Answer
-137.1 kJ
Work Step by Step
$\Delta H^{o}_{rxn}=\Sigma n_{p}\Delta H_{f}^{o}(products)-\Sigma n_{r}\Delta H_{f}^{o}(reactants)$
$=1(\Delta H^{o}_{f,\,C_{2}H_{6}(g)})-[1(\Delta H^{o}_{f,\,C_{2}H_{4}(g)})+1(\Delta H^{o}_{f,\,O_{2}(g)})]$
$=1(-84.68\,kJ)-[1(52.4\,kJ)+1(0)]=-137.1\,kJ$
