enthalpy (symbol H) In thermodynamics, amount of heat energy possessed by a substance. The enthalpy of a system equals the sum of its internal energy and the product of pressure and volume. In an endothermic reaction, there is an increase in enthalpy. The reverse occurs in an exothermic reaction.

enthalpy Symbol H. A thermodynamic property of a system defined by H = U + pV, where H is the enthalpy, U is the internal energy of the system, p its pressure, and V its volume. In a chemical reaction carried out in the atmosphere the pressure remains constant and the enthalpy of reaction, ΔH, is equal to ΔU + pΔV. For an exothermic reaction ΔH is taken to be negative.

enthalpy (H) Heat content per unit mass of a substance measured as the internal energy plus the product of its volume and pressure.