Question

Which of the following is an example of a redox reaction?

• A. NaCl dissolving in water
• B. 2H$_2$O$_2$ (aq) $\to$ 2H$_2$O (l) + O$_2$ (g)
• C. NaOH dissolving in water
• D. CaCO$_3$ (s) $\to$ CaO (s) + CO$_2$ (g)

Hint:

In a redox reaction, one species is reduced (gains electrons) and another is oxidized (loses electrons).

Answer 1

The Correct Option is B

Definition: A redox reaction is characterized by a change in the oxidation states of elements. In such a reaction, one species undergoes oxidation (electron loss), and another undergoes reduction (electron gain).

Option 1: Dissolution of NaCl in water

\( \text{NaCl} \rightarrow \text{Na}^+ + \text{Cl}^- \)

This is a physical process, not a redox reaction, as oxidation numbers do not change.

Option 2: Decomposition of hydrogen peroxide

Option 2: \( 2H_2O_2 \rightarrow 2H_2O + O_2 \)

The oxidation number of O in \( H_2O_2 \) is -1. In \( H_2O \), it is -2, and in \( O_2 \), it is 0.

• Some oxygen atoms are reduced: -1 → -2
• Other oxygen atoms are oxidized: -1 → 0

✔ This is a disproportionation redox reaction.

Option 3: Dissolution of NaOH in water

\( \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \)

This is a dissociation process. No redox occurs as there is no change in oxidation numbers.

Option 4: Thermal decomposition of calcium carbonate

Option 4: \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)

This is a thermal decomposition reaction. However, the oxidation states remain constant:

• Ca: +2 → +2
• C: +4 → +4
• O: -2 in all species

❌ Not a redox reaction.

✅ Correct Answer:

Option 2: \( \boxed{2H_2O_2 \rightarrow 2H_2O + O_2} \)