Introduction

Neutralisation reactions between acids and bases are fundamental processes in chemistry that always release energy into the surroundings. Understanding these reactions is crucial for HSC Chemistry students studying the Properties of Acids and Bases.

What is Enthalpy of Neutralisation?

The enthalpy of neutralisation is the energy change that occurs when one mole of an acid reacts completely with one mole of a base to form water and a salt. This process is always exothermic, meaning it releases energy into the surroundings.

Strong Acid-Base Neutralisation

When strong acids and strong bases react, they produce a consistent enthalpy change of $\Delta H = -57.1 \text{ kJ mol}^{-1}$. This consistency occurs because:

For example, when hydrochloric acid reacts with sodium hydroxide:

$\text{HCl}_{(aq)} + \text{NaOH}_{(aq)} \rightarrow \text{NaCl}_{(aq)} + \text{H}_2\text{O}_{(l)}$

The net ionic equation is:

$\text{H}^+_{(aq)} + \text{OH}^-_{(aq)} \rightarrow \text{H}_2\text{O}_{(l)} \quad \Delta H = -57.1 \text{ kJ mol}^{-1}$

Weak Acid Neutralisation

Weak acid neutralisation reactions release less energy than strong acid neutralisations. The enthalpy change is less negative because:

For example, with acetic acid (CH₃COOH):

First, the acid dissociation occurs:

$\text{CH}_3\text{COOH}_{(aq)} \rightleftharpoons \text{H}^+_{(aq)} + \text{CH}_3\text{COO}^-_{(aq)} \quad \Delta H = +1.0 \text{ kJ mol}^{-1}$

Then, the neutralisation with hydroxide ions:

$\text{CH}_3\text{COOH}_{(aq)} + \text{OH}^-_{(aq)} \rightarrow \text{H}_2\text{O}_{(l)} + \text{CH}_3\text{COO}^-_{(aq)} \quad \\\Delta H = -56.1 \text{ kJ mol}^{-1}$