Heat of Neutralisation

Heat of Neutralisation

1. Heat of neutralisation – the heat change when one mole of water is formed from the reaction between an acid and an alkali.
2. Neutralisation – a reaction between an acid reacts with a base (alkali) to form a salt and water.
3. Hydrogen ion from acid reacts with hydroxide ions from alkali to form water. H⁺(aq) + OH^-(aq) –> H₂O(l)
4. Neutralisation reaction gives out heat and always an exothermic reaction.

Example 1: (Strong acids – monoprotic acid and strong alkalis)
Chemical equation: HCl(aq) + NaOH(aq) –> NaCl(aq) + H₂O(l)
Ionic equation: H⁺(aq) + OH^-(aq) –> H₂O(l)
Heat of neutralisation of strong acids and strong alkalis are the same (ΔH = -57.3 kJ mol^-1)
Example 2: (Strong acids – diprotic acid and strong alkalis)
Chemical equation: H₂SO₄(aq) + NaOH(aq) –> Na₂SO₄(aq) + 2H₂O(l)
Ionic equation: 2H⁺(aq) + 2OH^-(aq) –> 2H₂O(l)
Heat of neutralisation of strong acids and strong alkalis are the same (ΔH = -57.3 kJ mol^-1)
Example 3: (Weak acids and strong alkalis)
Chemical equation: CH₃COOH(aq) + NaOH(aq) –> CH₃COONa(aq) + H₂O(l)
Ionic equation: H⁺(aq) + OH^-(aq) –> H₂O(l)
Heat of neutralisation of weak acids and strong alkalis are lower (ΔH = -55.0 kJ mol^-1) than heat of neutralisation of strong acids and strong alkalis (ΔH = -57.3 kJ mol^-1).
Example 4: (Strong acids and weak alkalis)
Chemical equation: HCl(aq) + NH₄OH(aq) –> NH₄Cl(aq) + H₂O(l)
Ionic equation: H⁺(aq) + OH^-(aq) –> H₂O(l)
Heat of neutralisation of strong acids and weak alkalis are lower (ΔH = -51.5 kJ mol^-1) than heat of neutralisation of strong acids and strong alkalis (ΔH = -57.3 kJ mol^-1).
Example 5: (Weak acids and weak alkalis)
Chemical equation: CH₃COOH(aq) + NH₄OH(aq) –> NH₄Cl(aq) + H₂O(l)
Ionic equation: H⁺(aq) + OH^-(aq) –> H₂O(l)
Heat of neutralisation of strong acids and weak alkalis are lower (ΔH = -50.4 kJ mol^-1) than heat of neutralisation of strong acids and strong alkalis (ΔH = -57.3 kJ mol^-1).