The enthalpy of neutralization of HCI by NH4OH at $25^{\circ}C$ will be:

Question

Accepted Answer

The enthalpy of neutralization is the heat change that occurs when one mole of water is formed from the reaction of an acid with a base. In the case of the neutralization of hydrochloric acid (HCl) by ammonium hydroxide (NH4OH), the reaction can be represented as:

$$ \text{HCl} + \text{NH}_4\text{OH} \rightarrow \text{NH}_4\text{Cl} + \text{H}_2\text{O} $$

1. **Explanation for Option 2 (-51.34 kJ):**

The enthalpy of neutralization for strong acids and strong bases is typically around -57 kJ/mol because the reaction essentially involves the formation of water from H$^+$ and OH$^-$ ions. However, NH4OH is a weak base, and the reaction involves additional steps, such as the dissociation of NH4OH and the formation of NH4Cl. These additional processes require energy, which reduces the overall exothermic nature of the reaction compared to the neutralization of a strong acid with a strong base. Therefore, the enthalpy of neutralization for HCl and NH4OH is less exothermic than -57 kJ/mol, making -51.34 kJ/mol a reasonable and correct value for this specific reaction.

2. **Explanation for why other options are incorrect:**

- **Option 1 (-57.32 kJ):** This value is typical for the neutralization of a strong acid with a strong base, where the reaction is straightforward and involves only the formation of water. Since NH4OH is a weak base, the enthalpy of neutralization is less exothermic than this value.

- **Option 3 (-61.34 kJ):** This value is more exothermic than the typical enthalpy of neutralization for strong acid-strong base reactions. It is not applicable to the reaction between HCl and NH4OH, as the presence of a weak base results in a less exothermic reaction.

- **Option 4 (+57.32 kJ):** A positive enthalpy value would indicate an endothermic reaction, which is not the case for neutralization reactions. Neutralization reactions are exothermic, releasing heat, so a positive value is incorrect.

In conclusion, Option 2 (-51.34 kJ) is the correct answer because it accurately reflects the enthalpy of neutralization for the reaction between HCl and NH4OH, considering the weak nature of NH4OH and the energy changes involved in the reaction.