Which of the following is NOT redox reaction?

Cr₂O₇²⁻ + 2 OH^⊝ ⟶ 2 CrO₄²⁻ + H₂O

To determine which reaction is NOT a redox reaction, we must first understand what constitutes a redox reaction. A redox reaction involves two simultaneous processes: oxidation (loss of electrons) and reduction (gain of electrons). This means there must be a change in the oxidation states of the elements involved. If no such change occurs, the reaction is not classified as a redox reaction.

Analysis of the given reactions:

1. Reaction: Cr₂O₇²⁻ + 2 OH^⊝ ⟶ 2 CrO₄²⁻ + H₂O
   In this reaction, the oxidation state of chromium remains unchanged at +6 in both dichromate (Cr₂O₇²⁻) and chromate (CrO₄²⁻) ions. Since there is no change in oxidation states, this reaction is NOT a redox reaction.
2. Reaction: SO₃²⁻ + H₂O + I₂ ⟶ SO₄²⁻ + 2 I^⊖ + 2 H^⊕
   Here, sulfur is oxidized as its oxidation state changes from +4 (in SO₃²⁻) to +6 (in SO₄²⁻), and iodine is reduced as its oxidation state changes from 0 (in I₂) to −1 (in I^⊖). This is a clear redox reaction.
3. Reaction: Ca(OH)₂ + Cl₂ ⟶ Ca(OCl)₂ + CaCl₂
   Chlorine undergoes both oxidation and reduction in this reaction, forming hypochlorite (OCl⁻) and chloride (Cl⁻). Hence, this is a redox reaction.
4. Reaction: PCl₅ ⟶ PCl₃ + Cl₂
   In this reaction, phosphorus changes its oxidation state from +5 (in PCl₅) to +3 (in PCl₃), and chlorine goes from −1 (in PCl₅) to 0 (in Cl₂). This confirms that it is a redox reaction.

After analyzing all the reactions, we conclude that:

• (1) is NOT a redox reaction, as there is no change in the oxidation state of chromium.
• (2), (3), and (4) are redox reactions.

This analysis clarifies the concept of redox reactions and helps in understanding why reaction (1) does not qualify as a redox reaction.