10 Questions You Should to Know about thermo-activated catalysts

Consider this scenario and answer the following questions: Chlorine atoms resulting from decomposition of chlorofluoromethanes, such as CCl2F2, catalyze the decomposition of ozone in the atmosphere. One simplified mechanism for the decomposition is: $$O_3\;^{\underrightarrow{\;sunlight\;\;}}\;O_2+O$$ $$O_3+Cl \rightarrow O_2+ClO$$ $$ClO+O \rightarrow Cl+O_2$$

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(a) Does this mechanism confirm that chlorine atoms are a catalyst in the gas-phase transformation of ozone to dioxygen?

Solution

Chlorine atoms are indeed a catalyst in this reaction mechanism because they react in the second step but are regenerated in the third step.

(b) Nitric oxide is also involved in the decomposition of atmospheric ozone by the mechanism: $$O_3\;^{\underrightarrow{\;sunlight\;\;}}O_2+O$$ $$O_3+NO\rightarrow NO_2+O_2$$ $$NO_2+O\rightarrow NO+O_2$$

Is NO a catalyst for the decomposition of ozone in this mechanism? Explain your answer.

Solution

Yes, NO is a catalyst, following the same logic as the Cl in part (a).

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Each pair of reaction coordinate diagrams below represents two versions of the same reaction: one catalyzed and one uncatalyzed. In each pair, identify which of the reactions shown is catalyzed:

Solution

Diagram (b) shows the reaction with the lowest activation energy; it is more likely to be the catalyzed reaction.

Solution

Diagram (b) shows the reaction with the lowest activation energy; it is more likely to be the catalyzed reaction.

Solution

Both reactions have a first (slow) step with a similar Ea. Changes to the overall rate of reaction by altering only the second step will be negligiable. However, diagram (b) shows the second step in the reaction with the lower activation energy; it is more likely to be the catalyzed reaction.

Solution

In (a), both reaction steps have a similar Ea (about 10 kJ). Based on the position of the lines, the first step may be slightly larger than the second, making it the rate determining step. The activation energy of this step is reduced in (b): the reaction shown in (b) will proceed faster and is the catalyzed reaction.

Solution

The right-hand figure has a larger activation energy for its single step than the rate-determining step (first step) in the reaction shown on the left. The left-hand diagram shows the catalyzed reaction.

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