Kirchhoff's equation relates the enthalpy of a reaction (heat of reaction at constant pressure) to temperature. It states that the variation of the enthalpy change of a chemical reaction with temperature is equal to the difference in heat capacities between the products and the reactants. Mathematically, it is expressed as $\Delta H_{2} - \Delta H_{1} = \Delta C_{p}(T_{2} - T_{1})$, where $\Delta C_{p}$ is the change in heat capacity at constant pressure. Therefore, temperature is the primary factor affecting the heat of reaction in this context.