Which of the following reaction involves the liberation of energy?

 $$\begin{gathered} {\mathrm{Na}}_{\left(\mathrm{s}\right)}\to {\mathrm{Na}}_{\left(\mathrm{g}\right)}^{+} ; ∆\mathrm{H}=\mathrm{I}.\mathrm{P}=+\mathrm{ve} \\ {{\mathrm{Cl}}_2}_{\left(\mathrm{g}\right)} \to 2{\mathrm{Cl}}_{\left(\mathrm{g}\right)} ; ∆\mathrm{H}=\mathrm{B}.\mathrm{E}=+\mathrm{ve} \\ {{\mathrm{Na}}^{+}}_{\left(\mathrm{g}\right)}+{{\mathrm{Cl}}^{-}}_{\left(\mathrm{g}\right)}\to {\mathrm{NaCl}}_{\left(\mathrm{s}\right)} ; ∆\mathrm{H}=-\mathrm{ve} \\ {\mathrm{NaCl}}_{\left(\mathrm{s}\right)}\to {{\mathrm{Na}}^{+}}_{\left(\mathrm{g}\right)}+{{\mathrm{Cl}}^{-}}_{\left(\mathrm{g}\right)} ; ∆\mathrm{H}=+\mathrm{ve} \end{gathered}$$ 

$\mathrm{If} ∆\mathrm{H}=-\mathrm{ve} \mathrm{then} \mathrm{heat} \mathrm{is} \mathrm{liberated}.$