The Atmosphere. Chemical Storylines. A1 What’s in the air?. It is a relatively thin layer of gas (about 100km thick) The two most chemically important regions are the STRATOSPHERE and the TROPOSPHERE 90% of all molecules in the atmosphere are in the bottom 15km ( the troposphere ).
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…there must be some reactions making it as well…..
O2 + hν O + O
dioxygen molecule oxygen atoms
(Bond Energy = +498 kJmol-1) (RADICALS)
O + O2O3ΔH = -100 kJmol-1
O + OO2ΔH = -498 kJmol-1
O + O32O2ΔH = -390 kJmol-1
O3 + hvO2 + O
Chlorine (Cl) and Bromine (Br) atoms
CH3Cl + hvCH3● + Cl ●
(both species have an unpaired electron;
they are radicals)
Chlorine reacts with ozone as follows…
Cl + O3ClO + O2
(radical) (new radical)
ClO + OCl + O2
Cl + O3 + ClO + OClO + O2 + Cl + O2
O3 + OO2 + O2
This is a catalytic cycle with Cl acting as the catalyst
In this way a single Cl atom can remove about 1 million ozone molecules
Hydroxyl radicals (HO●)
H2O + O2HO●
Nitrogen monoxide (NO)
X + O3XO + O2
(radical) (new radical)
XO + OX + O2
Catalytic cycle with X acting as the catalyst
X + O3 + XO + OXO + O2 + X + O2
O3 + OO2 + O2
CFCs: very handy compounds
In 1930 Thomas Midgley inhaled CCl2F2 (dichlorofluoromethane) and used it to blow out a candle.
This demonstrated that it was neither toxic nor flammable
It was invented to replace ammonia as a refrigerant (toxic and smelly)
CCl2F2 is an example of a chlorofluorocarbon (CFC)
They are very unreactive, have low flammabilities and toxicities and have a variety of different boiling points.
Their main uses have been as;
refrigerants and in air conditioning
propellants in aerosols,
blowing agents in expanded plastics such as polystyrene
The problem with CFCs is that they are now known to be too unreactive…
They have plenty of time to reach the stratosphere.
Scientists have now shown that, once in the stratosphere, the uv radiation causes them to photodissociate to form Cl radicals (see next page)
These then cause ozone depletion
The chemical industry has had the job of finding suitable replacements.
… ozone concs begin to fall sharply.
As ClO concs begin to rise sharply…
Travelling towards S pole
Other molecules in the stratosphere, react with chlorine
CH4 + Cl• CH3• + HCl
NO2 + ClOClONO2
HCl and ClONO2 (chlorine nitrate) are chlorine reservoir molecules because they ‘store’ the chlorine
Some of these will be carried down into the troposphere,
Most however, stay in the stratosphere…
Decreases in ozone concentration are
most dramatic in the Antarctic spring.
This is due to two changes occurring
during the winter…
1. Very low temperatures (below -80ºC)
2. A vortex of air forms
The low temps cause clouds to form made of solid particles of ice rich in nitric acid polar stratospheric clouds (see below)
The vortex isolates the air from the rest of the atmosphere
HCl and ClONO2 (chlorine reservoir molecules)are adsorbed onto the particles in the clouds and react…
ClONO2 + HCl HNO3 + Cl2
The HNO3 stays dissolved in the ice
Cl2 is released as a gas but stays trapped in the vortex
When sunlight returns in spring, the vortex breaks up
Cl2 molecules undergo photolysis to form Cl atoms
These react rapidly with the ozone (see A3)
Ozone is a vital sunscreen.
It absorbs harmful u.v. radiation.
Ozone depletion leads to;
Death of plankton
Effects on food chains
Changes in temperature and weather
1987 Montreal Protocol agreed restrictions on CFCs and similar bromine compounds (halons)
Since then it has been amended based on further research
Short term: hydrochlorofluorocarbons (HCFCs) e.g. CHClF2
C-H bond is broken in troposphere (often by OH radicals)
However, some still gets to stratosphere and forms Cl radicals
Will be phased out by 2020 in developed countries and 2040 in rest of world
Longer term: hydrofluorocarbons (HFCs) e.g. CH3CF3
No ozone depleting effect even if reaches stratosphere
No perfect solution as both are greenhouse gases!
Predictions are that ozone layer may just be beginning to recover but won’t be completely recovered until 2060-2070
Now turn our attention to the bottom 15km of the atmosphere…
Here methane (CH4) is less helpful.
It is made by methanogenic bacteria through anaerobic respiration (in the absence of oxygen)
It is therefore made wherever carbohydrate breaks down (or decays) anaerobically…
Marshes and compost heaps
Rice paddy fields
Digestive tracts (a cow releases 0.5 m3 of methane per day !!)
Methane’s concentration in the troposphere is now 2.5 times what it was in pre-industrial times
Methane is good in the stratosphere but bad in the troposphere
To see why, we need to look at how the sun keeps the Earth warm…
Hot objects emit electromagnetic
The sun (6000 K) radiates i.r., visible
and u.v. light
The Earth is much less hot (about 285 K)
It still emits radiation but only lower
energy i.r. radiation.
The end result is that a steady state is reached and the Earth’s temperature remains constant…
It is a delicate balance that can be easily disturbed if the amounts of certain gases in the atmosphere change.
