Internal Energy and Reversible Process

Internal energy is the sum of the kinetic and potential energies of the particles that make up a system. Reversible processes are those that can be reversed without any change in the system or the surroundings. In a reversible process, the system undergoes a series of small changes that can be reversed by an infinitesimal change in the external conditions.

Change in Internal Energy

The change in internal energy of a system is the difference between the initial and final internal energies. If the process is reversible, the change in internal energy is equal to the heat transferred to the system. Mathematically,

ΔU = Q

where ΔU is the change in internal energy, and Q is the heat transferred to the system. This equation is valid for a closed system undergoing a reversible process.

Constant Volume and Pressure Processes

For a closed system undergoing a reversible process at constant volume, the heat transferred is equal to the change in internal energy. Mathematically,

Q = ΔU

This is because there is no work done during the process, and all the heat transferred increases the internal energy of the system.

For a closed system undergoing a reversible process at constant pressure, the heat transferred is equal to the change in enthalpy. Mathematically,

Q = ΔH

This is because the work done during the process is equal to the change in enthalpy, and all the heat transferred increases the enthalpy of the system.

Conclusion

In a reversible process occurring in a closed system, the change in internal energy is equal to the heat transferred only if the process occurs at constant volume. If the process occurs at constant pressure, the heat transferred is equal to the change in enthalpy.