Elemental bromine,

CHEMISTRY · Multi-part · AP · Chemistry · Kinetics · Kinetics

Problem

Elemental bromine,

Br_2

, reacts in aqueous solution to form bromate,

BrO_3^{-}

, and bromide,

Br^{-}

, by a complex pathway. Two chemists independently propose the following alternative mechanisms that are each consistent with the overall balanced equation. Mechanism A Step 1

\mathrm{Br_2 + H_2O \rightarrow HOBr + Br^{-} + H^{+}}

Step 2

\mathrm{HOBr + Br^{-} + H^{+} \rightarrow Br_2 + H_2O}

Step 3

\mathrm{HOBr + H_2O \rightarrow BrO_2^{-} + 3\,H^{+} + 2\,e^{-}}

(slow) Step 4

\mathrm{BrO_2^{-} + HOBr + H^{+} \rightarrow BrO_3^{-} + Br^{-} + H_2O}

(fast) Mechanism B Step 1

\mathrm{Br_2 + H_2O \rightarrow BrOH^{+} + Br^{-}}

Step 2

\mathrm{BrOH^{+} + H_2O \rightarrow BrO_2^{-} + 3\,H^{+} + 2\,e^{-}}

(slow) Step 3

\mathrm{BrO_2^{-} + BrOH^{+} \rightarrow BrO_3^{-} + Br^{-} + 2\,H^{+}}

(fast) Assume that electrons (

e^{-}

) appear only as bookkeeping devices for redox balance and are neither reactants nor products. (a) For each proposed mechanism, list all species that qualify as reaction intermediates. Support each choice with an explicit reference to the elementary steps in which the species is produced and subsequently consumed. (b) Identify any species that functions as a catalyst in either mechanism and justify your answer. (c) Suppose spectroscopic analysis detects a transient absorption attributable to

BrOH^{+}

but finds no evidence for

HOBr

. Use this information to evaluate which mechanism is better supported by the experimental observation, and explain your reasoning. (d) Describe one additional experimental approach (other than spectroscopy) that could provide evidence to distinguish between Mechanism A and Mechanism B. Your response should clearly state the measurement to be taken and the expected outcome that would favor one mechanism over the other.

Related theory

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