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Internal energy is the microscopic energy contained in a substance, given by the random, disordered kinetic energy of the molecules. In addition it includes the potential energy between these molecules, and the energy contained in the atoms of these molecules. internal energy, in thermodynamics, is a feature or state function that determines the energy of a substance when there are no effects due to capillarity and external electricity, magnetism, and other fields.
Total energy contained in a system is called its internal energy or intrinsic energy.
It is denoted by symbol E or U.
Components of internal energy
The internal energy of a system is made up of a number of components, such as,
Translational kinetic energy of molecules: In thermodynamics, internal energy has two major components, and they are kinetic energy and potential energy. The kinetic energy is due to the motion of the system’s particles (e.g., translations, rotations, vibrations), some of these components are stated below.
Rotational energy of molecules: Rotational energy is mainly the rotation of the molecule around its center of gravity.
Vibrational energy of molecules: Vibrational energy is caused due to the periodic displacement of atoms of the molecule away from its equilibrium position.
Bond energy: The amount of energy required for breaking a mole of molecules into its component atoms, or the measure of strength of the chemical bond formed.
Electronic energy: (sum of the energies of the occupied orbitals)
Solved example: Assume that the system has constant pressure and the surrounding looses about 81 J of heat and does 521 J of work onto the system, then what will be the internal energy of the system?
Solution: q and w are positive in the equation ΔU = q + w, and we know that q = 81 J; w = 521 J
Thus ΔU = (81 J) + (521 J) = 602 J
Hence, the internal energy of the system will be 602 J.
Internal Energy
Internal energy of a system depends upon the state of the system, and not upon how the system attains that state, hence internal energy is a state function.
Each system possesses a definite amount of internal energy under a given set of conditions.
For example, internal energy of a system in state A be UA and in state B, be UB.
Then, change in internal energy (ΔU) of the system is given by
∆U = Ufinal – Uinitial
= UB – UA
Change in internal energy depends on initial & final state, and not on how this change is brought about.
Internal energy of a system can be changed in two ways:
Either by allowing heat (q) to flow into or out of the system.
By doing work (w) on or by the system.
If reaction is carried out at constant volume and constant temperature (ΔT = 0 and ΔV = 0), then no mechanical work is done, all the energy absorbed or evolved is equal to the change in internal energy.
Thus, change in internal energy (ΔU) in a chemical reaction is equal to heat absorbed or evolved under constant temperature and volume conditions. The internal energy possessed by a substance helps in differentiating its physical structure
Units
The SI unit of internal energy is Joule (J).
The CGS unit of internal energy is ergs. 1J=107 ergs.
Characteristics
A system’s internal energy is an extensive application. It is determined by the amount of compounds in the system. When the quantity is multiplied by two, the internal energy is multiplied by two.
A system’s internal energy is a state function. It is solely dependent on the system’s state variables (T, P, V, n). The process by which the final state is attained has no impact on the change in internal energy.
In a Cycle the change in process is Zero.
Significance of Internal Energy
Since, it depends on the quantity of a substance, hence it is an extensive property.
Basically, change in it represents the heat evolved or absorbed in a reaction at constant temperature and volume.
In a Isothermal Process change in it is equal to zero.
Important for understanding phase changes, chemical changes, and nuclear reactions.
Conclusion
Total internal energy U consists of two basic components, Uk, the total kinetic energy of the material particles moving in the interior space of the system, and Um, the total internal energy stored in other forms inside the materials contained by the system. However Internal energy is an extensive property. Internal energy becomes zero is an isothermal process. Work as a path function is not quantitatively related to ∆U.
