ENTROPY
Entropy (S) is a measure of the disorder, or randomness, of a system. The units of entropy are
energy/temperature, commonly J/K or cal/K. The greater the order in a system, the lower the
entropy; the greater the disorder or randomness, the higher the entropy. At any given temperature,
a solid will have lower entropy than a gas, because individual molecules in the gaseous state are
moving randomly, while individual molecules in a solid are constrained in place. Entropy is a state
function, so a change in entropy depends only on the initial and final states:
∆S = Sfinal − SinitialA change in entropy is also given by:
where qrev is the heat added to the system undergoing a reversible process (a process that proceeds
with infinitesimal changes in the system’s conditions) and T is the absolute temperature.
A standard entropy change for a reaction, ∆S°, is calculated using the standard entropies of
reactants and products:
∆S°rxn = (sum of S°products) − (sum of S°reactants)The Second Law of Thermodynamics
Entropy is an important concept because it determines whether a process will occur spontaneously.
The second law of thermodynamics states that all spontaneous processes proceeding in an isolated
system lead to an increase in entropy. Since the universe as a whole is one big isolated system, we
can also rephrase this law in a way that is perhaps more stimulating to our imagination—the
entropy of the universe either increases (spontaneous, irreversible processes) or stays the same
(reversible processes). It can never decrease.