Answer
$\Delta$H° (reaction) = -118 kJ/mol (-28 kcal/mol)
Work Step by Step
The overall reaction enthalpy can be calculated by summing the energy required to break the CH3CH2-H and Cl-Cl bonds (+652 kJ/mol or +156 kcal/mol) with the energy released from the CH3CH2-Cl and H-Cl bonds formation (-770 kJ/mol or -184 kcal/mol). The resulting value is -118 kJ/mol (-28 kcal/mol).