How is the standard reaction enthalpy estimated using enthalpies of formation?

The standard reaction enthalpy, denoted as Δ_rH, can be estimated using the enthalpies of formation (Δ_fH) of the reactants and products involved in a chemical reaction. The correct formulation states that the change in enthalpy for the reaction is equal to the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants.

This is based on Hess's law, which asserts that the total enthalpy change for a given reaction is the same regardless of the steps taken to carry out the reaction. Therefore, by using the enthalpy of formation, which represents the change in enthalpy for the formation of one mole of a compound from its elements at standard conditions, the overall reaction enthalpy can be calculated.

In the formulation, the summation of the products represents the total energy associated with forming the products from their reference states, while the summation of the reactants accounts for the energy needed to break them down from their standard states. By subtracting these two quantities, one can effectively find out how much energy is either absorbed or released during the reaction process, providing a clear insight into its thermodynamic favorability.