rate law

[Maleeha Amir 2E]

So I know that the rate law of something is determined by the slow step, but why do we not include intermediates and why do we need to substitute it?

[Melody L Lehman 2C]

Intermediates are not included because they are produced and used up in the overall reaction. The change in concentration of intermediates is small enough that it does not have an affect on the overall reaction. We substitute the intermediates for the concentration of other components needed to create the intermediate because the concentration of those components determine how fast the intermediates are formed and thus are involved in determining the rate of the overall reaction.

[Valeria Perez 3J]

Since intermediates are transient species that do not show up in the overall balanced reaction, they typically don't show up in the rate law because the rate law deals more with the reactants and products, not the intermediates, and is usually reflective of the overall balanced reaction. However, in terms of substituting it, I'm assuming you mean in the pre-equilibrium approach when the slow step comes later, the intermediate gets substituted so that the rate law shows the relationship between the overall reactants that lead to the end products, since the slow step would instead be using an intermediate that does not exist in the beginning of the reaction. Hope that makes sense!