Pre-Equilibrium Calculations

[Kailyn Missaghi]

hi everyone,
what are the rules/steps of calculating the pre-equilibrium values? How do we find k?

[Ishita S 2E]

When it comes to the pre-equilibrium approach, we must write the rate of reaction with multiple elementary steps in terms of the slowest reaction. Write this in the form rate= k * reactants. If there is an intermediate in the reactants then find a reaction where this intermediate is formed from non-intermediates. Set the rate law of this reaction equal to the reverse rate law of the same reaction. Solve for the intermediate and plug this into the first rate law. This helps us determine the rate of this reaction with multiple steps. The overall "k" for the reaction is k=k1k2/k1' because the first (fast) step is assumed to be at equilibrium, which means the reaction is happening in both the forward and reverse direction. This means we include both k1 and k1'. We only have k2, and not k2' because the second step is the slow step, so it is not assumed to be at equilibrium and therefore we only look at the forward reaction.

[Lexi Brownsberger 2E]

The pre-equilibrium approximation method can be used when finding the rate law (absent of any intermediates) for a reaction with a fast and reversible initial step. To do this, write the rate law based on the slow step, which is known as the rate-determining step. Then, we solve for the intermediate concentrations using the fast, initial step to eliminate them from the rate law.

[Sophia Lim 2A]

Calculating pre-equilibrium values involves determining the concentrations of reaction intermediates that form transiently before reaching equilibrium. These intermediates are formed in the rate-determining step of the reaction mechanism.