In the formation of HCl, the enthalpy falls drastically and the
entropy, or disorder, rises to a minor extent. Both effects favour
reaction, so the explosion is spontaneous.
Disordering contributes to reaction, so the free energy of 45.6
kcal, which makes the reaction spontaneous and which can potentially
be harnessed to do useful work, actually is greater than would
be expected from heat-energy arguments alone. This reaction, which
liberates only 44.2 kcal of heat, thus can do as much as 45.6 kcal
mole-l of useful work.
We can portray the rise and fall of energies and the drive toward
reaction in the arrow diagram on the right. The long downward-pointing
arrow at the left of the diagram represents the release of 44.2
kcal of enthalpy or heat (DH=-44.2
kcal); this favors the reaction.
The shorter arrow next to it symbolizes the decrease in order,
which leads to (-TDS) =-1.4 kcal.
The long DG arrow at the right of
the diagram is the sum of these two, and represents the overall
drop in free energy of 45.6 kcal (DG=-45.6
kcal). In this HCl example, both enthalpy and entropy contribute
to making the reaction spontaneous.