The heat of neutralisation of an acid by a base is highest when:

The absolute enthalpy of neutralisation of the reaction $MgO_{(s)} + 2HCl_{(aq)} \to MgCl_{2(aq)} + H_2O_{(l)}$ will be

On the basis of the following thermochemical data: $(\Delta_fG^o H^{+}_{(aq)} = 0)$
$H_2O_{(\ell)} \rightarrow H^{+}_{(aq)} + OH^{-}_{(aq)} \,; \, \Delta H = 57.32 \, kJ$
$H_{2(g)} + \frac{1}{2} O_{2(g)} \rightarrow H_2O_{(\ell)} \,; \, \Delta H = -286.20 \, kJ$
The value of enthalpy of formation of $OH^{-}$ ion at $25 \, ^oC$ is : .............. $kJ$

The reaction for the formation of $NaCl$ is:

State Hess's Law of Constant Heat Summation.

Explain the standard enthalpy of formation.