pre-equilibrium

[Charlotte Mignon]

Can someone please explain pre-equilibrium and provide me with an example? I am very confused.

[Lesley Perez 1F]

HI,

So from my understanding Pre-equilibrium is what happens before the final product. So, for example lets say we can have a reversible reaction , A+B ~ C. This reaction is gives us C but C is not our final product because we will use C for the second reaction, C + D ~ E. E then will be our final product. Pre-equilibrium is when a REVERSIBLE reaction happens before the final reaction. Not sure if this made sense but hope it helps !

[JeffreyL]

Just to expand on a few key points with pre-equilibrium, pre-equilibrium is caused by a fast step being followed by a slow step in the elementary steps where the product of the fast step is the intermediate that is used up in the slow step. The slow step causes a bottleneck because the reactants are slowly consumed while the products from the fast step are quickly produced. Therefore, since the production of the intermediate is faster than the rate it is being consumed as a reactant in the slow step, there will be a build up in concentration of that intermediate. As a result, the reverse reaction of the fast step will begin to run using the built-up intermediate as the reactant, causing a sort of "fake" equilibrium to occur where both forward and reverse reactions are occurring for the fast step. Hope this clears things up!