Brown's GCSE/IGCSE KS4 science-CHEMISTRY Revision Notes
Oil, useful products, environmental problems, introduction to
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
Index of KS4 Science GCSE/IGCSE
Chemistry Oil & Organic Chemistry Pages: 1.
Fossil Fuels : 2. Fractional distillation of crude oil & uses of fractions : 3.
ALKANES - saturated hydrocarbons and combustion : 4.
Pollution, carbon monoxide, nitrogen oxides, what
makes a good fuel?, climate change-global warming :
5. Alkenes - unsaturated hydrocarbons :
6. Cracking - a problem of supply and demand, other products :
7. Polymers, plastics, uses and problems :
8. Introduction to Organic Chemistry - Why so many series of
organic compounds? : 9. Alcohols - Ethanol
- properties, reactions, biofuels :
10. Carboxylic acids and esters : 11. Addition
polymers and condensation polymers :
12. Natural Molecules - carbohydrates - sugars
- starch : 13. Amino acids, proteins,
enzymes & chromatography : 14. Oils, fats,
margarine and soaps :
15. Vitamins, drugs-analgesic medicines & food
additives and aspects of cooking chemistry! : 16. Ozone, CFC's and free
radicals : 17. Extra notes, ideas and links on
Global Warming and Climate Change : Multiple Choice and Gap-Fill Quizzes:
m/c QUIZ on Oil Products (GCSE/IGCSE easier-foundation-level)
m/c QUIZ on Oil Products (GCSE/IGCSE harder-higher-level) :
IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry
3 Easy linked GCSE/IGCSE Oil Products word-fill worksheets
CFC's, Ozone and Free
what is so good about them? (before we get into the problems
If enough energy
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.
bond breaks unevenly
where the electron bond pair can stick with one fragment and a positive
and negative ion form.
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.
. 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!
==> CH3. + .Cl
(at Advanced Level this is called homolytic bond fission)
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
(CFC's for shorthand) are organic
molecules containing carbon, fluorine and chlorine
has the formula CCl2F2 (shown in right
They are very useful low boiling
organic liquids or gases, until recently, extensively used in refrigerators and
aerosol sprays e.g. repellents.
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.
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).
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 ...
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
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.
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!
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.
of chlorine atom radicals is the root of the problem because
they readily react with ozone and change it back to much more stable
O3 + Cl.
O2 + ClO.
bye bye ozone! and no uv
removed in the process!
+ O ==> Cl + O2 , which means the 'destructive' Cl
is still around!
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.
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.
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
but they are
However, all of
these molecules are greenhouse gases and will contribute to
Multiple Choice Quizzes and Worksheets
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry
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
... OCR 21st C GCSE Science
Worksheet gap-fill C1.1c Air
pollutants etc ...
... Edexcel 360 GCSE Science
Crude Oil and its Fractional distillation
... each set are interlinked,
so clicking on one of the above leads to a sequence of several quizzes
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)
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