20.2. Spontaneous Reactions and Free Energy http://www.ck12.org
Now we can talk about the entropy changes in the universe simply by looking at data about the system of interest. If
the results of the equation are less than zero, the reaction will be spontaneous. The American physicist Josiah Gibbs
took this idea and introduced a thermodynamic quantity that combines enthalpy and entropy into a single value that
has been named after him.Gibbs free energyis defined by the following equation:
∆G =∆Hsys- T∆Ssyswhere∆G is the change in free energy,∆H is the change in enthalpy, T is the temperature (in Kelvin), and∆S is
the change in entropy. As we can see by comparing this to the derived equations above, the value of∆G must be
less than zero for a spontaneous process. Stated another way, systems have a natural tendency to move towards a
minimum amount of free energy.
FIGURE 20.3
Gibbs postulated that a system pro-
gressed towards the equilibrium direction
which exhibited the highest entropy.