The decomposition of hydrogen peroxide is catalyzed by...

The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism: $$ \begin{aligned} \mathrm{H}_{2} \mathrm{O}_{2}(a q)+\mathrm{I}^{-}(a q) & \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{IO}^{-}(a q) & \text { (slow) } \\ \mathrm{IO}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}_{2}(a q) & \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)+\mathrm{I}^{-}(a q) & \text { (fast) } \end{aligned} $$ (a) Write the rate law for each of the elementary reactions of the mechanism. (b) Write the chemical equation for the overall process. (c) Identify the intermediate, if any, in the mechanism. (d) Assuming that the first step of the mechanism is rate determining, predict the rate law for the overall process.

The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:
$$
\begin{aligned}
\mathrm{H}_{2} \mathrm{O}_{2}(a q)+\mathrm{I}^{-}(a q) & \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{IO}^{-}(a q) & \text { (slow) } \\
\mathrm{IO}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}_{2}(a q) & \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)+\mathrm{I}^{-}(a q) & \text { (fast) }
\end{aligned}
$$
(a) Write the rate law for each of the elementary reactions of the mechanism. (b) Write the chemical equation for the overall process. (c) Identify the intermediate, if any, in the mechanism. (d) Assuming that the first step of the mechanism is rate determining, predict the rate law for the overall process.

Key Concepts

Elementary Reaction

An elementary reaction is a single step process in a reaction mechanism that occurs in one collision or rearrangement event. In an elementary reaction, the rate law can be directly derived from its molecularity, meaning that the stoichiometric coefficients indicate how the rate depends on the concentrations of the reactants involved in that step.

Rate Law

A rate law expresses the relationship between the reaction rate and the concentrations of the reactants. For an elementary reaction, the rate law is written with the concentration of each reactant raised to a power equal to its coefficient in the elementary step. This law is a critical tool in chemical kinetics to understand how changes in concentration affect the speed of the reaction.

Overall Reaction

The overall reaction is the net chemical equation obtained by summing all the elementary steps in a mechanism while canceling out any intermediates. It represents the complete transformation of reactants to products in the overall process, regardless of the intermediate species that are formed and consumed along the way.

Reaction Intermediate

A reaction intermediate is a species that is formed in one step of a reaction mechanism and consumed in a subsequent step. These intermediates do not appear in the overall balanced equation because they are transient and their concentrations remain very low. Their identification is significant for understanding the detailed pathway of the reaction.

Rate-Determining Step

The rate-determining step is the slowest step in a reaction mechanism and thus controls the overall reaction rate. Since it has the highest energy barrier, the overall rate law is often based on its kinetics. Recognizing the rate-determining step is essential for predicting changes in the reaction conditions and for designing catalysts that might lower the activation energy of the slow step.

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