Some reactions are not 100% complete, even after an
infinite period of time - they remain as a mixture of reactants
after all visible reaction has ceased. A competition exists between
forward and reverse reactions, and equilibrium is a condition of
balance between these opposing processes.
We now come to the question: Why do some reactions,
which by their free
energy values should be spontaneous and far from equilibrium,
sit inert and unreactive for years, whereas other reactions go with
explosive rapidity? The decomposition of NO to nitrogen and oxygen
is thermodynamically spontaneous, so why do we have photochemical
smog from oxides of nitrogen?
The activation energy is symbolized by a hill that the boulder
must surmount before it can roll off the mountain.