O2
+ hv
O +
OChemical storylines - The Atmosphere
The
chemical and physical processes going on in the atmosphere have a profound
influence on life on Earth. This topic looks at two major problems – global
warming and the depletion of the ozone layer – by studying some fundamental
chemical principles.
A1 What’s in
the air?
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It is a
relatively thin layer of gas (about 100km thick)
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90% of all
molecules in the atmosphere are in the bottom 15km (TROPOSPHERE)
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Concentrations
of some substances are very small so they are measured in parts per million by
volume (ppm)
A2 Screening the Sun
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The sun
radiates a wide spectrum of energy.
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Part of this
corresponds to the energy required to break chemical bonds.
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This includes
molecules such as DNA.
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This can
damage genes and lead to skin cancer and damage proteins and lead to wrinkles
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The most
damaging region is the ultra violet
region – high frequency and high energy.
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Some chemicals
absorb this radiation e.g. glass and manufactured chemicals-sunscreens.
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The best
sunscreen of all is the atmosphere itself
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Atmospheric
gases in the stratosphere absorb ultra violet (uv) radiation very well
(“strongly”)
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Ozone
(O3)
is particularly good at this.
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Only present
in the atmosphere tiny amounts.
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Protects us in
stratosphere but is harmful in troposphere (see DF)
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It is a very
reactive gas and a powerful oxidising agent (causes oxidation, gets reduced)
If
ozone is so reactive, why hasn’t it all run out?
…there
must be some reactions making it as well
O2
+ hv
O +
O
dioxygen molecule
oxygen atoms
(Bond
Energy = +498 kJmol-1)
(RADICALS)
This
process is called PHOTODISSOCIATION.
It occurs when molecules absorb radiation of the correct frequency (hv)
or by electrical discharges or in photochemical smog (see DF).
The O
atoms (radicals) produced are very very reactive. There are three possibilities
of what they will do next…
O
+ O2
O3
DH
= -100 kJmol-1
O
+ O
O2
DH
= -498 kJmol-1
O
+ O3
2O2 DH
= -390 kJmol-1
When
ozone ABSORBS radiation (in the region 10.1 x 1014 to 14.0 x 1014
Hz) we have another photodissociation.
O3
+ hv
O2 + O
This is
the vital reaction that protects us from the harmful u.v. radiation.
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These would
eventually reach a STEADY STATE where ozone is made as quickly as it is used up.
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Chemists can
use this, plus knowledge about RATES of the reactions, to calculate what the
conc. of ozone in the atmosphere should be.
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Measured
amounts are a lot lower than
the calculated amounts.
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Ozone must
be being used up by reactions with some of the other radicals in the
stratosphere….
In
general;
X
+
O3
XO +
O2
(radical)
(new radical)
then..
XO +
O
X + O2
(regenerated)
**
This is a catalytic cycle with X acting as the catalyst **
Overall:
X
+ O3
+ XO
+ O
XO
+ O2
+ X
+ O2
O3 +
O
O2 +
O2
1)
Chlorine
atoms (Cl)
Small
amounts of chloromethane (CH3Cl) are released by oceans and burning
vegetation and reach the stratosphere…
CH3Cl
+ hv
CH3l +
Cl
Similar
for other chlorine compounds produced in human activities such as CFCs.
2)
Hydroxyl
radicals (HOl)
Water
in the stratosphere…
H2O
+ O
2HOl
3)
Nitrogen
monoxide (NO)
This is
made in car engines from N2 and also from N2O released by
bacteria in the soil and oceans.
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These
reactions would not matter if they were slower than the reaction of O3
with O. However, Cl + O3 is 1500 times faster.
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Despite the
fact that the Cl radicals are present in much lower concentrations than O atoms,
they still make a very large contribution to the removal of ozone.
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What’s more,
they are regenerated (catalytic
cycle) and can therefore destroy lots of O3.
A4 The CFC story
Read
“Sherry Rowland’s predictions” and “Joe Farman’s story”
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In 1930 Thomas
Midgley inhaled CCl2F2 (dichlorofluoromethane) and used it
to blow out a candle.
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This
demonstrated that it was neither toxic nor flammable
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It was
invented to replace ammonia as a refrigerant (toxic and smelly).
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CCl2F2
is an example of a
chlorofluorocarbon (CFC)
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They are very
unreactive, have low flammabilities and toxicities and have a variety of
different boiling points.
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Their main
uses are as; i) propellants
in aerosols
ii)
refrigerants
iii)
blowing agents in expanded plastics
iv)
cleaning solvents
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They are too
unreactive.
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They have
plenty of time to reach the stratosphere.
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Here the u.v.
radiation causes them to photodissociate to form Cl radicals.
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These then
cause ozone depletion
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The chemical
industry has had the job of finding suitable replacements.
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Replacement
compounds are hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs)
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These contain
a C-H bond and can be broken in the troposphere
so they don’t reach the stratosphere and the ozone layer
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Unfortunately,
these aren’t perfect as they are greenhouse gases and contribute to global
warming (see later)
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A new proposal
is to use C3H3F5.
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Bromine
radicals will behave in the same way as chlorine radicals.
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They are
released from bromomethane which is made in the same ways as chloromethane.
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They are also
released from halons (e.g. CBrClF2) which are used in fire
extinguishers
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Methane (CH4)
is very important because it reacts with Cl atoms in the stratosphere;
CH4
+ Cl
CH3 +
HCl
(removed
in rain)
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Bromine
doesn’t react with methane in this way
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There still
isn’t enough methane to stop Cl levels rising
A5 How bad is the ozone crisis?
