Energy and spontaneity

The spontaneous reaction takes place in a direction in which the energy of the system is lowered; it is accompanied by a release of energy. The reaction between NaOH and HCl is exothermic ($\Delta_r H^0 = -57$ kJ) and is spontaneous.

On the other hand:

i. Ice melts spontaneously above 0 °C by absorbing heat from the surroundings. It is endothermic.

ii. Likewise, NaCl dissolves spontaneously in water with the absorption of heat from the surroundings:

$\mathrm{NaCl(s)} + \mathrm{aq} \longrightarrow \mathrm{Na^{+}(aq)} + \mathrm{Cl^{-}(aq)}, \quad \Delta H^0 = +3.9\ \mathrm{kJ\,mol^{-1}}$ …