In which of the following reactions, the enthalpy is the least ?

Options

A. `CH_(3)COOH + NaOH to CH_(3)COONa + H_(2)O`
B. `HCl + NH_(4)OH to NH_(4)Cl + H_(2)O`
C. `HCl + NaOH to NaCl + H_(2)O`
D. `HCN + NH_(4)OH to NH_(4)CN + H_(2)O`

Explanation

To determine which reaction has the least enthalpy among the given options, we will analyze the nature of the acids and bases involved in each reaction.

### Step-by-Step Solution:

1. **Identify the Nature of Acids and Bases**:
- We need to categorize each reaction based on whether they involve strong acids, weak acids, strong bases, or weak bases.

2. **Understand Enthalpy in Acid-Base Reactions**:
- In acid-base reactions, the enthalpy change is influenced by the strength of the acid and base. Strong acids and bases ionize completely, releasing more energy, while weak acids and bases do not ionize completely, resulting in less energy release.

3. **Analyze Each Option**:
- **Option 1**: Weak acid + Strong base
- **Option 2**: Strong acid + Weak base
- **Option 3**: Strong acid + Strong base
- **Option 4**: Weak acid + Weak base

4. **Determine the Enthalpy for Each Case**:
- **Option 1**: The reaction involves a weak acid and a strong base, which will have a moderate enthalpy.
- **Option 2**: The reaction involves a strong acid and a weak base, which will have a higher enthalpy due to the strong acid.
- **Option 3**: The reaction involves a strong acid and a strong base, which will have the highest enthalpy due to complete ionization.
- **Option 4**: The reaction involves a weak acid and a weak base, which will have the least enthalpy because both do not ionize completely, resulting in minimal energy release.

5. **Conclusion**:
- Based on the analysis, **Option 4 (Weak acid + Weak base)** has the least enthalpy.

### Final Answer:
The reaction with the least enthalpy is **Option 4: Weak acid + Weak base**.

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