Which of the following reaction is oxidation- reduction?

Options

A. `H^(o+)+OH^(ɵ)toH_(2)O`
B. `(1)/(2)H_(2)+(1)/(2)Cl_(2)toHCl`
C. `CaCO_(3)toCaO+CO_(2)`
D. (a) and (c)

Explanation

To determine which of the given reactions is an oxidation-reduction (redox) reaction, we need to analyze the changes in oxidation states of the elements involved in each reaction. Here’s a step-by-step solution:

### Step 1: Understand Oxidation and Reduction
- **Oxidation** is defined as an increase in oxidation number.
- **Reduction** is defined as a decrease in oxidation number.
- A reaction is classified as a redox reaction if both oxidation and reduction occur simultaneously.

### Step 2: Analyze Each Reaction
We will analyze the four reactions provided in the question.

#### Reaction A: H⁺ + OH⁻ → H₂O
- **Left Side:**
- H⁺: +1 oxidation state
- OH⁻: O is -2, H is +1
- **Right Side:**
- H₂O: O is -2, H is +1
- **Change in Oxidation States:** No change in oxidation states.
- **Conclusion:** This is NOT a redox reaction.

#### Reaction B: H₂ + Cl₂ → 2HCl
- **Left Side:**
- H₂: H is 0 (neutral)
- Cl₂: Cl is 0 (neutral)
- **Right Side:**
- HCl: H is +1, Cl is -1
- **Change in Oxidation States:**
- H changes from 0 to +1 (oxidation).
- Cl changes from 0 to -1 (reduction).
- **Conclusion:** This IS a redox reaction.

#### Reaction C: CaCO₃ → CaO + CO₂
- **Left Side:**
- CaCO₃: Ca is +2, C is +4, O is -2
- **Right Side:**
- CaO: Ca is +2, O is -2
- CO₂: C is +4, O is -2
- **Change in Oxidation States:** No change in oxidation states.
- **Conclusion:** This is NOT a redox reaction.

#### Reaction D: (Not provided in detail, but inferred)
- Since reactions A and C are not redox reactions, option D is also not a redox reaction.

### Step 3: Identify the Correct Answer
From the analysis, we conclude that:
- **Only Reaction B (H₂ + Cl₂ → 2HCl) is an oxidation-reduction reaction.**

### Final Answer
The correct option is **B**.

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