Looking for classes? Ksquare Career Institute, Bengaluru →
The absolute enthalpy of neutralisation of the reaction :
$MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$
will be : -
$MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$
will be : -
A
57.33 kJ mol$^{-1}$
B
$-57.33$ kJ mol$^{-1}$
C
Greater than $-57.33$ kJ mol$^{-1}$
D
Less than $-57.33$ kJ mol$^{-1}$
Detailed Solution
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 $-57.33$).
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 $-57.33$).