Rate Law

[Madeline Ho 1C]

Why is it that for elementary steps, the rate law can simply be found using molecularity but this does not apply to the overall balanced reaction?

[2c_britneyly]

With elementary steps there's no ambiguity. If the equation is A+A--> B, then you know it requires two moles of A for B to form. If they only give you the overall reaction with no elementary steps, then there could be an intermediate you didn't know about.

[Jayasuriya Senthilvelan 4I]

With an overall reaction, we have no idea what steps go behind it. It could happen just as it is written, or there could be a number of elementary steps occurring in the background that are not obvious. For this reason, when we are given an overall reaction, we must examine its reaction mechanism (set of proposed elementary steps) in order to find the rate law. For each elementary step, we are assuming that it happens in a single step with no intermediate whatsoever.