Aim for FiveChemistry

Short free-response question

Chlorine atoms and ozone

  • Unit 5
  • 4 points
  • About 9 minutes

You can use a calculator on this question, just like on exam day.

A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

In the upper atmosphere, chlorine atoms speed up the destruction of ozone. A proposed mechanism is shown below. Both steps are exothermic.

Proposed mechanism

Step 1: Cl(g) + O₃(g) → ClO(g) + O₂(g) (slow)

Step 2: ClO(g) + O(g) → Cl(g) + O₂(g) (fast)

Source: Simplified mechanism for this question

Suggested time: 9 minutes

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Part (a)

1 point

Write the balanced equation for the overall reaction.

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Part (b)

1 point

Identify the catalyst and the intermediate in the mechanism. Justify your answer.

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Part (c)

1 point

Write the rate law for the overall reaction that is consistent with the proposed mechanism. Justify your answer.

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Part (d)

1 point

Describe the reaction energy profile you would draw for this two-step mechanism. Include the number of peaks, which peak is higher, where the intermediate appears, and the relative energies of the reactants and products.

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