The heat of neutralisation is highest in

Options

A. `NaOH + CH_3COOH`
B. `HCl + NaOH`
C. `NH_4OH + HCl`
D. `CH_3COOH + NH_4OH`.

Explanation

To determine which reaction has the highest heat of neutralization, we need to analyze the nature of the acids and bases involved in each option. The heat of neutralization is defined as the heat change that occurs when an acid and a base react to form water and a salt.

### Step-by-Step Solution:

1. **Identify the Nature of Acids and Bases**:
- The heat of neutralization is highest when a strong acid reacts with a strong base. This is because strong acids and strong bases completely dissociate in solution, leading to a more exothermic reaction.

2. **Examine Each Option**:
- **Option 1**: NaOH (strong base) + Acetic Acid (weak acid)
- This reaction will not produce as much heat because acetic acid does not fully dissociate in solution.
- **Option 2**: HCl (strong acid) + NaOH (strong base)
- Both HCl and NaOH are strong, meaning they will completely dissociate and react, producing a significant amount of heat.
- **Option 3**: Ammonium Hydroxide (weak base) + HCl (strong acid)
- While HCl is a strong acid, ammonium hydroxide is a weak base, which will result in less heat being released compared to a strong base.
- **Option 4**: Acetic Acid (weak acid) + Ammonium Hydroxide (weak base)
- Both reactants are weak, leading to minimal heat of neutralization.

3. **Conclusion**:
- The reaction that produces the highest heat of neutralization is **Option 2: HCl + NaOH**. This is because both reactants are strong, leading to complete dissociation and maximum heat release.

### Final Answer:
The heat of neutralization is highest in the reaction between **HCl and NaOH** (Option 2).

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