Internal Energy

INTERNAL ENERGY (U)

Internal Energy is the total of the kinetic energy due to the motion and vibration of atoms/molecules and the potential energy associated with the electric attractive interactions among atoms/molecules. It includes the energy of all the chemical bonds, and the energy of the free, conduction electrons metals. Internal energy does not include the translational or rotational kinetic energy of a body as a whole. It also does not include the relativistic-energy equivalent E = mc2. The kinetic part of U varies markedly with temperature, whilst the potential part remains nearly constant.

Internal energy is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic scale. For example, a room temperature glass of water standing on a table has no apparent energy, either potential or kinetic . But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second

During a tranformation like a chemical reaction, the variation in Internal Energy of a body(system) is essentially defined by the first law of thermodynamics which states that energy is conserved:

ΔU = Q + W + W'

where :
ΔU is the change in internal energy of a system during a process.
Q is the heat added to a system (measured in joule); that is, a positive value for Q represents heat flow into a system while a negative value denotes heat flow out of a system.
W is the mechanical work done on a system (measured in joule) and W' is energy added by all other processes (like electrochemical reactions, photoinduced reactions, etc.).

As a consequence Internal Energy is expressed in joule. For a process with constant volume and no other form of energy transfer, ΔU represents the total heat transferred to the surrounding.