If ε represents the energy of one molecule, then

1.16

If N is the number of molecules in the system, then the total internal energy is

1.17

For an ideal gas there are no intermolecular forces of attraction and repulsion, and the internal energy depends only on temperature. Thus, for an ideal gas

U = ƒ(T)

1.18

Other forms of energy which can also be possessed by a system are magnetic energy, electrical energy and surface tension energy. In absence of these forms, the total energy E of a system is given by

E = (EK + EP) + U

1.19

where E_K + E_P Macro energy and U = Micro energy.

In absence of motion and gravity,

EK = 0, EP = 0

1.20

Hence,

E = U

1.21

Hence, Eq. (1.4) becomes

Q = ΔU + W

1.22

For open system, the enthalpy H = U + pV is a property. The specific enthalpy h = u + pv is an intensive property.