standard enthalpy of formation

Meaning: Standard enthalpy of formation, ΔH°f, is the enthalpy change when one mole of a compound forms from its elements, each in its standard state (the most stable form at 1 bar, usually quoted at 25 °C). An element in its standard state is not being made from anything, so its ΔH°f is zero: O₂(g), C(graphite), Br₂(l), Hg(l). A negative ΔH°f means forming the compound from its elements releases heat.

Equation: ΔH°rxn = ΣnΔH°f(products) − ΣmΔH°f(reactants), where n and m are the stoichiometric coefficients (mol) and ΔH°f is in J/mol (usually listed in kJ/mol). ΔH°rxn is in J (or kJ) for the reaction as written.

On the MCAT: A passage gives a table of ΔH°f values and asks for ΔH°rxn of a combustion or another reaction, or asks which reaction is more exothermic. Reversing a reaction flips the sign of ΔH, and doubling the coefficients doubles it. This is Hess's law in practice.

Common confusion: Students subtract in the wrong order (it is products minus reactants) or forget to multiply by the coefficients. Another slip is giving a nonzero value to an element in its standard state. Only the most stable form is zero, so C(graphite) and Br₂(l) are zero, but C(diamond) and Br₂(g) are not.