H₂SO4 acts as a strong oxidising agent. In which of the reaction, is it not acting as an oxidising agent?

The correct option is: CaF₂ + 2H₂SO₄ → CaSO₄ + 2HF

Understanding Oxidising Agents:
An oxidising agent is a substance that oxidises other substances by accepting electrons, which means it undergoes reduction itself. In terms of oxidation numbers, the oxidation state of the key element in the oxidising agent decreases during the reaction. In sulfuric acid (H₂SO₄), sulfur (S) is in its highest oxidation state of +6.

If H₂SO₄ acts as an oxidising agent, the oxidation state of sulfur must decrease (from +6 to a lower value, typically +4 in SO₂). If the oxidation state of sulfur remains unchanged at +6, and the oxidation states of all other elements also remain unchanged, then the reaction is not a redox reaction, and H₂SO₄ is not acting as an oxidising agent.

Let's analyze the given reactions step-by-step to see why H₂SO₄ does not act as an oxidising agent in the correct option:

1. Analysis of the reaction: CaF₂ + 2H₂SO₄ → CaSO₄ + 2HF
Let us determine the oxidation states of the elements on both sides of the reaction:

In CaF₂:
Calcium (Ca) has an oxidation state of +2.
Fluorine (F) has an oxidation state of -1.

In H₂SO₄:
Hydrogen (H) has an oxidation state of +1.
Oxygen (O) has an oxidation state of -2.
Sulfur (S) has an oxidation state of +6.

In CaSO₄:
Calcium (Ca) has an oxidation state of +2.
Oxygen (O) has an oxidation state of -2.
Sulfur (S) has an oxidation state of +6.

In HF:
Hydrogen (H) has an oxidation state of +1.
Fluorine (F) has an oxidation state of -1.

Comparing the oxidation states before and after the reaction:
Ca: +2 → +2 (No change)
F: -1 → -1 (No change)
H: +1 → +1 (No change)
S: +6 → +6 (No change)
O: -2 → -2 (No change)

Since there is no change in the oxidation state of any element, this is a non-redox (specifically, a double displacement or acid-base type) reaction. Therefore, H₂SO₄ does not act as an oxidising agent here.

Let's briefly verify the other options where H₂SO₄ does act as an oxidising agent:

• In C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O:
Carbon (C) is oxidised from 0 in C to +4 in CO₂.
Sulfur (S) is reduced from +6 in H₂SO₄ to +4 in SO₂. Thus, H₂SO₄ acts as an oxidising agent.

• In S + 2H₂SO₄ → 3SO₂ + H₂O:
Elemental sulfur (S) is oxidised from 0 to +4 in SO₂.
Sulfur in H₂SO₄ is reduced from +6 to +4 in SO₂. Thus, H₂SO₄ acts as an oxidising agent.

• In Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O:
Copper (Cu) is oxidised from 0 in Cu to +2 in CuSO₄.
Sulfur in H₂SO₄ is reduced from +6 to +4 in SO₂. Thus, H₂SO₄ acts as an oxidising agent.