Short Answer:

Enthalpy and internal energy are two important energy terms used in thermodynamics, especially for gases. Internal energy (U) is the total energy stored inside the gas due to the motion and interactions of its molecules. It includes only the energy inside the system. On the other hand, enthalpy (H) is the sum of internal energy and the flow energy (P × V). It represents the total energy content including the energy needed to push the gas out of the system.

In simple words, internal energy is related to the heat inside the gas, while enthalpy is used when the gas is flowing or doing pressure-volume work, like in turbines or compressors. Enthalpy is more useful in open systems, and internal energy is used mainly in closed systems.

Detailed Explanation:

Difference between enthalpy and internal energy for gases

In thermodynamics, energy exists in different forms, and two of the most commonly used forms for gases are internal energy (U) and enthalpy (H). Both help us understand how gases store and transfer energy, but they are used in different situations based on whether the system is open or closed and whether it involves volume change or flow.

Internal Energy (U)

Internal energy is the total microscopic energy of all the gas molecules. It includes:

• Kinetic energy (due to motion of molecules)
• Potential energy (due to molecular forces)

It does not include any energy related to external work or pressure.

Key points:

• Denoted by U
• Units: kJ or kJ/kg
• Property of the system only
• Changes with temperature
• Used in closed systems where no mass enters or leaves

When we heat a gas in a sealed container, its molecules move faster, and its internal energy increases. No gas leaves the container, so we only consider internal energy.

Enthalpy (H)

Enthalpy is the total energy content of the gas, which includes:

• Internal energy (U)
• Flow energy or pressure-volume work (P × V)

Mathematically:
H = U + P × V

Enthalpy is especially useful when the gas is flowing in or out of a system, like in turbines, boilers, compressors, and heat exchangers.

Key points:

• Denoted by H
• Units: kJ or kJ/kg
• Useful for open systems
• Takes into account the energy required to move the gas
• Changes with both temperature and pressure

Main Differences

1. Definition:
   • Internal energy is the energy within the gas molecules.
   • Enthalpy includes internal energy plus the work done by the gas to push against surroundings.
2. Formula:
   • U = Internal energy
   • H = U + P × V
3. Application:
   • Internal energy is used in closed systems (no mass flow).
   • Enthalpy is used in open systems (mass enters or leaves).
4. Dependence:
   • U depends mainly on temperature.
   • H depends on temperature and pressure.
5. System type:
   • U is easier for non-flow processes (like heating in a cylinder).
   • H is better for flow processes (like steam through a turbine).

Example for Better Understanding

Let’s say steam flows through a boiler. Inside the boiler:

• Steam gains energy (internal energy increases).
• It also does expansion work as it flows out (this is flow energy).

So, the energy we measure at the outlet is not just internal energy—it is enthalpy. That’s why steam tables usually list enthalpy values for different pressure and temperature.

In contrast, if you heat gas in a closed rigid tank, only the internal energy changes, because volume stays constant and no mass flows.

Conclusion

The difference between enthalpy and internal energy for gases is based on the type of system and whether flow work is involved. Internal energy is the energy stored inside the gas, while enthalpy includes that energy plus the energy needed for the gas to flow or expand. Internal energy is important in closed systems with no flow, and enthalpy is used in open systems where gases enter and leave. Both are essential for analyzing thermodynamic processes involving gases.