Some reactions are not 100% complete, even after an infinite period of time - they remain as a mixture of reactants and products 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?