Internal Energy

What is Internal Energy

Explain Internal Energy.

All forms of energy associated with a system is called its internal energy or simply energy (E or U) of the system. This is expressed in Joule (J). This arises due to the movement of molecules, arrangement of atoms in molecules, number and arrangement of electrons in atoms etc. It is neither possible nor necessary to calculate the absolute value of internal energy of a system. We are mainly interested with the energy changes (ΔE) when a system goes from one state to another. It is a state function and therefore does not depend upon the path by which the changes have been brought about. Internal energy is an extensive quantity. The concept of internal energy is based on our common observations e.g., a liquid freezes into solid giving heat, Zn and CuSO4 solution producing electrical energy in Daniel cell, steam expands from higher to lower pressure producing mechanical work.

For a closed system the internal energy is defined as-
ΔE = q + W
E is the change in internal energy of a system during a process
q is the heat
W is the mechanical work.
If an energy exchange occurs because of temperature difference between a system and its surroundings, this energy appears as heat otherwise it appears as work. When a force acts on a system through a distance the energy is transferred as work. The above equation shows that energy is conserved.

Let EA and EB are the internal energies in states A and B respectively. Then the difference between the internal energies in the two states will be-
ΔE = EB – EA
The difference in internal energies has a fixed value and will be independent of the path taken between two states A and B. For the chemical reaction, the change in internal energy may be considered as the difference between the internal energies of the products and that of the reactants.
ΔE = Eproducts – Ereactants
Thus, the internal energy, ΔE is a state function.

Internal energy for an ideal gas is proportional to the number of particles per mole and the temperature
E = cnT
E is internal energy
c is heat capacity at constant volume
n is moles
T is temperature.
For monatomic ideal gas, internal energy-
E = (3/2)RnT.
For diatomic ideal gas, internal energy-
E = (5/2)RnT.

Internal energy for endothermic reaction is positive.
Eproducts – Ereactants = +ve
i.e. ΔE = +ve.
Internal energy for exothermic reaction is negative.
Eproducts – Ereactants = −ve
i.e. ΔE = −ve.

Calorie to Joule and Erg