Methane is a greenhouse gas…
A greenhouse gas will absorb i.r. radiation but not u.v. or visible radiation.
They will let the sun’s radiation IN, but will absorb some of the Earth’s i.r. radiation that would otherwise go into space.
As a result the atmosphere gets warmer which makes the Earth warmer. This is the greenhouse effect.
Once the energy is absorbed, two things can happen;
Some i.r. is re-emittedby the molecules
This occurs in all directions; some towards Earth, some into space
i.r. increases the vibrational energy of the molecules
Bonds vibrate more
This vibration can be transferred to other molecules in the air (e.g. O2 and N2) by collisions
They move faster, so have more kinetic energy
So temperature of the air is raised
Carbon dioxide and several other substances in the troposphere are also greenhouse gases
Some have a greater effect than others depending on…
How good it is at absorbing i.r.
Its lifetime in the troposphere
One way of comparing these gases is by determining their global warming potential
This compares everything to CO2 which is given a value of 1
The greenhouse effect is good for you
The greenhouse effect keeps the average temperature high enough to support life.
Moon – no atmosphere – v. hot days, v. cold nights
Venus – 90% CO2– huge greenhouse effect (about 450ºC)
1880-1940 average temperature rose
Then 1940-1970 fell by 0.2°C
So why are we worried?
During 1970s CO2 levels rose significantly
Very difficult to make predictions due to
huge number of variable factors…
Concentrations of gases
All possible chemical reactions occurring and their rates
Changes in solar radiation
Changes in human activities
Interactions between the atmosphere and the oceans
Feed all this data into powerful computers to generate models of how the climate might change
In 1988 the Intergovernmental panel on Climate Change (IPCC) was set up
This led, in 1997 to the Kyoto Protocol
In this 169 countries agreed to proposed limits on the emissions of greenhouse gases
It came into effect in 2004
Records suggest that the 11 years from 1995-2006 were among the 12 warmest years on record
Using modelling studies, the IPCC have said it is 95% certain that the global pattern of warming over the last 50 years cannot be explained without including warming due to human emissions
The two most significant greenhouse gases are CO2 and H2O, mainly because they are so abundant.
Water, however, is different from other greenhouse gases…
Usually it’s a liquid and so isn’t a problem but, if the Earth gets warmer…
we will get more water vapour…
…so greater greenhouse effect - BAD
Water as droplets in clouds will block out the sun…
This makes it difficult to predict what will happen
CO2 and H2O absorb in two ‘bands’ of
the Earth’s radiation spectrum
Between these two bands is a window
where 70% of the Earth’s radiation
can escape (as it isn’t absorbed)
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 not naturally present
These absorb radiation in the vital ‘window’ region
They have a very large global warming potential and so small amounts have a big effect
At least half the expected increase in the greenhouse effect due to human activities is likely to be caused by carbon dioxide.
We must therefore control the amount of CO2 we produce.
CO2 in the atmosphere is about 0.038%
We need to be able to detect tiny changes to this…
1) Qualitative(to show that it is present – of no use here)
Turns lime water cloudy
2) Quantitative(to show how much is there)
Infra red spectroscopy
– the more CO2 present, the more i.r. gets absorbed
(that’s the whole problem of the greenhouse effect!)
Calculations suggest that the increase in CO2 in the atmosphere should be twice what it actually is.
Not all the CO2 produced is going into the atmosphere. Where is it going?
Oceans soak up carbon dioxide
CO2 is fairly soluble in water.
Large amounts of CO2 (g) dissolve in the oceans
CO2(g) +aq CO2(aq)
This is a REVERSIBLE REACTION (it can occur in both directions)
CI 7.1 “Equilibria”
Phytoplankton use up most of the CO2
which goes into the sea
The concentration of CO2(aq) is
therefore kept small and so CO2(g)
is encouraged to dissolve
A very small proportion of the CO2(aq) (about 0.4%) reacts with the water…
CO2(aq) + H2O(l) HCO3-(aq) + H+(aq)
H+ is the species which causes solutions to be acidic.
But, reaction is in equilibrium and only 0.4% of the CO2 reacts
A solution of CO2 will therefore be only weakly acidic
A9.1 Chemical equilibria
pH is related to the concentration of H+ ions
We can therefore link pH to the amount of CO2 present in solution…
…and then relate this to the amount of CO2 present in the air
Over last 20 years pH has gone down by about 0.04 pH units
Coping with carbon
Oceans do a good job but CO2 is still rising
The steep rise in the 20th century is unprecedented
In 1750s CO2 conc was 280 ppm…
…it is now 383 ppm…
…if we don’t take drastic action it may have doubled to 560ppm within your lifetime
Climate change models predict this
will result in a temperature rise of
between 2ºC and 4.5ºC
The link between CO2 and global warming is now well supported by scientific evidence.
It is uncertain how these changes will affect the planet
However, the IPCC has warned of;
Reduction of snow cover
Thawing of arctic permafrost
Melting of polar sea ice
Rising sea levels
Increases in extreme weathers such as heat waves
More rain in northern latitudes
Less rain in tropical regions
More intense typhoons and hurricanes
This will cause the sea level to rise due to the melting of ice.
Some people believe we are heading for disaster from accelerating global warming.
Others believe that the Earth will develop ways of compensating for any serious departure from the equilibrium