16. CFC's, Ozone and Free Radicals

• CFC's - what is so good about them? (before we get into the problems they cause!)

  • A CFC is a covalently bonded relatively small molecule of carbon, chlorine and fluorine atoms.

• If enough energy is supplied by heat or by visible/uv electromagnetic radiation, or the is weak enough, a covalent bond can break in two ways. This illustrated with the molecule chloromethane CH₃Cl.

• The bond breaks unevenly where the electron bond pair can stick with one fragment and a positive and negative ion form.

  • e.g. CH₃Cl ==> CH₃⁺ + Cl^-   (at Advanced Level this is called heterolytic bond fission)

  • shows what happens to the molecule, or

• The bond breaks evenly, where the bonding pair of electrons are equally divided between two highly reactive fragments called free radicals.

  • Free radicals are characterised by having an unpaired electron not involved in a chemical bond.

  • The ^. means the 'lone' electron on the free radical, which is not part of a bond anymore, and wants to pair up with another electron to form a stable bond - that's why free radicals are so reactive!

  • e.g.  CH₃Cl ==> CH₃^. + ^.Cl    (at Advanced Level this is called homolytic bond fission)

  •   shows what happens to the molecule

• In the stratosphere small amounts of unstable ozone O₃ (trioxygen) are formed by free radical reactions.

• The chemistry of free radicals is important in the current environmental issue of ozone layer depletion.

  • Chlorofluorocarbons (CFC's for shorthand) are organic molecules containing carbon, fluorine and chlorine

  • e.g.  dichlorodifluoromethane has the formula CCl₂F₂ (shown in right diagram).

  • They are very useful low boiling organic liquids or gases, until recently, extensively used in refrigerators and aerosol sprays e.g. repellents.

  • They are relatively unreactive, non-toxic and have low flammability, so in many ways they are 'ideal' for the job they do.

  • However it is their chemical stability in the environment that eventually causes the ozone problem but first we need to look at how ozone is formed and destroyed in a 'natural cycle'. This presumably has been in balance for millions of years and explains the uv ozone protection in the upper atmosphere.

  • Ozone is formed in the stratosphere by free radical reactions.

    • 'ordinary' stable oxygen O₂ (dioxygen) is split (dissociates) into two by high energy ultraviolet radiation (uv photon energy 'wave packets) into two oxygen atoms (which are themselves radicals) and then a 'free' oxygen atom combines with an oxygen molecule to form ozone (trioxygen).

      • O₂ + uv ==> 2O^. then O^. + O₂ ==> O₃ 

    • The ozone is a highly reactive and unstable molecule and decomposes into dioxygen when hit by other uv light photons. The oxygen atom radical can do several things including ...

      • O₃ + uv ==> O₂ + O^. 

    • This last reaction is the main uv screening effect of the upper atmosphere and the ozone absorbs a lot of the harmful incoming uv radiation from the Sun.

    • If the ozone levels are reduced more harmful uv radiation reaches the Earth's surface and can lead to medical problems such as increased risk of sunburn and skin cancer and it also accelerates skin aging processes.

    • There is strong evidence to show there are 'holes' in the ozone layer with potentially harmful effects, so back to the CFC problem for some explanations and solutions!

  • The chemically very stable CFCs diffuse up into the stratosphere and decompose when hit by ultraviolet light (uv) to produce free radicals, including free chlorine atoms, which themselves are highly reactive free radicals.

    • e.g. CCl₂CF₂ ==> CClF₂^. + Cl^.  (note the C-Cl bond is weaker than the C-F bond and breaks more easily)

  • The formation of chlorine atom radicals is the root of the problem because they readily react with ozone and change it back to much more stable ordinary oxygen.

    • O₃ + Cl^. ==> O₂ + ClO^. bye bye ozone! and no uv removed in the process!

    • and then: ClO + O ==> Cl + O₂ , which means the 'destructive' Cl is still around!

    • The two reactions above involving chlorine atoms are known as a catalytic cycle because the chlorine atoms from CFC's etc. act as a catalyst in the destruction of ozone. 

  • Therefore many countries are banning the use of CFCs, but not all despite the fact that scientists predict it will take many years for the depleted ozone layer to return to its 'original' O₃ concentration and alternatives to CFC's are already being marketed.

    • BUT at least the ozone layer is recovering thanks to some world-wide co-operation and the work of chemists in developing less environmentally harmful alternatives.

  • Alternatives to CFCs

    • The idea is to use replacement compounds that are less harmful to the ozone layer.

    • The molecules listed below contain C-H bonds and are broken down in the lower troposphere before they reach the ozone layer in the stratosphere.

    • Hydrochlorofluorohydrocarbons (HCFCs)

      • e.g. CH₃CFCl₂ 1,1-dichloro-1-fluoroethane

    • Hydrofluorocarbons (HFCs)

      • e.g. CH₂FCF₃ 1,1,1,4-tetrafluoroethane

    • Alkanes

      • e.g. butane CH₃CH₂CH₂CH₃

      • but they are flammable!

    • However, all of these molecules are greenhouse gases and will contribute to global warming!

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Advanced Level Organic Chemistry revision notes

Revise KS4 Science GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10)