Answer
$$\Delta H^o_{rxn} = -1.0 \ kJ/mol$$
Work Step by Step
$$\Delta H^o_{rxn} = \Delta H^o_f [Products] - \Delta H^o_f[Reactants]$$
$$\Delta H^o_{rxn} = (-4551.1 \ kJ/mol) -(-4550.1 \ kJ/mol) = -1.0 \ kJ/mol$$
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