Understanding neutralisation reactions

Neutralisation is a chemical reaction where an acid reacts with a base to form a salt and water. A common misconception is that this reaction always results in a solution with a pH of 7. However, even when equal amounts of acid and alkali are mixed, the resulting pH may differ from 7 depending on the strengths of the acid and base involved.

The pH of salt solutions

The pH of a salt solution depends on the strengths of the acid and base from which the salt is derived:

1. Strong acid + strong base - The resulting salt solution is neutral (pH = 7). For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride (NaCl), a neutral salt.

HCl_(aq) + NaOH_(aq) ➔ NaCl_(aq) + H₂O_(l)

1. Weak acid + strong base - The resulting salt solution is alkaline (pH > 7). For example, ethanoic acid reacts with sodium hydroxide to form sodium ethanoate (CH₃COONa), an alkaline salt.

CH₃COOH_(aq) + NaOH_(aq) ➔ CH₃COONa_(aq) + H₂O_(l)

1. Strong acid + weak base - The resulting salt solution is acidic (pH < 7). For example, hydrochloric acid reacts with ammonia to form ammonium chloride (NH₄Cl), an acidic salt.

HCl_(aq) + NH_3(aq) ➔ NH₄Cl_(aq)

In summary, when either the parent acid or base is weak, the resulting salt solution will not be neutral. The stronger of the two will determine whether the solution is acidic or alkaline.

Enthalpy changes during neutralisation

The standard enthalpy change of neutralisation (ΔH^⦵_neut) refers to the heat energy released when an acid neutralises a base to form one mole of water under standard conditions.

For example, consider the neutralisation of hydrochloric acid with sodium hydroxide:

HCl_(aq) + NaOH_(aq) ➔ NaCl_(aq) + H₂O_(l) ΔH^⦵_neut = $-$57.5 kJ mol^-1

Strong acids and bases:

• The enthalpy change (ΔH^⦵_neut) for the neutralisation of strong acids with strong bases is typically around $-$57.5 kJ mol^-1.
• This is because both strong acids and bases are fully ionised in solution, meaning the reaction primarily involves the combination of H⁺ and OH^- ions to form water: H⁺_(aq) + OH^-_(aq) ➔ H₂O_(l) ΔH = $-$57.5 kJ mol^-1

Weak acids and bases:

• The enthalpy change (ΔH^⦵_neut) for the neutralisation of weak acids or bases is usually less negative than $-$57.5 kJ mol^-1.
• For example, the reaction between ethanoic acid (a weak acid) and sodium hydroxide (a strong base) has a ΔH^⦵_neut of $-$56.1 kJ mol^-1.
• This is because weak acids and bases are only partially ionised in solution, so the resulting salt solutions are often not neutral at the equivalence point.