A-Level Enthalpy, entropy and the feasibility of reactions: worked solution

Question

In a further neutralisation, HCl at the same concentration as that used in (a)(iii) was fully neutralised by 0.400 mol dm⁻³ NaOH. The peak temperature recorded was higher than that observed when NaC₆H₇O₆ was neutralised with HCl. Account for this difference in the maximum temperature reached.

HCl + NaOH is a strong acid + strong base reaction: both are fully dissociated, so the reaction is essentially H+(aq) + OH-(aq) -> H2O(l), which releases the full (maximum) enthalpy of neutralisation, giving a larger temperature rise. Reacting HCl with NaC6H7O6 (the salt of a weak acid) instead forms molecular ascorbic acid: H+ + C6H7O6- -> C6H8O6. Because ascorbic acid is a weak acid, this is much less exothermic (energy is effectively taken up in keeping the acid mostly un-ionised / re-forming un-ionised acid rather than forming water), so less heat is released and the peak temperature is lower.