Looking at specific examples, we find that in some cases endothermic
reactions occur when the products provide greater randomness or positive
entropy. This reaction is an example:
CaCO 3 (s) CaO(s) + CO 2 (g)The production of the gas and thus greater entropy might be expected to take
this reaction almost to completion. However, this does not occur because another
force is hampering this reaction. It is the absorption of energy, and thus the
increase in enthalpy, as the CaCO 3 is heated.
The equilibrium condition, then, at a particular temperature, is a compromise
between the increase in entropy and the increase in enthalpy of the system.
The Haber process of making ammonia is another example of this
compromise of driving forces that affect the establishment of an equilibrium. In
this reaction
N 2 (g) + 3H 2 (g) 2NH 3 (g) + heatthe forward reaction to reach the lowest heat content and thus release energy
cannot go to completion because the force to maximum randomness is driving the
reverse reaction.
Change in Free Energy of a System—the Gibbs Equation
These factors, enthalpy and entropy, can be combined in an equation that
summarizes the change of free energy in a system. This is designated as ΔG. The
relationship is
ΔG = ΔH − TΔS (T is temperature in kelvins)TIP
Free energy, ΔG, depends on ΔH (enthalpy) and ΔS (entropy).and is called the Gibbs free-energy equation.
The sign of ΔG can be used to predict the spontaneity of a reaction at
constant temperature and pressure. If ΔG is negative, the reaction is (probably)
spontaneous; if ΔG is positive, the reaction is improbable; and if ΔG is 0, the
system is at equilibrium and there is no net reaction.
The ways in which the factors in the equation affect ΔG are shown in this
table: