Enthalpy Changes of Neutralisation

Specification Reference Topic 12, point 23

Quick Notes

Standard Enthalpy Change of Neutralisation is the energy change when 1 mole of water is formed in a reaction between an acid and a base:
- H⁺(aq) + OH⁻(aq) → H₂O(l)
- Standard conditions assumed:
  - Temperature = 298 K
  - Pressure = 100 kPa
Strong Acids (e.g. HCl, HNO₃) are fully ionised in solution:
- All H⁺ ions are freely available and neutralisation is straightforward.
- Entire enthalpy change goes into forming water.
Weak Acids (e.g. CH₃COOH) are only partially ionised in water:
- Most acid remains as CH₃COOH molecules and fewer free H⁺ ions available.
- OH⁻ reacts first with free H⁺.
- More H⁺ ions must be produced via further ionisation.
- Ionisation of weak acid is endothermic (absorbs energy) and this reduces the total energy released.
- Overall neutralisation appears less exothermic than with strong acids.

Full Notes

The enthalpy change of neutralisation refers to the energy released when 1 mole of hydrogen ions (H⁺) reacts with hydroxide ions (OH⁻) to form water:

H⁺(aq) + OH⁻(aq) → H₂O(l) ΔH ≈ –57 kJ mol⁻¹

This is a standard enthalpy change, so the values refer to 100 kPa pressure and 298 K temperature, unless stated otherwise.

Example Addition of HCl to NaOH

This shows a strong acid neutralising a strong base.

Edexcel A-Level Chemistry diagram of enthalpy change of neutralisation for HCl reacting with NaOH to form water.

Explaining the Difference Between Strong and Weak Acids

For a strong acid like HCl or HNO₃, the acid is fully ionised in solution. This means that all the H⁺ ions from the acid are already present and free in solution, ready to react with OH⁻. As a result, the neutralisation reaction happens directly.

In contrast, a weak acid like ethanoic acid (CH₃COOH) is only partially ionised in water. Most of it remains as CH₃COOH molecules rather than separating into H⁺ and CH₃COO⁻ ions.

So when you add a base, the OH⁻ reacts with the small number of free H⁺ ions first. To continue the neutralisation, more H⁺ ions are needed, and these must come from further ionisation of CH₃COOH. However, this ionisation is an endothermic process (it takes in energy).

As a result, some of the energy released during neutralisation is used up to ionise more acid molecules.

This means the overall enthalpy change appears smaller (less exothermic) than with a strong acid.

Summary

Enthalpy change of neutralisation = energy released when 1 mole of H₂O forms from H⁺ and OH⁻.
Standard value ≈ –57 kJ mol⁻¹ under 298 K and 100 kPa.
Strong acids: fully ionised, all energy goes into forming water.
Weak acids: partially ionised, some energy is used for ionisation, so neutralisation appears less exothermic.