How is a standard reaction enthalpy calculated from standard enthalpies of formation?

1. Picture the reaction as decomposing the reactants into elements, then building the products. 2. Decomposing reactants contributes the negatives of their formation enthalpies. 3. Building products contributes their formation enthalpies directly. 4. Each term is first multiplied by the stoichiometric coefficient of that species. 5. So the reaction enthalpy is the product sum minus the reactant sum. 6. Reversing the order of subtraction flips the sign of the whole answer. _Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_