Answer
$-106.5\,kJ/mol$
Work Step by Step
We find:
$\Delta H^{\circ}_{rxn}=\Sigma n_{p}\Delta H_{f}^{\circ}(products)-\Sigma n_{r}\Delta H_{f}^{\circ}(reactants)$
$=[\Delta H_{f}^{\circ}(O_{3},g)]-[\Delta H_{f}^{\circ}(O_{2},g)+\Delta H_{f}^{\circ}(O,g)]$
$=142.7\,kJ/mol-[0\,kJ/mol+249.2\,kJ/mol]$
$=-106.5\,kJ/mol$
