$\begin{aligned} & \Delta H_{R E E}=\Delta H_{f(\exp )}-\Delta H_f(T h e o) \\ & \Delta H_{f(\exp )} \text { for } X_2 Y_{(g)}=80 \mathrm{~kJ} / \mathrm{mole} \\ & \text { for } \Delta H_f(T h e o) \\ & X_{2(\mathrm{~g})}+\frac{1}{2} Y_{2(\mathrm{~g})} \rightarrow X_2 Y_{(\mathrm{g})} \Delta H_f=? \\ & \Delta H_{f(T h e o)}=\left(B E_{X=X}+\frac{1}{2} B E_{Y=Y}\right)-\left(B E_{X=X}+B E_{X=Y}\right) \\ & =\left(940+\frac{1}{2} \times 500\right)-(410+602) \\ & =178 \mathrm{~kJ} / \mathrm{mole} \\ & \Delta H_{R . E}=80-178 \\ & \quad=-98 \mathrm{~kJ} / \mathrm{mol} \\ & \left|\Delta H_{R . E}\right|=98\end{aligned}$