Difference between enthalpy and internal energy

Enthalpy and internal energy are both used in thermodynamics to describe the energy of a system. Enthalpy is a measure of the total energy of a system, whereas internal energy is a measure of the energy within a system. Enthalpy includes both the internal energy of the system, as well as the work done by the system on its surroundings. Internal energy is the sum of the kinetic and potential energies of the molecules in a system. To learn more about both terms, let us dive deep into the difference between enthalpy and internal energy. 

Difference between enthalpy and internal energy


Enthalpy is the heat energy that is absorbed or evolved during the progression of a chemical reaction. 

The internal energy is the sum of that system's potential and kinetic energy. 


Enthalpy is represented by H and internal energy is represented by U. 


Enthalpy is calculated by H = U + PV

Internal energy is calculated by U = q - w


Enthalpy is defined as the relationship between the system and its surrounding. Thus, enthalpy is used for open systems. 

Internal energy is defined as the total energy in a system. Thus, the internal energy is used for closed systems. 


The endothermic process has positive H and the exothermic process has negative H. 

Internal energy increases when the system absorbs energy and decreases when the system releases energy to the surrounding. 


The unit for enthalpy is Joule/mole whereas the unit for internal energy is Joule. 


Enthalpy is a function of temperature and pressure and internal energy is a function of temperature only.

Ideal gas:

The Enthalpy of an ideal gas is 40% more than internal energy. 

The internal energy of the ideal gas is 40% less than enthalpy. 


Enthalpy can be applied to refrigerators for freezing and boiler for boiling. 

A glass of water has no apparent energy, but it does have internal energy. 



Enthalpy is associated with systems in contact with their surroundings, whereas internal energy is the overall energy of a specific system. However, changes in enthalpy and internal energy are critical in understanding the kind and nature of chemical processes in a system. As a result, it is essential to grasp the distinction between enthalpy and internal energy.

Explore more information: 

  1. Enthalpy Vs Entropy