Consider the following mechanism for the oxidation of bromide ions by hydrogen peroxide in aqueous acid solution. Which of the following rate laws is consistent with the mechanism?

Question

Consider the following mechanism for the oxidation of bromide ions by hydrogen peroxide in aqueous acid solution.  Which of the following rate laws is consistent with the mechanism?

Quizplus · Accepted Answer

The rate-determining step in the mechanism involves the reaction between Br¯ and HOCl to form HOBr and Cl¯. Therefore, the rate law must contain [Br¯]. The other reactants, H2O2 and H+, are present in excess and do not appear in the rate law. The proposed mechanism suggests that the reactions between H2O2 and HOBr, and between HOBr and Br¯, are fast and in equilibrium, so their concentrations will remain constant. Therefore, the rate law must involve only the slow step.

The mechanism suggests that the slow step is a bimolecular reaction between Br¯ and HOCl. This reaction involves collision between two species, each with a concentration of [Br¯] and [HOCl]. Therefore, the rate law must have a term that involves the product of these two concentrations, which rules out answer choices A, B, and D.

Answer choice E is also incorrect because it is too simple and does not consider the complex mechanism proposed.

Answer choice C is consistent with the proposed mechanism and satisfies the conditions mentioned above. It has a term that involves the product of the concentrations of the two reactants in the slow step, [Br¯] and [HOCl], and a term that involves the concentration of the catalyst, H+. Therefore, answer choice C is the best choice.

Consider the Following Mechanism for the Oxidation of Bromide Ions