Understanding Pre-Equilibrium Approximation in Reaction Mechanisms

The video tutorial explains the pre-equilibrium approximation to determine the rate law for a reaction mechanism with a fast initial step. Using the reaction between nitric oxide and bromine as an example, it describes the steps of the mechanism, identifies intermediates, and derives the overall rate law. The tutorial emphasizes the importance of substituting intermediate concentrations and clarifies that coefficients in the overall equation cannot be directly used as exponents in the rate law, except for elementary reactions.

What is the primary purpose of using the pre-equilibrium approximation in reaction mechanisms?

• To balance chemical equations

• To simplify the calculation of equilibrium constants

• To determine the rate law for reactions with a fast initial step

• To identify the products of a chemical reaction

In the reaction mechanism involving nitric oxide and bromine, what role does NOBr2 play?

• It is a product

• It is an intermediate

• It is a reactant

• It is a catalyst

Why is the second step of the reaction mechanism considered the rate-determining step?

• Because it is the fastest step

• Because it produces the most products

• Because it involves the most reactants

• Because it is the slowest step

Why can't the rate law for the overall reaction be left in terms of the intermediate NOBr2?

• Because intermediates are unstable

• Because intermediates are not present in the initial reactants

• Because intermediates do not affect the rate

• Because intermediates are not part of the final products

What assumption is made about the first step of the mechanism to use the pre-equilibrium approximation?

• It is irreversible

• It reaches a fast equilibrium

• It is the slowest step

• It involves a catalyst

How is the concentration of the intermediate NOBr2 expressed in terms of reactants using the pre-equilibrium approximation?

• By assuming it is constant

• By using the equilibrium constant of the first step

• By using the rate constant of the second step

• By measuring it directly

What is the final form of the rate law for the overall reaction?

• Rate = k[NO][Br2]

• Rate = k[NO]^2[Br2]

• Rate = k[NO][Br2]^2

• Rate = k[NOBr2][NO]

Why is it incorrect to use the coefficients of the overall balanced equation as exponents in the rate law?

• Because they only apply to elementary reactions

• Because they are only used for products

• Because they are always zero

• Because they are not related to reaction rates

What does the coincidence of coefficients in the balanced equation and the rate law exponents indicate?

• It indicates a mistake in the calculation

• It is a coincidence specific to this reaction

• It shows the reaction is at equilibrium

• It is a common occurrence in all reactions

What remains constant when the rate of the forward reaction equals the rate of the reverse reaction?

• The concentration of reactants

• The rate constant

• The concentration of products

• The concentration of the intermediate