Chemical energy is released or absorbed from reactions in various forms. the most easily measurable form of energy comes in the form of heat, or enthalpy. the enthalpy of a reaction can be calculated from the heats of formation of the substances involved in the reaction: δh∘rxn=σ npδh∘f(products)−σ nrδh∘f(reactants) where n represents the stoichiometric coefficients.