNO₂
Answer and solution
Answer (a) H₃PO₄
Solution
Chemistry ma’am · 3 steps
Find the oxidation state of the key atom. If it is already at its maximum, it can only go down.
- The rule. To act as a reducing agent, an atom must be able to lose electrons and rise in oxidation state. To act as an oxidising agent, it must be able to gain electrons and fall. An atom in an intermediate oxidation state can do both. An atom at its maximum state can only be reduced.
Compound Key atom's oxidation state Possible range Can do both? H₃PO₄ P: +5 -3 to +5 No, P is at its maximum H₂SO₃ S: +4 -2 to +6 Yes H₂O₂ O: -1 -2 to 0 Yes HNO₂ N: +3 -3 to +5 Yes - Why P is stuck. Phosphorus is in group 15 and has 5 valence electrons. In H₃PO₄ all 5 are already used, so P cannot be oxidised any further. It can only act as an oxidising agent.
- So. H₃PO₄ is the compound that cannot act as both oxidising and reducing agent, so the answer is (a).
The idea to keep. Maximum oxidation state means oxidant only, minimum means reductant only, and anything in between can be both. For main-group elements, the maximum is usually the group's valence electron count.
Answer from the NTA final answer key. Solution by PrepWiser.