AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: use equations, such as the following, to…
Full specification wording
Ozone depletion: Students should be able to: use equations, such as the following, to explain how chlorine atoms catalyse decomposition of ozone:
Here you must work with the equations themselves.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Here you must work with the equations themselves. Write the two steps, Cl• + O₃ → ClO• + O₂ and ClO• + O₃ → 2O₂ + Cl•, then add them. Cl• appears on the left of the first and the right of the second, and ClO• appears on the right of the first and the left of the second, so both cancel, leaving 2O₃ → 3O₂. That cancellation is the proof of catalysis: the chlorine radical is consumed and then regenerated, so it does not appear in the overall equation and a tiny amount keeps working. The species that is made and then used up inside the cycle, ClO•, is the intermediate. The same pattern is used for other radicals, such as nitrogen monoxide, so recognise the shape of the argument rather than memorising one pair of equations.
What earns the marks
- Writing both propagation steps with radical dots shown.
- Cancelling Cl• and ClO• because each appears on both sides of the added equations.
- The overall equation 2O₃ → 3O₂ correctly balanced.
- Identifying Cl• as the catalyst because it is regenerated, and ClO• as the intermediate.
Common mistakes and how to put them right
Cancelling ozone rather than the radicals when adding the two steps.
Leaving a chlorine species in the overall equation.
Balancing the overall equation with oxygen atoms instead of O₂ molecules.
Omitting the radical dots so the equations describe ions.
Exam tips
- Set the two equations out one above the other before adding; the cancellation is then obvious.
- State in words that the radical is regenerated, as well as showing it in the equations.
- If given a different radical such as NO, apply the same two-step pattern rather than recalling chlorine.
Try the lesson
The start of the Ozone depletion: Students should be able to: use equations, such as the following, to explain how chlorine atoms catalyse decomposition of ozone: lesson, made from checked teaching notes for this specification point.
Have a go first
In the chlorine-catalysed decomposition of ozone, which equation shows the step that regenerates the chlorine atom catalyst?
Listen
Here you must work with the equations themselves. Write the two steps, Cl• + O₃ → ClO• + O₂ and ClO• + O₃ → 2O₂ + Cl•, then add them. Cl• appears on the left of the first and the right of the second, and ClO• appears on the right of the first and the left of the second, so both cancel, leaving 2O₃ → 3O₂. That cancellation is the proof of catalysis: the chlorine radical is consumed and then regenerated, so it does not appear in the overall equation and a tiny amount keeps working. The species that is made and then used up inside the cycle, ClO•, is the intermediate. The same pattern is used for other radicals, such as nitrogen monoxide, so recognise the shape of the argument rather than memorising one pair of equations. A strong answer earns marks for these points. writing both propagation steps with radical dots shown. cancelling Cl• and ClO• because each appears on both sides of the added equations. the overall equation 2O₃ → 3O₂ correctly balanced. identifying Cl• as the catalyst because it is regenerated, and ClO• as the intermediate. Exam tip: Set the two equations out one above the other before adding; the cancellation is then obvious.
See it
Key words
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adding propagation steps
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cancelling catalyst and intermediate
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overall equation
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catalyst regeneration
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See every definition
- adding propagation steps
- Writing the two steps one above the other and then adding them, so that species appearing on both sides cancel.
- cancelling catalyst and intermediate
- Removing the chlorine radical and ClO• because each appears on both sides of the added equations, leaving 2O₃ → 3O₂.
- overall equation
- The balanced equation 2O₃ → 3O₂ that remains after the chlorine species cancel from the added steps.
- catalyst regeneration
- The chlorine radical is consumed and then regenerated, so it does not appear in the overall equation and a tiny amount keeps working.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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