Ozone depletion and CFCsAQA A-Level Chemistry: Revision notes
Section 1
Ozone and ultraviolet radiation
Ozone, O₃, forms naturally in the upper atmosphere (the stratosphere). It is beneficial because it absorbs ultraviolet (UV) radiation from the Sun, which would otherwise reach the surface and damage living cells, causing problems such as skin cancer.
Ozone is continually formed and broken down, so the amount in the atmosphere is steady unless something upsets the balance.
Section 2
CFCs and chlorine atoms
Chlorofluorocarbons (CFCs) are compounds of carbon, chlorine and fluorine, such as CCl₂F₂. They were used as solvents and refrigerants because they are unreactive, non-flammable and non-toxic, so they survive to reach the upper atmosphere.
There, UV radiation provides enough energy to break the C–Cl bond by homolytic fission, forming chlorine atoms (radicals):
CCl₂F₂ → ·CClF₂ + Cl·
C–F bonds are stronger and are not broken in this way.
Say C–Cl, not C–F, is broken by UV: C–Cl is the weaker bond.
Section 3
Chlorine atoms as a catalyst
Chlorine atoms catalyse the decomposition of ozone:
- Cl· + O₃ → ClO· + O₂
- ClO· + O₃ → 2O₂ + Cl·
Adding the steps, the overall equation is 2O₃ → 3O₂. The Cl· is regenerated, so it is not used up and one chlorine atom can decompose many ozone molecules. ClO· is an intermediate. The loss of ozone contributes to the hole in the ozone layer, which lets more UV radiation reach the surface.
Cl· is the catalyst and ClO· is the intermediate. Do not mix them up, and do not say the chlorine atoms are used up.
Section 4
Evidence and legislation
Measurements by different research groups, using different methods, gave consistent evidence of ozone loss and of the part played by chlorine from CFCs. Independent, reproducible results of this kind persuaded governments to introduce legislation banning CFCs as solvents and refrigerants.
Scientific knowledge develops in this way: new evidence is checked by other groups before action is taken.
Section 5
Alternatives to CFCs
Chemists developed chlorine-free alternatives such as hydrofluorocarbons (HFCs), for example CH₂FCF₃. They contain no chlorine, so they cannot form chlorine atoms, and their C–F bonds are too strong to be broken by the UV that reaches the upper atmosphere.
Therefore they do not damage the ozone layer.
Must Know
- Ozone absorbs UV radiation
- UV breaks the C–Cl bond in CFCs by homolytic fission, forming Cl·
- Cl· + O₃ → ClO· + O₂ and ClO· + O₃ → 2O₂ + Cl·
- Cl· is regenerated, so it is a catalyst
- Overall: 2O₃ → 3O₂
- Consistent evidence from different groups led to legislation banning CFCs
- Chlorine-free alternatives were developed
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ozone depletion and CFCs
- 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.Write an equation for the formation of a chlorine atom from CCl₂F₂ and state the type of bond fission involved.2 marks
- Chlorine atoms in the upper atmosphere decompose ozone by a two-step sequence:
Cl· + O₃ → ClO· + O₂
ClO· + O₃ → 2O₂ + Cl·Explain why one chlorine atom can lead to the decomposition of many ozone molecules.2 marks
- 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.Explain how the results of research by different groups provided evidence that led to legislation banning CFCs.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).