The enthalpy of formation is defined as the energy, in the form of heat, given off or absorbed during a chemical reaction under the most stable conditions for the reaction to occur from pure elements. The basic formula for the enthalpy of formation is the sum of the enthalpy of the reactants subtracted from the sum of the enthalpy of the products. Standard enthalpies of formation occur under standard conditions.
The enthalpy of formation is measured in joules or kilojoules per mole. A joule is a unit of energy, and a mole measures the amount of atomic particles in one substance. Enthalpies with negative numbers are exothermic reactions, meaning heat is given off into the surrounding environment. Enthalpies with positive numbers are endothermic, meaning heat is absorbed by the reactants from the surrounding environment. Exothermic reactions feel warm to the touch, but endothermic reactions feel cool.
The enthalpy of formation of pure elements is zero. These pure elements then contribute enthalpies to future reactions with other elements and molecules. The enthalpy of formation for water is -286 kilojoules per mole, which means 286 kilojoules of energy are given off for every mole of water formed from hydrogen and oxygen. One mole of water is slightly more than 18 milliliters or 18 grams. Enthalpies of formation are determined through laboratory experiments.