Step 1: Redox Reaction Fundamentals
A chemical reaction where atomic oxidation states are altered is classified as a redox reaction. Oxidation signifies electron loss, while reduction signifies electron gain.
Step 2: Redox Reaction Identification
- Option (1): \( \text{H}_2 + \text{Cl}_2 \rightarrow 2 \text{HCl} \). Hydrogen (\( \text{H}_2 \)) undergoes oxidation, and chlorine (\( \text{Cl}_2 \)) undergoes reduction. This constitutes a redox reaction.
- Option (2): \( \text{NaCl} + \text{AgNO}_3 \rightarrow \text{NaNO}_3 + \text{AgCl} \). No alteration in oxidation states occurs; this is a double displacement reaction, not a redox reaction.
- Option (3): \( \text{C}_2 \text{H}_5 \text{OH} + \text{O}_2 \rightarrow 2 \text{H}_2 \text{O} \). There is no discernible change in oxidation states.
- Option (4): \( \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2 \text{O} \). Oxygen is reduced, but hydrogen is not oxidized.
Answer: Option (1) is the correct answer: \( \text{H}_2 + \text{Cl}_2 \rightarrow 2 \text{HCl} \).