Final answer:
The standard enthalpy of formation (ΔfH°) is the heat change when one mole of a substance is formed from its elements in their most stable states under standard conditions, with the value being zero for elements in their most stable forms.
Step-by-step explanation:
The standard enthalpy of formation (ΔfH°) is an enthalpy change that occurs when exactly 1 mole of a pure substance is formed from its elements in their most stable states under standard state conditions. These conditions are typically a pressure of 1 atm for gases and a concentration of 1 M for species in solution, with pure substances being in their standard states at 1 atm pressure and the reference temperature (usually 25°C or 298 K). The standard enthalpy of formation of any element in its most stable form is zero.
Using the standard enthalpy of formation values, we can calculate the standard enthalpy of reaction (ΔH°rxn) for any chemical reaction. This is done through the "products minus reactants" rule, where we sum the standard enthalpies of formation of the products, each multiplied by its stoichiometric coefficient, and subtract the sum of the standard enthalpies of formation of the reactants, also each multiplied by their stoichiometric coefficients.