Internal energy and enthalpy are thermodynamic properties used to describe the state and behavior of a system, such as a gas or a fluid. They are essential concepts in the field of thermodynamics and have significant practical applications in various scientific and engineering disciplines. Let’s explore their definitions, provide examples, and discuss their significance:

Internal Energy (U):

  1. Definition: Internal energy is the total energy contained within a system due to the microscopic motion and interactions of its particles (atoms and molecules). It includes both kinetic energy (energy of motion) and potential energy (energy due to forces between particles).
  2. Examples:
    • In a gas, the internal energy is associated with the random motion of gas molecules.
    • In a solid, it is associated with the vibrational and rotational motion of atoms.
    • In a liquid, it includes the kinetic and potential energies of the molecules.
  3. Significance: Internal energy is crucial for understanding energy transfer processes and the behavior of a system. It plays a key role in:
    • Determining temperature changes during heating or cooling processes (ΔU = q – w, where q is heat and w is work).
    • Analyzing phase transitions, such as melting or boiling, where internal energy remains constant.
    • Calculating changes in internal energy in chemical reactions (ΔU) and determining if a reaction is exothermic or endothermic.
    • Studying the relationship between temperature and pressure in ideal gases (ideal gas law).

Enthalpy (H):

  1. Definition: Enthalpy is a thermodynamic property defined as the sum of a system’s internal energy (U) and the product of its pressure (P) and volume (V). Mathematically, H = U + PV.
  2. Examples:
    • Enthalpy is often used in the study of chemical reactions, particularly in the context of chemical thermodynamics. For example, the change in enthalpy (ΔH) of a chemical reaction represents the heat released or absorbed during the reaction.
  3. Significance: Enthalpy is significant in various ways:
    • It simplifies energy calculations in processes at constant pressure, where ΔH directly represents the heat exchange between a system and its surroundings.
    • In chemistry, the enthalpy change of a reaction (ΔH) helps determine whether a reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0) and quantifies the heat associated with the reaction.
    • Enthalpy is used in engineering applications, such as HVAC (heating, ventilation, and air conditioning) systems, to calculate energy requirements and efficiency.




In summary, internal energy and enthalpy are fundamental thermodynamic properties used to understand and quantify energy changes within a system. Internal energy describes the total energy content of a system, while enthalpy incorporates pressure-volume work into the energy equation, making it particularly useful for processes at constant pressure and in chemical reactions. These concepts are vital for the study of thermodynamics and have wide-ranging applications in science and engineering.

The relation between internal energy (U) and enthalpy (H) can be derived from the definition of enthalpy, which is the sum of internal energy and the product of pressure (P) and volume (V). Mathematically, H = U + PV.

Now, to derive the relationship between changes in internal energy and enthalpy (ΔU and ΔH) for a system, we can start with the definition of enthalpy and use the first law of thermodynamics, which states:

ΔU = q – w


  • ΔU is the change in internal energy of the system.
  • q is the heat added to the system.
  • w is the work done by the system on its surroundings.

Now, let’s consider a system undergoing a process at constant pressure (P). In this case, the work done by the system can be expressed as:

w = -PΔV


  • ΔV is the change in volume of the system.

Now, let’s substitute this expression for work into the first law of thermodynamics:

ΔU = q + PΔV

Now, we’ll use the definition of enthalpy (H = U + PV):

H = U + PV

Next, let’s consider the change in enthalpy (ΔH) for the same process:

ΔH = H_final – H_initial

Substitute the expression for H:

ΔH = (U_final + PV_final) – (U_initial + PV_initial)

Now, let’s subtract U_initial from both sides of the equation:

ΔH – ΔU = (U_final + PV_final) – U_initial – PV_initial

Now, we can see that U_final – U_initial is simply ΔU, and we can factor out P as it’s constant in this process:

ΔH – ΔU = ΔU + P(V_final – V_initial)

Now, we can simplify this equation:

ΔH – ΔU = ΔU + PΔV

Finally, let’s move ΔU to the left side of the equation:

ΔH = ΔU + ΔU + PΔV

ΔH = 2ΔU + PΔV

So, the relationship between the change in enthalpy (ΔH) and the change in internal energy (ΔU) for a system undergoing a process at constant pressure is:

ΔH = 2ΔU + PΔV

This equation is particularly useful for analyzing processes in which pressure is held constant, such as many chemical reactions and phase changes in open systems. It relates the change in enthalpy to the change in internal energy and the work done by or on the system.