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Ozone is a
vital sunscreen.
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It absorbs
harmful u.v. radiation.
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Ozone
depletion leads to;
-
Skin cancer
- Eye cataracts
- Death of plankton
- Effects on food chains
-
Changes in temperature and weather
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Numerous
treaties meant that developed nations had almost phased out CFC use by 1998 and
funds are being provided to help developing countries to do likewise.
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Hopefully
amounts of Cl and Br atoms in the stratosphere should reach a maximum early this
century and then decline slowly.
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The ozone
layer should then return to normal by the middle of this century
A6 Trouble in
the Troposphere
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The bottom
15km of the atmosphere
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Here methane
(CH4) is less helpful.
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Methane is
good in the stratosphere but bad in the troposphere
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To see why, we
need to look at how the sun keeps the Earth warm
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Hot objects
emit electromagnetic radiation
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The sun (6000
K) emits i.r., visible and u.v. light
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The Earth also
emits radiation but is much less hot (285 K) and so only emits lower energy i.r.
radiation.
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The end result
is that a STEADY STATE is reached and the Earth’s temperature remains constant
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It is a
delicate balance that can be easily disturbed if the amounts of gases in the
atmosphere change. e.g. methane
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Methane is a GREENHOUSE
GAS
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It absorbs
some of the earth’s radiation that would otherwise go into space.
Made by
methanogenic bacteria by anaerobic respiration
So
methane is made wherever carbohydrate breaks down (or decays) anaerobically;
Marshes and compost heaps
Rice paddy fields
Biogas digesters
Digestive tracts (a cow releases 0.5 m3 of methane per day !!)
A greenhouse gas will absorb i.r. radiation but not u.v. or visible radiation. This means that they will let the sun’s radiation IN to warm up the Earth, but will not let (some of) the Earth’s radiation out. As a result the atmosphere gets warmer which makes the Earth warmer. This is the greenhouse effect.
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The greenhouse
effect keeps the average temperature high enough to support life.
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Moon – no
atmosphere – v. hot days, v. cold nights
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Venus – 90%
carbon dioxide – huge greenhouse effect (about 450ºC)
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The Earth’s
climate is a very complex system and this makes it difficult to make
predictions.
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An enormous
amount of data is fed into powerful computers which then give a mathematical
model of the climate
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Over the last
100 years, the 5 hottest years have all been in the 1990s
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There is now a
great deal of evidence that global warming is taking place and that this is due
to man made emissions of greenhouse gases
A7 Keeping the
window open
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The two most
significant greenhouse gases are CO2 and H2O, mainly
because they are so abundant.
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CO2
and H2O absorb in two ‘bands’ of the Earth’s radiation spectrum
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Between these
tow bands is a ‘window’ where 70% of the Earth’s radiation can escape (as
it isn’t absorbed)
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Gases made by
human activity can increase the natural greenhouse effect in two ways:
¯
Increasing
amounts of gases already present. e.g. CO2 from burning fossil fuels.
¯
Adding other
gases such as CFCs
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These absorb
radiation in the vital ‘window’ region
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They have a
v.v. large “greenhouse factor” and so small amounts have a big effect
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Water is
different.
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Usually it’s
a liquid and so isn’t such a problem but….
¯
If
the Earth gets warmer
more water vapour – BAD
¯
Water as
droplets in clouds will block out the sun - GOOD
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This makes it
difficult to predict what will happen
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At least half
the expected increase in the greenhouse effect due to human activities is likely
to be caused by carbon dioxide.
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We must
therefore control the amount of CO2 we produce.
A8 Focus on
Carbon Dioxide
Detecting
CO2
1) Qualitative
(to show that it is present)
2) Quantitative
(to show how much is there)
Infra
red spectroscopy
–
more CO2 present, more i.r. gets absorbed (that’s the whole
problem
of the greenhouse effect!)
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Calculations
suggest that the increase in CO2 in the atmosphere should be twice
what it actually is.
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Not all the CO2
produced is going into the atmosphere. Where is it going?
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CO2
is fairly soluble in water.
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Large amounts
of CO2 (g) dissolve in the oceans
CO
2(g)
+aq
CO2(aq)
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This is a
REVERSIBLE REACTION (it can occur in both directions)
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Phytoplankton
use up most of the CO2 which goes into the sea
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[CO2(aq)]
is therefore kept small and so CO2(g) is encouraged to dissolve
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A very small
proportion actually reacts with the water;
CO2(aq)
+ H2O(l)
HCO3-(aq) + H+(aq)
hydrogencarbonate
ion
hydrogen
ion
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H+
is the species which causes solutions to be acidic.
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A solution of
CO2 will therefore be WEAKLY ACIDIC
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pH is related
to the concentration of H+ ions
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We can
therefore relate pH to the amount of CO2 present in solution and then
relate this to the amount of CO2 present
in the air
A9 Coping with
carbon
|
Year |
Atmospheric
concentration of CO2/ppm |
|
1850 |
270 |
|
2000 |
370 |
|
2080 |
540 |
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It is thought
that doubling the amount of CO2 will cause an average temperature
increase of 2ºC by the 2030s
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This will
cause the sea level to rise due to the melting of ice.
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Some people
believe we are heading for disaster from accelerating global warming.
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Others believe
that the Earth will develop ways of compensating for any serious departure from
the equilibrium.