Turn on thread page Beta

Chemistry QUICK question watch

    • Thread Starter

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


    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.


University open days

  • University of East Anglia
    All Departments Open 13:00-17:00. Find out more about our diverse range of subject areas and career progression in the Arts & Humanities, Social Sciences, Medicine & Health Sciences, and the Sciences. Postgraduate
    Wed, 30 Jan '19
  • Aston University
    Postgraduate Open Day Postgraduate
    Wed, 30 Jan '19
  • Solent University
    Careers in maritime Undergraduate
    Sat, 2 Feb '19
Brexit: Given the chance now, would you vote leave or remain?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.