CHM 2046C Module Nine Sample Exam Page 5 Name:______________

Part G: Reaction Mechanisms 20 points

a) The decomposition of ozone in the upper atmosphere to dioxygen occurs by a two-step mechanism. The first step is a fast reversible step and the second is a slow reaction between an oxygen atom and an ozone molecule:

Step 1: O₃ (g) ß à O₂ (g) + O (g) Fast. Reversible, reaction

Step 2: O₃ (g) + O (g) à 2 O₂ (g) slow

a. Which is the rate determining step?

b. Write the rate expression for the rate-determining step.

c. Write the rate equation for the overall reaction.

d. Which specie(s) is (are) reaction intermediates?

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b) The conversion of cis-2-butene to trans-2-butene is catalyzed by iodine I₂.

The mechanism is believed to involve the dissociation of the iodine molecules to two iodine atoms, a process that quickly reaches an equilibrium, followed by the addition of iodine (atom) to one of the carbon atoms of the butane. This allows the rotation around the carbon-carbon bond. (carbon=carbon can not rotate). The iodine then dissociates from the intermediates to facilitate the conversion of another butene molecule.

a. Write the equations for the various steps of this mechanism.

b. Derive the rate equation.

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c) Consider the following reaction in basic solution:

I ^1- + ClO ^1- à Cl ^1- + IO ^1-

The rate equation is: Rate = k [I^1-][ ClO ^1-] / [OH^1-]

Suggest a mechanism: