Enthalpy
Definition
The enthalpy of a system is equal to the internal energy of a system plus the product of its pressure and volume. For processes at constant pressure, the heat absorbed or released equals the change in enthalpy. The specific enthalpy is the enthalpy per unit mass. It can be related to the temperature and pressure through

(1) 
Where the term fraction is part of the isobaric coefficient of thermal expansion. The above form is derived using differential relations such as the Triple Product Rule and Maxwell's relations to arrive at an expression that relates enthalpy in terms of pressure and temperature.
If the fluid is incompressible, and

(2) 
The enthalpy is greater than the total internal energy of the system by pressures times’ volume of the system. However, it absolute value of enthalpy could not calculated, but the changes in enthalpy can be calculated. The enthalpy change is the sum of change in internal energy of the system and the pressure time’s change in volume of the system.

(3) 
In the above equation, the pressure is kept constant for simplicity. Here, the change in enthalpy is given by symbol ∆H, the change in total internal of the system is given by symbol ∆E, the change in volume of the system is given by the symbol ∆V, and the pressure is given by the sumbol P ^{[1]}.
 ↑ [Enthalpy . (n.d.). Retrieved April 24, 2019, from Enthalpy Change: http://www.westfield.ma.edu/cmasi/gen_chem1/Energy/enthalpy_definition_folder/enthalpy_definition.htm