A reaction whose heat of reaction shows the bond energy of \(\mathrm{HCl}\) is (a) \(\mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{H}(\mathrm{g})+\mathrm{Cl}(\mathrm{g})\) (b) \(2 \mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{Hl}(\mathrm{g})+\mathrm{Cll}(\mathrm{g})\) (c) \(\mathrm{HCl}(\mathrm{g}) \longrightarrow \frac{1}{2} \mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{Cl}_{2}(\mathrm{~g})\) (d) \(\mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{H}^{+}(\mathrm{g}) \longrightarrow \mathrm{Cl}^{-}(\mathrm{g})\)

Bond energy refers to the amount of energy required to break a bond between two covalently bonded atoms. In this case, we want an equation that shows breaking the bond in HCl.

We examine each of the given reactions individually:(a) \( \mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{H}(\mathrm{g})+\mathrm{Cl}(\mathrm{g}) \) - In this reaction, the bond between hydrogen and chlorine is broken to form gaseous hydrogen and chlorine. This reaction shows breaking the bond in HCl.(b) \( 2 \mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{Hl}(\mathrm{g})+\mathrm{Cll}(\mathrm{g}) \) - This reaction does not make chemical sense. Since there are no elements named Hl and Cll, we can disregard this option.(c) \( \mathrm{HCl}(\mathrm{g}) \longrightarrow \frac{1}{2} \mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{Cl}_{2}(\mathrm{~g}) \) - In this reaction, HCl is broken down into half a molecule each of hydrogen and chlorine gas. Physically, there is no such thing as half a molecule, so this reaction is not feasible.(d) \( \mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{H}^{+}(\mathrm{g})\longrightarrow \mathrm{Cl}^{-}(\mathrm{g}) \) - This reaction seems to break HCl into a hydrogen and a chloride ion. However, ions do not exist freely in gaseous state. So this option is incorrect.

Based on the analysis, the first reaction is the correct answer. This reaction represents the breaking of the bond in HCl, which gives the bond energy of HCl. The rest of the reactions are either chemically incorrect or physically impossible.