The first law of thermodynamics postulates that the variation of  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork Δu of a closed system interacting with its environment and undergoing an evolution, depends only on the initial state and the final state , which means that u is a  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork.

The first law indicates that if the internal energy u of the system varies, there is an exchange of energy with the outside in the form of  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork W and / or  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork Q , and that the sum of W and Q is equal to the internal energy variation of the system.

The first law can thus be written for a  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork system in the form: Δu = W + Q

By convention, the energy  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork by a system is counted positively (its internal energy increases), and that given to the outside is counted negatively.

By revealing a quantity h called  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork such as Δh = Δu + Δ(Pv), the first law in an  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork system is written: Δh = τ + Q.

h is also a state function.

The enthalpy thus appears simply as the  closedenthalpygeneralizationheatinternal energyopenreceivedstate functionwork to open systems of internal energy for closed systems .