Rate determining step

[Gabby Magat 3F]

Can someone explain why the rate determining step of a reaction mechanism is the slowest step? How do we determine which step is the slowest given a reaction mechanism? Thanks!!

[Selena Quispe 2I]

The rate determining step is the slowest step because it limits the overall rate of the reaction, a reaction cannot occur any faster than its slowest step which has a high energy barrier. In the example shown in class we knew Step 1 was the slowest step because the 2 NO2's were in the rate law and therefore the overall rate is proportional to [NO2]^2.

[Khoa Vu 3l]

The rate-determining step of a reaction mechanism is its slowest step because the other steps cannot proceed without that step first occurring. Lavelle, for example, uses brownie baking to show us what the rate-determining step is. While you might split up the parts of the cooking process to your friends (mixing, measuring, molding, etc.), no matter how fast you and your friends work, you will not be able to make as many brownies as can be baked in the oven at a single time. Therefore, the rate-determining step for brownie baking is the actual baking time. Likewise, in a chemical reaction, the step with the highest energy barrier, the slow step, must first occur before all the other steps can occur. We are typically told what is the slow step in a multi-step reaction, but in the case we are not, we are usually given the general rate law of the overall reaction with the order number of each reactant species. We can then derive the slowest step's reactants. However, it becomes a bit harder to determine all the intermediaries which occur between each reaction (species that are present in intermediary steps but ultimately cancel out in the overall reaction). Lavelle has stated though that we will be given these intermediaries.