enthalpy (ĕnˈthălpē) [key], measure of the heat content of a chemical or physical system; it is a quantity derived from the heat and work relations studied in thermodynamics. As a system changes from one state to another the enthalpy change, Δ H, is equal to the enthalpy of the products minus the enthalpy of the reactants. If heat is given off during a transformation from one state to another, then the final state will have a lower heat content than the initial state, the enthalpy change Δ H will be negative, and the process is said to be exothermic. If heat is absorbed during the transformation, then the final state will have a higher heat content, Δ H will be positive, and the process is said to be endothermic. The enthalpy change accompanying a chemical reaction is called the heat of the reaction. For a reaction in which a compound is formed from its composite elements, the enthalpy increase or decrease is called the heat of formation of the compound. Changes of state, or phase, of matter are also accompanied by enthalpy changes; the change associated with the solid-liquid transition is called the heat of fusion and the change associated with the liquid-gas transition is called the heat of vaporization (see latent heat). The enthalpy change for a given reaction often may be used to tell how favorable the reaction is; an exothermic reaction involves a loss of heat and a consequent lower final energy and thus tends to be favorable, while an endothermic reaction tends to be unfavorable because it involves an increase in energy. However, there are other factors, such as entropy changes, which must also be taken into account in determining whether or not a given process can occur.