ABSTRACT
In order to take into account the spontaneity and irreversibility of real processes (heat always goes from a hot substance to a cold one, but not the reverse), thermodynamics invokes the notion of the entropy S. In statistical terms entropy is defined as the probability of accessible states for each molecule in the system:
VkS ln . (5.5) In equation (5.5), k is the Boltzmann constant (1.38066 × 10−23 J K−1), which is related to the (thermal) energy of one molecule, and V is the number of all accessible states of the molecule. Generally, in physical chemistry, we consider not a single molecule (or particle), but rather one mole, i.e., 6 1023 molecules (or particles). Thus equation (5.5) becomes VRS ln , (5.6) where R = kNA = 8.314 J K
1 mol1 is the universal gas constant, and NA is Avogadro’s number.
