Enthalpy, Entropy, and the Laws of Thermodynamics

In thermodynamics, the total energy of a system is given by the sum of the total kinetic and potential energies of the molecules in the system. 

The first law of thermodynamics is the application of the conservation of energy principle. In geochemistry, the first law considers that the change in internal energy (dU) is equal to the heat added to the system (dq) plus the work (dw) done on the system  

Berkowitz et at. Contaminant Geochemistry Transport and Fate in the Subsurface Environment. 

The first law of thermodynamics applied to an adiabatic system may be expressed as the work done on a system by an adiabatic process, which is equal to the increase in its internal energy, and a function of the state of the system. 

In any natural system, the system boundary is prescribed as a nonadiabatic (insulated) wall that allows the passage of heat to and from the surrounding system. The change in internal energy is not equal to the heat supplied when the system is open and free to change its volume. Under this condition, part of the energy supplied is returned to the surroundings and the heat supplied at constant pressure is equal to the enthalpy (H) of the system, which is defined as 

---