∆Suniverse = ∆Ssystem + ∆Ssurroundings ≥ 0Note that the entropy of a system can decrease, as long as it is compensated for by a larger increase
in entropy in the surroundings. The mechanism of refrigeration decreases the entropy within the
refrigerator by maintaining a low temperature that would not persist if left to nature; heat is,
however, generated and dumped outside into the kitchen that increases the entropy. Our cells are
constantly engaging in biochemical reactions that increase the order locally: Synthesizing large
biomolecules from disordered “building blocks,” sequestering ions in different compartments when
it would be more “natural” for them to diffuse over a larger volume, et cetera. All these processes
come at the expense of entropy increases elsewhere: for example, disorder that was generated
when we digested our meal the previous evening. A system will spontaneously tend toward an
equilibrium state (one of maximum entropy) if left alone.
The Third Law of Thermodynamics
Instead of just working with changes or relative magnitudes (as in the case of enthalpy), there is a
standard with which one can assign the actual value of entropy of a substance. The third law of
thermodynamics states that the entropy of a pure crystalline substance at absolute zero is zero. This
corresponds to a state of “perfect order” because all the atoms in this hypothetical state possess no
kinetic energy and do not vibrate at all; thus, there is absolutely no randomness and no disorder in
the spatial arrangement of the atoms.
GIBBS FREE ENERGY
What makes a reaction favorable? In the quantity of entropy, we have an unambiguous criterion of
whether a reaction would occur spontaneously: The total entropy of the universe has to increase.
The only problem with this is that it is not very practical—who knows how to keep track of the
entropy of the entire universe? It would be nice to have a quantity that deals only with the system
itself that we can examine to determine the favorability of a reaction. The thermodynamic state
function, G (known as the Gibbs free energy), is just such a quantity. It combines the two factors that
affect the spontaneity of a reaction—changes in enthalpy, ∆H, and changes in entropy, ∆S, of the
system. The change in the free energy of a system, ∆G, represents the maximum amount of energy
released by a process, occurring at constant temperature and pressure, that is available to perform
useful work. ∆G is defined by the equation: