synthesis is an example of the first kind of reaction: fast, and
apparently complete (see previous page). If hydrogen gas and chlorine
gas are mixed in a container with a window and kept in the dark,
no reaction will occur. But light will trigger an explosion:
H (g) + Cl
- 45.54 kcal per 2 moles of HCl
After the explosion almost no detectable quantities of H
and Cl will
remain. The large negative free energy change indicates that the
reaction should be spontaneous, and the light-triggered explosion
shows that this is so. Then why is there no reaction in the dark?
The answer is that the reaction in the dark is still spontaneous,
but is so slow that we do not notice any changes.