Explain the concept of internal energy.

Internal energy is the total energy stored in the particles of a system, including kinetic and potential energy.

Internal energy, often symbolised as U, is a fundamental concept in thermodynamics. It is the total energy contained within a system and is the sum of the kinetic and potential energies of the particles that make up the system. The kinetic energy is due to the motion of the particles, while the potential energy is due to the forces between the particles.

In a gas, for example, the kinetic energy is associated with the random motion of the gas molecules, while the potential energy is associated with the forces of attraction or repulsion between the molecules. In a solid or liquid, the potential energy also includes the energy associated with the forces that hold the atoms or molecules in place.

The internal energy of a system can change as a result of heat transfer, work done by or on the system, or a combination of both. For instance, when heat is added to a system, the kinetic energy of the particles increases, leading to an increase in the internal energy. Conversely, when work is done by the system (for example, when a gas expands), the internal energy decreases.

It's important to note that the internal energy of a system is a state function, meaning it depends only on the current state of the system and not on the path taken to reach that state. This means that the change in internal energy of a system is the same regardless of the process or processes that caused that change.

In summary, internal energy is a key concept in understanding how energy is stored and transferred in chemical systems. It provides a framework for analysing energy changes in reactions and processes, and is fundamental to the study of thermodynamics.