In many reactions the contributions of enthalpy and entropy (disorder)
reinforce one another, as in the arrow diagram at the upper right.
In such a reaction heat is released and disorder is created, both
of which favor the spontaneous reaction. We will see examples of
such reactions shortly.
In other reactions enthalpy and entropy may work against one
another, as at the lower right. In this particular example, heat
is given off, thereby favoring the reaction; but order increases
(the -TDS arrow points up), thus
hindering the reaction.
The enthalpy effect dominates in this case and the reaction
still is spontaneous, but this need not always be true. The examples
we cited at the beginning of the chapter to show the fallacy of
deciding spontaneity on the basis of enthalpy alone were in this
How do the changes in enthalpy and entropy compare when ammonium
chloride dissolves in water? Which effect predominates? Re-call
from Chapter 12 that heat is absorbed: DH0
= +3.62 kcal mole-1.This by itself
works against the dissolving process.
But the molar entropy increases from 22.6 e.u. to 40.2 e.u., an
increase in disorder per mole of 17.6 e.u., or cal deg-1.
If DS = +17.6 cal deg-1
mole-1, then at room temperature
TDS = 298° (+17.6 cal deg-1
= 5245 cal mole-1
TDS = 5.25 kcal mole-1
The free energy change per mole then can be calculated:
D G = DH
- TDS = +3.62 kcal - 5.25 kcal =
Enthalpy applies 3.62 kcal worth of opposition to the dissolving
of ammonium chloride in water, but entropy, or disorder, favors
the dissolving process by 5.25 kcal. The net effect is that NH4Cl
dissolves with an overall free energy drive of 1.63 kcal mole-l.
The key to chemical spontaneity is not what enthalpy or entropy
may do individually, but what happens to the free energy during
the process. The results are easier to understand, however, if we
realize that two components are involved, H and S.
It obviously is unnecessary to tabulate free energies if heats
of formation and third-law entropies are available, but standard
free energies of formation of compounds from their elements are
so useful that they normally are included in tables of the type
given in appendix. Let
us use these tabulated values for some reactions that will illustrate
how free energy behaves.