A-Level pH, dissociation constants and buffers: worked solution

Question

(b) Assume for this part that the acid is monobasic and can be written HA. A student wants to find out whether HA is strong or weak, and plans a set of experiments in which hydrochloric acid is neutralised by sodium hydroxide and, separately, HA is neutralised by sodium hydroxide. Explain how comparing the results of these experiments could reveal whether HA is a strong or a weak acid.

Use equal volumes and equal concentrations of the two acids, each neutralised by the same NaOH, and measure the enthalpy of neutralisation (the maximum temperature rise). For a strong acid, fully ionised, the reaction is essentially H+(aq) + OH-(aq) -> H2O and gives a fixed enthalpy of neutralisation of about -57 kJ mol-1. A weak acid is only partly ionised, so some energy is absorbed to ionise the remaining HA molecules, giving a less exothermic neutralisation and a smaller temperature rise. So: if HA produces the same temperature rise as HCl, HA is strong; if HA gives a noticeably smaller temperature rise, HA is weak.