Question
What do you mean by internal energy and internal energy change?
Solution
The total energy stored by a fixed amount of the substance is called its internal energy or intrinsic energy and is denoted by U. It is the sum of various forms of energies such as kinetic energy of translation, rotation and vibration, potential energy due to intermolecular forces, the chemical bond energy and electronic and nuclear energies of the constituent atoms.
Internal energy is a function of state only. It is not possible to determine the absolute value of internal energy as most of the constituent energies cannot be precisely determined. However, we are more interested in knowing the change in internal energy (∆U).
Change in internal energy (∆U): It may be defined as The difference in internal energies of the product and reactant species taking part in a chemical reaction at constant volume. For example, if UR represents the internal energy of the reactants and Up represents the internal energy of the products, then
where ∆U gives the change in internal energy.
In a chemical reaction, ∆U has always noticed in the form of heat evolved or absorbed during the reaction in case the reaction is carried out at constant volume. Hence internal energy change (∆U) is equal to the heat evolved or absorbed in a chemical reaction at constant volume and constant temperature.
Clearly, if
the extra energy possessed by the system in the initial state (or the reactants) would be given out and 
will be negative.
Similarly, if
energy will be absorbed in the process and 
will be positive.
Units of U: The units of energy are ergs (in CGS units) or joules (in S.I. units)
1 Joule = 107 ergs.
Internal energy is a function of state only. It is not possible to determine the absolute value of internal energy as most of the constituent energies cannot be precisely determined. However, we are more interested in knowing the change in internal energy (∆U).
Change in internal energy (∆U): It may be defined as The difference in internal energies of the product and reactant species taking part in a chemical reaction at constant volume. For example, if UR represents the internal energy of the reactants and Up represents the internal energy of the products, then
where ∆U gives the change in internal energy.
In a chemical reaction, ∆U has always noticed in the form of heat evolved or absorbed during the reaction in case the reaction is carried out at constant volume. Hence internal energy change (∆U) is equal to the heat evolved or absorbed in a chemical reaction at constant volume and constant temperature.
Clearly, if
the extra energy possessed by the system in the initial state (or the reactants) would be given out and will be negative. Similarly, if
energy will be absorbed in the process and will be positive.Units of U: The units of energy are ergs (in CGS units) or joules (in S.I. units)
1 Joule = 107 ergs.
