Which one of the following is a redox reaction ?

Options

A. `H_(2)+Br_(2)rarr2HBr`
B. `2NaCl+H_(2)SO_(4)rarrNa_(2)SO_(4)+2HCl`
C. `HCl+AgNO_(3)rarrAgCl+HNO_(3)`
D. `NaOH+HClrarrNaCl+H_(2)O`

Answer

To determine which of the given reactions is a redox reaction, we need to identify if there is a change in oxidation states of the elements involved in the reactions. A redox reaction involves both oxidation (loss of electrons, increase in oxidation state) and reduction (gain of electrons, decrease in oxidation state).

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to analyze the provided reactions to see if they involve changes in oxidation states. The reactions typically involve elements in their elemental form and their compounds.

2. **Examine the First Reaction (H2 + Br2 → 2HBr)**:
- In the reactants, both hydrogen (H2) and bromine (Br2) are in their elemental forms, thus their oxidation states are 0.
- In HBr, the oxidation state of hydrogen is +1 and bromine is -1.
- **Change in Oxidation States**:
- H: 0 (in H2) → +1 (in HBr) → Oxidation
- Br: 0 (in Br2) → -1 (in HBr) → Reduction
- Since both oxidation and reduction occur, this is a redox reaction.

3. **Examine the Second Reaction (Na + Cl2 → 2NaCl)**:
- Sodium (Na) is in its elemental form with an oxidation state of 0, and chlorine (Cl2) is also 0.
- In NaCl, sodium has an oxidation state of +1 and chlorine has -1.
- **Change in Oxidation States**:
- Na: 0 → +1 → Oxidation
- Cl: 0 → -1 → Reduction
- This is also a redox reaction.

4. **Examine the Third Reaction (H2SO4)**:
- In sulfuric acid (H2SO4), the oxidation state of sulfur is +6.
- There are no changes in oxidation states when comparing the reactants and products.
- Thus, this is **not a redox reaction**.

5. **Examine the Fourth Reaction (HCl)**:
- In hydrochloric acid (HCl), hydrogen is +1 and chlorine is -1.
- There are no changes in oxidation states when comparing the reactants and products.
- Thus, this is **not a redox reaction**.

6. **Examine the Fifth Reaction (HNO3)**:
- In nitric acid (HNO3), nitrogen has an oxidation state of +5.
- Similar to the previous reactions, there are no changes in oxidation states.
- Thus, this is **not a redox reaction**.

7. **Examine the Neutralization Reaction**:
- Neutralization reactions typically involve an acid and a base forming water and a salt.
- There are no changes in oxidation states in a simple neutralization reaction.
- Thus, this is **not a redox reaction**.

### Conclusion:
The first reaction (H2 + Br2 → 2HBr) is a redox reaction because it involves both oxidation and reduction processes.

### Final Answer:
**The correct answer is the first reaction (H2 + Br2 → 2HBr).**