Spontaneity and Gibbs Free Energy
A spontaneous reaction is one that will occur at a given temperature without the
input of energy. Strangely enough, however, sometimes endothermic reactions
(which require the input of energy in order to take place) occur spontaneously.
Why? Because, as we said, the universe likes entropy—disorder. If a particular
reaction is endothermic (∆H is positive) but creates greater disorder (∆S is also
positive), and the disorder the reaction creates exceeds the energy it requires,
then the reaction may occur spontaneously although it’s endothermic. Similarly,
if a reaction creates order instead of disorder, it may occur spontaneously as long
as it’s exothermic and the negative enthalpy change exceeds the negative entropy
change.
What determines whether a reaction will or won’t occur spontaneously? The
combination of ∆H and ∆S. This combination of ∆H and ∆S is called Gibbs free
energy, and is symbolized by ∆G. The actual formula for determining ∆G is ∆G
= ∆H – T∆S (where T is temperature, measured in degrees Kelvin). Remember
the following points about Gibbs free energy:
• If ∆G for the reaction is negative, then that reaction occurs
spontaneously in the forward direction.
• If ∆G for the reaction is positive, then that reaction occurs
spontaneously in the reverse direction.
• If ∆G for the reaction is zero, then the reaction is in equilibrium.
(We’ll discuss equilibrium later.)Important Facts About Gibbs Free EnergyΔG = ΔH − TΔSIf ΔG < 0, then the reaction is spontaneous in the forward direction.If ΔG > 0, then the reaction is spontaneous in the reverse direction.Review everything we’ve talked about in this chapter, and then answer the
following set of questions. The answers can be found in Part III.