Heat of neutralisation

The heat of neutralisation is the heat released when one mole of water is formed from the neutralisation of an acid by an alkali. Malay: haba peneutralan · Chinese: 中和热.

Neutralisation is exothermic, so the value is always negative, and it is defined per mole of water because water is the substance common to every acid-alkali reaction. The reaction that actually gives out the heat is the combination of hydrogen ions and hydroxide ions to form water, H⁺ + OH⁻ → H₂O, whatever the particular acid and alkali were.

Example. A strong acid such as hydrochloric acid with a strong alkali such as sodium hydroxide gives a higher heat of neutralisation than a weak acid such as ethanoic acid with the same alkali. Both are fully ionised in the strong-strong case, so all the ions are ready to combine; but a weak acid is only partly ionised, and some of the heat released must be spent ionising the rest of it, which lowers the measured value.

Confusion to avoid. Do not define the heat of neutralisation per mole of acid or per mole of alkali. It is defined per mole of water formed. This matters when a diprotic acid such as sulfuric acid reacts: one mole of the acid produces two moles of water, so the heat released is for two moles, not one.

Because it always comes back to one mole of water, the heat of neutralisation is the clearest of the four heats to set up with Q = mcθ, and SPM Chemistry often uses it to test whether you can explain the strong-versus-weak difference. In the experiment you add the acid to the alkali in a polystyrene cup, take the total volume as the mass of the surroundings, and read the highest temperature reached to find the temperature change. When you explain the lower value for a weak acid, say plainly that some of the released heat is used to ionise the acid molecules that are not yet ionised, because that sentence is what earns the explanation mark in the scheme.