## Appendix C

## A Molecular Approach to Internal Energy and Enthalpy

Boltzmann’s distribution law (sec. 7.c) can be written as

*n*_{i} = *n*_{0} exp [–(∊_{i} – ∊_{0})/kT] (C.1)

Here *n*_{i} is the number of molecules with energy *ε*_{i} and *n*_{0} is the number of molecules in the lowest allowed energy level,* i.e.* the ground state having the zero point energy, ∊_{0}. Eqn. C.1 represents how a large number of particles distribute themselves among a set of allowed energy levels. At all temperatures above absolute zero, some molecules occupy higher energy levels. Consequently the system has an excess energy over the zero point energy. This excess energy is thermal energy.

For a system consisting of an Avogadro number of molecules, the total energy* E* is the sum of* E*_{0}, the ...