Question

why are the exponents in the rate law for an overall reaction only determined experimentally but the rate law for a mechanisms elementary step comes from the reaction coefficients?

Answer 1

A reaction mechanism is composed of a number of elementary steps and each elementary step is a one-step reaction i.e you cannot devise a mechanism for an elementary reaction. This is why we can raise the stoichiometric coefficients to exponents of the reactant concentrations in case of elementary reaction only because they are the one-step reaction.

For an overall reaction, there is always a number of steps involved in it (i.e the mechanism), so we cannot just raise the reaction stoichiometry to exponents as a reaction can involve several possible mechanisms. We can get the information that exactly which path is followed only if we have the experimental data of the reaction proceedings handy. We can then judge whether it is a zero, first, 2nd etc order by plotting the graphs!