H₂ + Cl₂ $\stackrel{ }{\to }$ 2HCl + 44 K.Cal. Heat of decomposition of HCl is

Heat of decomposition = $\frac{{∆}_{\mathrm{r}}\mathrm{H}}{\mathrm{number} \mathrm{of} \mathrm{moles} \mathrm{formed}}$

= $\frac{44}{2}$=22 K.cal

Heat of decomposition and heat of formation of a compound is numerically same but differ in sign convention