Ozone depletion and CFCsAQA A-Level Chemistry: Subtopic test

1. 1
   Ozone, O₃, forms naturally in the upper atmosphere. Dichlorodifluoromethane, CCl₂F₂, is a chlorofluorocarbon (CFC) that was widely used as a refrigerant, and its molecules can drift up into the same region of the atmosphere.

   (a)

   Why is ozone in the upper atmosphere beneficial to life on Earth?

   [1 mark]

   • AIt absorbs infrared radiation and keeps the Earth warm
   • BIt reacts with CFCs to form harmless products
   • CIt absorbs ultraviolet radiation from the Sun
   • DIt reflects visible light back into space

   (b)

   How are chlorine atoms formed from CCl₂F₂ in the upper atmosphere?

   [1 mark]

   • AUltraviolet radiation breaks the C–Cl bond
   • BVisible light breaks the C–F bond
   • COzone oxidises the CFC molecule
   • DThe CFC dissolves in rain water and releases chloride ions

   (c)

   Write an equation for the formation of a chlorine atom from CCl₂F₂ and state the type of bond fission involved.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Chlorine atoms in the upper atmosphere decompose ozone by a two-step sequence:
   Cl· + O₃ → ClO· + O₂
   ClO· + O₃ → 2O₂ + Cl·

   (a)

   What is the overall equation for the two-step sequence?

   [1 mark]

   • AO₃ → O₂
   • B2O₃ → 3O₂
   • CO₃ + Cl· → ClO· + O₂
   • D2O₃ + 2Cl· → 2ClO· + 3O₂

   (b)

   Which species is an intermediate in the sequence?

   [1 mark]

   • ACl·
   • BO₃
   • CO₂
   • DClO·

   (c)

   Explain why one chlorine atom can lead to the decomposition of many ozone molecules.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Until the 1980s CFCs were widely used as solvents and refrigerants. In 1985 researchers working in Antarctica reported unexpectedly low ozone concentrations above the continent. Other research groups, using different measuring methods, then published their own results.

   (a)

   Explain how the results of research by different groups provided evidence that led to legislation banning CFCs.

   [3 marks]

   (b)

   Chemists developed chlorine-free replacements such as CH₂FCF₃. Explain why CCl₂F₂ releases chlorine atoms in the upper atmosphere but CH₂FCF₃ does not damage the ozone layer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Scientists investigating the ozone layer measured the amount of ozone and of chlorine-containing species at high altitude. They found that a single chlorine atom can remove a very large number of ozone molecules before it is eventually removed from the cycle.

   (a)

   Explain, with the help of equations, how chlorine atoms are formed from CFCs and decompose ozone, and why the loss of ozone is harmful.

   [6 marks]

   (b)

   Evaluate the claim that the evidence justified banning CFCs as solvents and refrigerants.

   [6 marks]

   Total for question 4: 12 marks

End of questions