Why does enthalpy change differ in a chemical reaction because of temperature?
From Hess' law, the heat of reaction at temperatures $T_1$ and $T_2$ are related by $$\Delta H_{T_1}=C_{p,\mathrm r}(T_2-T_1)+\Delta H_{T_2}+C_{p,\mathrm p}(T_1-T_2),$$where $C_{p,\mathrm r}$ is the heat capacity of a stoichiometric molar mixture of reactants and $C_{p,\mathrm p}$ is the heat capacity of a stoichiometric molar mixture of products.