Pre-Equilibrum Approach

[Jonathan Shalom 3C]

When would we use the pre-equilibrium approach?

[alexandra]

Use the pre equilibrium approach to find the rate law if slow step is not first step because you can’t have an intermediate in the rate law

[Jasmin Gonzalez]

You use the pre-equilibrium approach when dealing with reactions that occur in multiple steps and involve the formation of an intermediate species. Its usually useful in situations where one forms much faster than it reacts further to the establishment of equilibrium within the intermediate step.

[606158388]

We would use the pre-equilibrium approach when the proposed reaction mechanism has an intermediate in the slow step. The pre-equilibrium approach is used by assuming the elementary step above the aforementioned slow step is fast in both directions (meaning you can assume that there is a build up of product so you can assume an equilibrium forms). Remember that this is an approximation.

[Valeria Perez 3J]

In order to eliminate the intermediates and have the rate law be reflective of the slow step, you would want to use the pre-equilibrium approach. This situation typically arises when you have a fast step before your slow step, meaning that your slow step rate law contains an intermediate, but not the other species that is reflected in the experimentally determined rate law. This is when you would use the pre-equilibrium approach to eliminate the intermediate and substitute it with the species that reflects the overall balanced redox reaction. Hope that makes sense!