Ozone, effect of CFC's, free radicals
Brown's GCSE/IGCSE/O Level 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
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.
Index of KS4 Science GCSE/IGCSE/O Level
Chemistry Oil & Organic Chemistry revision notes pages: 1.
Fossil Fuels & carbon Cycle : 2.
Fractional distillation of crude oil & physical
properties and uses of fractions,
what makes a good fuel? : 3.
ALKANES - saturated hydrocarbons, structure, uses, combustion : 4.
Pollution, carbon monoxide, sulfur/nitrogen oxides, climate change-global warming,
carbon footprint :
5. Alkenes - unsaturated hydrocarbons,
structure and chemistry :
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? : 9a. Alcohols,
Ethanol, manufacture, physical properties & chemical reactions
Biofuels & alternative fuels,
hydrogen, biogas, biodiesel
: 10a. Carboxylic
acids - chemistry and uses
: 10b. Esters, chemistry and uses including perfumes
: 11. Condensation polymers, Nylon & Terylene,
comparing thermoplastics, fibres and thermosets
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
destruction, CFC's and free
radicals : 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
ALL my Advanced
Level Organic Chemistry revision notes
CFC's, Ozone and Free
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.
what is so good about them? (before we get into the problems
are covalently bonded relatively small organic molecule of carbon,
chlorine and fluorine atoms e.g.
dichlorodifluoromethane, (CFC–12), trichlorofluoromethane,
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
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
It was in the early
1970s that scientists found out that chlorine could be
involved with destroying ozone in the ozone layer of the
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
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 very reactive!
==> CH3▪ + ▪Cl
(at Advanced Level this is called homolytic bond fission)
what happens to the molecule
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
(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.
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.
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).
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 ...
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.
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 some of the ozone! and no uv
removed in the process!
AND the chlorine oxide
radical, ClO, is just as reactive as the chlorine
radical itself ...
+ O3 ==> Cl▪ + 2O2
free radical reactions removing ozone, AND the 'destructive' Cl
is still around!
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.
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
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
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.
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
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.
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
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
dichlorofluoromethane (a HCFC)
more on HCFCs ?
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.
However, all of
these molecules are greenhouse gases and will contribute to
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
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
ALL my Advanced
Level Organic Chemistry revision notes
ozone layer depletion free
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