heat of formations

Meaning: The heat of formation (standard enthalpy of formation, ΔH°f) is the enthalpy change when 1 mole of a compound forms from its elements, each in its standard state (the most stable form at 1 bar, usually 25 °C). Enthalpy is a state function, so tabulated values let you treat any reaction as reactants breaking down to elements, then elements building products. An element in its standard state, such as O₂(g), C(graphite) or Br₂(l), has ΔH°f = 0 by definition.

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

On the MCAT: A passage or question gives a table of ΔH°f values and asks for ΔH°rxn, often for a combustion. Multiply each value by its coefficient, subtract as shown, then read the sign: negative is exothermic, positive is endothermic. Reversing a reaction flips the sign of ΔH. Questions may also compare the relative stability of compounds.

Common confusion: Students subtract in the wrong order or skip the coefficients. With heats of formation, it is products minus reactants. With bond enthalpies the order is reversed (bonds broken minus bonds formed). Also, only the standard-state form of an element is zero: O₃(g) and Br₂(g) are not.