The enthalpy change of neutralisation of a weak acid is the heat released when one mole of water is formed from the reaction between a weak acid and a base under standard conditions. It is smaller in magnitude compared to the neutralisation of a strong acid with a strong base.
Understanding Enthalpy of Neutralisation
Neutralisation is a chemical reaction where an acid reacts with a base to produce a salt and water. This reaction is typically exothermic, meaning it releases heat energy. The enthalpy change of neutralisation ($\Delta H_{neut}$) is defined as the energy change that occurs when one mole of water is formed by the reaction of an acid with a base under standard conditions (usually 298 K and 1 atm pressure).
For the reaction between a strong acid and a strong base in dilute aqueous solution, the net ionic equation is always:
H⁺(aq) + OH⁻(aq) → H₂O(l)
The enthalpy change for this specific reaction is remarkably constant, approximately -57.3 kJ/mol. This consistency arises because strong acids and strong bases are completely dissociated into ions in water.
Why is the Enthalpy Different for Weak Acids?
Unlike strong acids, weak acids do not completely dissociate in water. They exist in equilibrium between the undissociated molecule and its ions. When a weak acid is neutralised by a strong base (which provides OH⁻ ions), the OH⁻ ions react with the H⁺ ions present in the solution. This reaction reduces the concentration of H⁺ ions, shifting the weak acid's dissociation equilibrium to produce more H⁺ ions (Le Chatelier's principle).
The Role of Dissociation
As stated in the reference: "The enthalpy change of neutralisation involving weak acids are smaller in magnitude compared to that between strong acids and strong bases."
This difference in magnitude is primarily because weak acids are only partially dissociated. When the weak acid reacts with a base, the equilibrium HA ⇌ H⁺ + A⁻ is pushed to the right. The further dissociation of the weak acid requires energy input. This energy input is subtracted from the total energy released by the formation of water, resulting in a less exothermic (smaller magnitude) overall enthalpy change for the neutralisation of a weak acid.
- Strong Acid + Strong Base: H⁺(aq) + OH⁻(aq) → H₂O(l) (ΔH ≈ -57.3 kJ/mol) - Energy released primarily from H⁺ + OH⁻ forming water.
- Weak Acid + Strong Base: HA(aq) + OH⁻(aq) → A⁻(aq) + H₂O(l) (ΔH < -57.3 kJ/mol, i.e., smaller negative value or smaller magnitude) - Energy released from H⁺ + OH⁻ formation minus energy absorbed for HA → H⁺ + A⁻ dissociation.
This means that "not all hydrogen ions are free to react with hydroxide ions" initially, and energy is needed to 'release' more H⁺ ions from the undissociated acid molecules.
Comparison: Weak vs. Strong Acid Neutralisation
Here's a comparison of typical enthalpy values:
| Reaction Type | Example Reaction | Typical ΔHneut (kJ/mol) | Magnitude Comparison |
|---|---|---|---|
| Strong Acid + Strong Base | HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) | -57.3 | Larger Magnitude |
| Weak Acid + Strong Base | CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l) | Around -55 to -56 | Smaller Magnitude (less exothermic) |
| Strong Acid + Weak Base | HCl(aq) + NH₄OH(aq) → NH₄Cl(aq) + H₂O(l) | Around -51 to -52 | Smaller Magnitude (less exothermic) |
| Weak Acid + Weak Base | CH₃COOH(aq) + NH₄OH(aq) → CH₃COONH₄(aq) + H₂O(l) | Even Smaller Magnitude | Smallest Magnitude |
(Note: The exact value for weak acid neutralisation depends on the specific acid and its degree of dissociation.)
Practical Implications
Understanding the enthalpy change helps in various applications, such as calorimetry experiments to determine the heat of neutralisation or predicting temperature changes in industrial processes involving neutralisation. The lower heat output from weak acid neutralisation is a direct consequence of the energy cost associated with their incomplete dissociation.
