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    why would a weaker acid give a less exothermic enthalpy of neutralisation per mole of water, than a stronger acid

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    Perhaps it will make sense to ask what's the difference between a weak and a strong acid in the first place: weak acids only dissociate slightly in water, while strong acids are virtually fully dissociated.

    When you neutralise an acid, you're trying to "grab" a H+ from it to form H2O. In the strong acid scenario, the H+ is already removed and "free" to form H2O. In the weak acid scenario, you need to break bonds between the H and the weak acid before you could take the H to form H2O.

    One more piece of information is that breaking bonds is endothermic, and making bonds is exothermic.

    Assuming you've got the same number of moles of protons in the weak and strong acid solution, the number of moles of H2O you form will be the same, so the energy given out from making H2O is the same.

    On the other hand, you used up next to no energy to break the H+ from the strong acid, while you used some energy to break the H+ from the weak acid. In both cases, the energy given out from forming H2O is greater than that used up in breaking the H+ from the acid. Therefore, overall the neutralisation reaction is exothermic (forming bonds outweigh breaking bonds). However, the weak acid neutralisation is less exothermic because some of the energy given out has first been "invested" in breaking the H+ from the weak acid molecule.

    selkie
 
 
 

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