We know it is possible to determine whether a reaction is spontaneous or not, that is, whether it should occur under specific conditions. If the free energy change (DG) is negative then the reaction should occur.

And yet....We have seen the thermite reaction:

Fe₂O₃ + 2 Al Ž 2 Fe + Al₂O₃ The standard free energy change for this reaction is -835.4 kJ/mol. But when the two reactants are placed together nothing happens. A significant energy input is required to get this "spontaneous" reaction to occur. Why?

This example is only one of many which points to the insufficiency of thermodynamics in explaining why some chemical processes occur and others don't (or at least don't occur without intervention of some sort) and why some are so rapid and others so slow.

The missing piece of this puzzle is kinetics: the study of rates of reactions.