Overall Rate Law

[Jaira_Murphy_2D]

If I'm given a reaction mechanism with 2 steps I can just combine products on one side and reactants on the other side and cancel out like terms to get the overall reaction? And for a reaction mechanism with 3 or more steps, I have to use the pre-equilibrium approach?

Or, how do I know when to use the pre-equilibrium approach?

[Joonsoo Kim 4L]

Summing the elementary steps to get the overall reaction goes for any mechanism of any number of steps. If the sum matches with the overall chemical equation, the mechanism has satisfied one of the requirements of being a plausible mechanism for that reaction.

You use the pre-equilibrium approach when the slow step (the rate-determining step) contains an intermediate. You find the equilibrium in the fast step to then substitute the intermediate concentration in terms of the reactants that match the experimentally determined rate law.