I read somewhere that for example the neutralisation reaction between sodium hydroxide and acetic acid is less exothermic than those of sodium hydroxide with hydrochloric and nitric acid because some energy is needed to cause the weak acid (acetic acid) to completely ionise. Is this true?

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    $\begingroup$ Think carefully. What is the main reason why a neutralization reaction is exothermic? $\endgroup$ – Pritt says Reinstate Monica Jun 5 '17 at 9:02
  • $\begingroup$ Correct me if I'm wrong please, but I'd say that most neutralisations simply involve the reaction between a hydronium ion and a hydroxide ion to form two water molecules which is an exothermic process. But, I still don't see what the difference is between the neutralisation of HCl with NaOH than that of CH3COOH with NaOH. $\endgroup$ – shA3245699 Jun 5 '17 at 9:46
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    $\begingroup$ You're on the right track. Acetic acid is a weak acid. It furnishes lesser $\ce{H3O+}$ in solution compared to a typical strong acid. So, less hydronium ions, less combination of hydronium and hydroxide, and less energy released. $\endgroup$ – Pritt says Reinstate Monica Jun 5 '17 at 9:48
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    $\begingroup$ I still don't understand why the fact that a weak acid does not ionise completely is an explanation as to why it is neutralised less exothermically. If we have equimolar solutions of HCl and CH3COOH both of which are monoprotic, won't we still need an equal number of moles of NaOH to neutralise both? $\endgroup$ – shA3245699 Jun 5 '17 at 10:55

I think the answer is closer to what was stated in the original question: "some energy is needed to cause the weak acid (acetic acid) to completely ionise". Stated differently, less energy is released from making acetate ion from acetic acid than from making chloride ion from hydrochloric acid (or water from hydronium ion).

Despite the lesser tendency of acetic acid to ionize, the overall stoichiometry of the two reactions is the same--as pointed out in a comment by the OP.

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