The decomposition of hydrogen peroxide in the presence of iodide ion is represented…

CHEMISTRY · Multi-part · A Level · Chemistry · Rate equations · Rate equations

Problem

The decomposition of hydrogen peroxide in the presence of iodide ion is represented overall by

2\,\text{H}_2\text{O}_2(aq)\;\longrightarrow\;2\,\text{H}_2\text{O}(l)+O_2(g).

A chemist proposes the following two-step mechanism at neutral pH: (1) H

_2

_2

+ I

^-

\longrightarrow

^-

(aq) + OH

^-

(slow) (2) IH

^-

(aq) + H

_2

_2

\longrightarrow

^-

+ H

_2

O(l) + O

_2

(g) (fast) An experiment conducted at 20 °C with

[\text{H}_2\text{O}_2]_0 = 0.10\,\text{mol L}^{-1}

and

[\text{I}^-]_0 = 0.020\,\text{mol L}^{-1}

gives an initial rate of

4.0\times10^{-5}\,\text{mol L}^{-1}\text{s}^{-1}

. (a) Derive the rate law predicted by the proposed mechanism, expressing it in terms of the concentrations of measurable species. (b) Using the experimental data provided, calculate the value of the rate constant

k

for the reaction at 20 °C, including correct units. (c) The chemist repeats the experiment with the same

[\text{H}_2\text{O}_2]_0

but halves the initial iodide concentration. Predict quantitatively how the initial rate will change according to the rate law derived in part (a). Explain your reasoning. (d) Explain qualitatively how the presence of a different halide ion (e.g., Br

^-

) might affect the observed rate if the slow step requires specific interaction between H

_2

_2

and the halide.

Related theory

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