This means that the change in enthalpy under such conditions is the heat absorbed (or released) by a chemical reaction. Δ H of a system is equal to the sum of non-mechanical work done on it and the heat supplied to it.įor quasistatic processes under constant pressure, Δ H is equal to the change in the internal energy of the system, plus the work that the system has done on its surroundings.

The change Δ H is positive in endothermic reactions, and negative in heat-releasing exothermic processes.

Thus, change in enthalpy, Δ H, is a more useful quantity than its absolute value. The total enthalpy, H, of a system cannot be measured directly. This is because a change in enthalpy takes account of energy transferred to the environment through the expansion of the system under study. The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements, because it simplifies certain descriptions of energy transfer. The unit of measurement for enthalpy in the International System of Units (SI) is the joule, but other historical, conventional units are still in use, such as the British thermal unit and the calorie. It is a state function and an extensive quantity. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Įnthalpy is a thermodynamic potential. Enthalpy is a measure of the total energy of a thermodynamic system.

Enthalpy vs entropy definition