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Enthalpy of Neutralisation
Concepts tested here
- Neutralisation involving a weak base
All Questions
2005 AIPMT 1 question
-
The absolute enthalpy of neutralisation of the reaction :
$MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$
will be : -The enthalpy of neutralisation of a strong acid by a strong base is constant, $-57.33$ kJ per mole of water formed, because the net reaction is always $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$.
Here the base is MgO, an insoluble solid, which is not present as free $OH^-$ ions in solution.
Part of the energy released is used up in breaking the lattice of MgO (dissociating it to give the base), i.e. in a process that absorbs energy.
So less heat is evolved than in the neutralisation of a strong acid by a strong base.
The enthalpy of neutralisation is therefore less negative than $-57.33$ kJ mol$^{-1}$, i.e. it is greater than $-57.33$ kJ mol$^{-1}$ (for example $-50$ kJ mol$^{-1}$ is greater than $-57.33$).