Question

The pre-equilibrium and the steady-state approximations are two different approaches to deriving a rate law from a proposed mechanism.

Which statement best describes these approaches?

A) The pre-equilibrium approximation focuses on the concentration of reactants at the start of the reaction, while the steady-state approximation considers the concentrations at an intermediate stage.

B) The pre-equilibrium approximation assumes that the reaction reaches equilibrium quickly, allowing the derivation of a rate law, while the steady-state approximation involves assuming that certain reaction intermediates are at a constant concentration.

C) The pre-equilibrium approximation is used for reactions with a fast initial step, while the steady-state approximation is employed for reactions with a slow initial step.

D) The pre-equilibrium approximation and the steady-state approximation are interchangeable methods used to derive rate laws from proposed mechanisms.

Answer 1

The pre-equilibrium approximation assumes a fast initial step reaches equilibrium, permitting use of equilibrium constants for rate law derivation, while the steady-state approximation holds that intermediate concentrations remain constant for simplifying rate law derivation.

Step-by-step explanation:

The best description of the pre-equilibrium and steady-state approximations in deriving a rate law from a proposed mechanism is that the pre-equilibrium approximation assumes that a fast equilibrium is established between reactants and intermediate species, while the steady-state approximation involves the assumption that the concentration of intermediates remains constant throughout the reaction process.

In the pre-equilibrium approach, the fast initial step reaches equilibrium, allowing for the use of equilibrium constants to derive rate laws for the slow step that follows.

On the other hand, in the steady-state approximation, the rate of formation and the rate of consumption of an intermediate are equal, leading to a constant concentration of that intermediate, which simplifies the derivation of the rate law.