(c) doc b

Ozone, effect of CFC's, free radicals

Doc Brown's GCSE/IGCSE/O Level KS4 science–CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

16. Ozone, CFC's and free radicals

What is ozone? What are CFCs? How does a CFC destroy ozone? What are free radicals? Why do CFCs cause ozone depletion in the upper atmosphere? Why is the ozone layer important to our health? What is the danger of less ozone in the upper atmosphere? All questions answered below! These revision notes on ozone depletion and the effect of CFCs and free radicals should prove useful for the NEW AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.


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16. CFC's, Ozone and Free Radicals

  • The use of CFC liquids and gases in refrigerators and aerosols etc. has had a very negative effect on the ozone concentration in the upper atmosphere. Ozone absorbs the most harmful high energy uv radiation that can cause skin cancer. BUT, here we have a success story, by stricter regulation and using alternative chemicals, the situation is being reversed.

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

    • Chlorofluorocarbons (CFCs) are covalently bonded relatively small organic molecule of carbon, chlorine and fluorine atoms e.g.

    • (c) doc b dichlorodifluoromethane, (CFC–12),(c) doc b trichlorofluoromethane, (CFC–11).

    • both these CFCs are made by replacing the hydrogen atoms in methane with chlorine and fluorine atoms.

    • CFCs have very useful in many applications with their advantageous properties. They are non–toxic, non–flammable and insoluble in water. They also have low boiling points – typical of small covalent molecules.

    • CFCs were used as heat exchanging coolants in refrigerators, in air–conditioning systems, propellants in aerosol spray cans (now banned in Europe and US), foams, cleaning solvents e.g. cleaning agents for electrical components.

  • It was in the early 1970s that scientists found out that chlorine could be involved with destroying ozone in the ozone layer of the upper atmosphere.

    • In the 1980s scientists produced evidence for a decrease in ozone levels in the atmosphere above Antarctica.

    • These findings were described as a 'hole in the ozone layer'.

    • Further tests on chemicals in the upper atmosphere showed the presence of CFCs breaking down and this chemistry was facilitating the breakdown of ozone itself.

    • As the scientific evidence accumulated, we are now sure that CFCs are one cause of depletion in ozone concentration in the upper atmosphere.

  • An introduction to free radical chemistry – its essential to know about this if you want to understand why CFCs destroy ozone

    • 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 very reactive!

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

      •   shows what happens to the molecule

      • Free radicals can be fragments of molecules or single atoms.

      • The single dot (▪)represents the unpaired electron on the free radical.

  • 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.

    • How is ozone formed? Why is the ozone layer so important to life on Earth?

    • 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 one of these 'free' oxygen atoms combines with an oxygen molecule to form ozone (trioxygen).

        • O2 + uv ==> 2O

        • then O + O2 ==> O3 to make ozone

      • Ozone molecules are found in the 'ozone layer' high up in the stratosphere, part of the upper atmosphere.

      • 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 

        • The oxygen molecule and oxygen atom can then rejoin to make ozone, so have a natural 'recycling system' of ozone decomposition and ozone formation.

      • 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. It is the higher energy uv photons that are most likely to be absorbed by the ozone, and this is the most harmful part of the ultraviolet radiation spectrum.

      • 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. CCl2F2 ==> Cl + CClF2

      • Note the C–Cl bond is weaker than the C–F bond and breaks more easily to give the very reactive chlorine atom free radical.

    • 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 some of the ozone! and no uv light removed in the process!

      • AND the chlorine oxide radical, ClO, is just as reactive as the chlorine radical itself ...

      • ClO + O3 ==> Cl + 2O2

      • another free radical reactions removing ozone, AND the 'destructive' Cl is still around!

      • The two reactions above involving chlorine atoms are an example of a chain reactions and make a catalytic cycle of ozone destruction, because the chlorine atoms from CFC's etc. go through the cycle many times acting as a catalyst in the destruction of ozone. 

        • One chlorine atom can destroy hundreds/thousands? of ozone molecules before it joins up with another radical giving a molecule that isn't a reactive free radical.

  • BANNING CFCs any effect? are there alternatives to CFCs?

    • Obviously the ban on using CFCs is needed, the case for ozone depletion due to CFCs has been made.

    • But why did the problem persist? and why, even now, the ozone layer has not fully recovered?

      • The problem is that CFCs are not very reactive and are quite stable in the lower atmosphere where the CFC molecules don't get hit by the high energy uv photons to give chlorine atoms.

      • BUT, they will still drift up into the stratosphere and contribute to ozone destruction.

      • In other words, its going to take a long time for all the CFC bans to have complete effect because (i) the 'long-life' of CFCs and (ii) so few CFC molecules, giving chlorine atom formation can be responsible for destroying so many more ozone molecules.

    • The conclusive evidence of ozone destruction by CFCs did cause much concern, spreading from the scientific community to the wider public at large, and most importantly, for any action to be taken, politicians also realised something needed to be done.

      • things didn't happen fast as governments, quite rightly, demanded a good body of proven evidence e.g. fully evaluated peer reviewed research papers.

      • Even by the late 1970s Canada, Norway, Sweden and the USA had banned the use of CFCs as aerosol propellants.

      • Once the 'ozone hole' was discovered other countries (including other European countries like the UK) reduced chlorofluorocarbon production and have now banned the use of CFCs completely.

    • 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

        • and (c) doc b, dichlorofluoromethane (a HCFC)

        • more on HCFCs ?

      • Hydrofluorocarbons (HFCs)

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

        •  and (c) doc bdifluoromethane

        • HFCs are similar to CFCs but they don't contain chlorine in the molecule, so even if they reached the stratosphere, they can't give rise to the catalytic cycle of destruction caused by the chlorine atom free radicals.

        • HFCs are now considered to be safe to use.

      • Alkanes

        • e.g. butane CH3CH2CH2CH3

        • but it is a hydrocarbon gas and they are very flammable!

      • Advanced Chemistry Page Index and LinksHowever, all of these molecules are greenhouse gases and will contribute to global warming!

      • Its sometimes very difficult to win 100% on these complex environmental issues, its great to be an idealist, but a compromise has to be accepted sometimes!

GCSE/IGCSE/O Level Oil Products & Organic Chemistry INDEX PAGE

ALL my Advanced A Level Organic Chemistry revision notes

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 (c) doc b 3 linked easy Oil Products gap–fill quiz worksheets

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

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b 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


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