HCl 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 (g) 2HCl (g)
G= - 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.