You are interested in a simple reaction: Stoichiometry: A rightarrow B Mechanism

Question

You are interested in a simple reaction: Stoichiometry: A rightarrow B Mechanism: A rightarrow I step 1, k_1 I rightarrow A step 2, k_2 I rightarrow B step 3, k_3 a) Under what specific mechanistic conditions would a steady approximation would work? Answer this question specific to mechanism given. No math is required, answer the question using your conceptual understanding of rate constants. b) Under what specific mechanistic conditions would it be appropriate to use a pre-equilibrium assumption? Once again, no math is required, answer the question using your conceptual understanding of rate constants.

Explanation / Answer

a) When a reaction mechanism has several steps of comparable rates, the rate-determining step is often not obvious. However, there is an intermediate in some of the steps. An intermediate is a species that is neither one of the reactants, nor one of the products. The steady-state approximation is a method used to derive a rate law. The method is based on the assumption that one intermediate in the reaction mechanism is consumed as quickly as it is generated. Its concentration remains the same in a duration of the reaction.

b) The pre-equilibrium approximation assumes that the reactants and intermediates of a multi-step reaction exist in dynamic equilibrium. The pre-equilibrium approximation method derives an expression for the rate of product formation with approximated concentrations. Unlike the steady state method, the pre-equilibrium approximation does so by assuming that the reactants and intermediate are in equilibrium.

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