Question

Which of the following will not show disproportionation reaction?

• A. Sulphurous acid
• B. Hydrogen sulphide
• C. Orthorombic sulphur in basic medium
• D. Orthophosphorous acid

Answer 1

Answer: (B)

Hydrogen sulphide

Answer 2

A disproportionation reaction is a redox reaction where an element in an intermediate oxidation state is simultaneously oxidized and reduced. For an element to undergo disproportionation, it must be able to exist in at least three different oxidation states, with its current state being intermediate.

1. Sulphurous acid (H₂SO₃): The oxidation state of Sulphur (S) is +4. This is an intermediate state (between -2 and +6). Thus, it can disproportionate.
2. Hydrogen sulphide (H₂S): The oxidation state of Sulphur (S) is -2. This is the lowest possible oxidation state for Sulphur. Therefore, it cannot be reduced further and hence cannot undergo disproportionation.
3. Orthorhombic sulphur (S): The oxidation state of Sulphur (S) is 0. This is an intermediate state (between -2 and +6). In basic medium, it disproportionates into S²⁻ (-2) and SO₃²⁻ (+4).
4. Orthophosphorous acid (H₃PO₃): The oxidation state of Phosphorus (P) is +3. This is an intermediate state (between -3 and +5). On heating, it disproportionates into PH₃ (-3) and H₃PO₄ (+5).

Therefore, Hydrogen sulphide (H₂S) will not show a disproportionation reaction.