gce chemistry the atomosphere context study edexcel

8/9/2019 GCE Chemistry the Atomosphere Context Study EDEXCEL

1/18

GCE

Chemistry

Edexcel Advanced Subsidiary GCE in Chemistry (8CH01)

Edexcel Advanced GCE in Chemistry (9CH01)

The Atmosphere

Issue 2July 2008

Context study


2/18

Edexcel, a Pearson company, is the UKs largest awarding body, offering academicand vocational qualifications to more than 25,000 schools, colleges, employers and

other places of learning in the UK and in over 100 countries worldwide. Qualificationsinclude GCSEs, AS and A Levels, NVQs, Diplomas and our BTEC suite of vocational

qualifications from entry level to BTEC Higher National Diplomas, recognised byemployers and higher education institutions worldwide.

We deliver 9.4 million exam scripts each year, with more than 90% of exam papers

marked onscreen annually. As part of Pearson, Edexcel continues to invest in cutting-edge technology that has revolutionised the examinations and assessment system.

This includes the ability to provide detailed performance data to tutors and studentswhich helps to raise attainment.

This document is Issue 2. Key changes are sidelined. We will inform centres of any

changes to this issue. The latest issue can be found on the Edexcel website:

www.edexcel.org.uk

References to third-party material made in this document are made in good faith.Edexcel does not endorse, approve or accept responsibility for the content ofmaterials, which may be subject to change, or any opinions expressed therein.

(Material may include textbooks, journals, magazines and other publications andwebsites.)

Authorised by Roger Beard

Prepared by Sarah Harrison

All the material in this publication is copyright Edexcel Limited 2008


3/18

Contents

Introduction 1

The atmosphere 2Regions of the atmosphere 2

Greenhouse gases 3Carbon dioxide 4CFCs and HCFCs 5Methane 6Nitrogen oxides (NOX) 8Ozone 8Water vapour 10Other gases 11Their effect on the environment 12Resources 13


4/18


5/18

Context study (The Atmosphere) Edexcel AS/A GCE in Chemistry (8CH01/9CH01) Issue 2 July 2008 1

Edexcel Limited 2008

Introduction

This document is designed to help teachers to understand the contemporary context of the

atmosphere. It gives teachers information on this context and on how to research it further ifthey wish. This document could also be given to students as introductory material.


6/18

2 Context study (The Atmosphere) Edexcel AS/A GCE in Chemistry (8CH01/9CH01) Issue 2 July 2008


The atmosphere

Regions of the atmosphere

The atmosphere is divided into four regions.

The troposphere This is the lowest layer (up to 10 km from the Earths surface) andcontains about 80 per cent of the total mass of air and almost all the water vapour. Thetemperature decreases linearly with distance from the Earth.

The stratosphere This is the next layer and goes up to 50 km above the Earth. Thetemperature rises as the distance from the Earth increases, reaching about 10 C at50 km.

The mesosphere This is above the stratosphere and here the temperature decreaseswith increasing altitude.

The ionosphere (or thermosphere) This is the outermost layer, stretching up to 500 kmfrom the Earths surface. This has a high temperature which is caused by thebombardment of the few air molecules by particles from the Sun.

Figure 1 The Earths atmosphere, as seen from space (Source: NASA)


7/18



Greenhouse gases

The Sun is constantly radiating energy and the gases in our atmosphere absorb some of this

radiation, which then heats the Earth. Some of this energy is radiated back into space, and ifthe amount absorbed equalled the amount radiated the Earth would remain at a steady

temperature. However, some gases in the atmosphere trap radiation, preventing it from beingradiated into space, and these are known as greenhouse gases. The effect of trapping theSuns radiation is called the greenhouse effect.

The natural greenhouse effect is a very good thing for life on Earth. Without it, the averagetemperature of the planet would be about -20C and there would be little liquid water at anytime, water that is essential for life. The problem is with the enhanced greenhouse effect.

Many, but not all, climate scientists believe that the addition of greenhouse gases to the

atmosphere is largely responsible for global warming. This remains a contentious issue. Thegases responsible are produced both naturally and as a result of human activity (anthropogenic

activity).

The greenhouse effect is concerned with the troposphere. The majority of the radiation fromthe Sun is in the range of wavelengths from 100 to 5000 nm, but nearly all of it is in the visible

range of 400 to 700 nm. Some of this radiation is reflected back into space by snow and

icefields but most of this radiation is absorbed by the Earths surface, and some is absorbed byclouds. The Earth radiates energy back into space, but this radiation is in the infrared regionof the spectrum, with a wavelength of over 4000 nm.

Clouds trap some of this radiation and some is absorbed by molecules in the air. Only

molecules with polar bonds can absorb infrared radiation. To do this the molecule mustchange its dipole moment during the vibration or bending of a bond in the molecule.

Evidence from ancient ice cores drilled in the Antarctic and from rocks around the worldindicates that the Earth has been much warmer in the past.

Figure 1 An Antarctic ice core (Source: USGS)

Average global temperatures in the Cretaceous period, when chalk rock was being formed,were significantly higher than today. Clearly these temperature variations must have had a

natural cause. A number of gases are able to absorb radiation within the infrared region of thespectrum. These include the greenhouse gases such as carbon dioxide, water vapour, methaneand ozone.


8/18



Carbon dioxide

From natural sources

Carbon dioxide is released in vast amounts as a result of volcanic activity. There have been

many periods of intense vulcanism in the Earths history, each likely to have beenaccompanied by major releases of carbon dioxide. Hydrothermal activity on the ocean flooralso releases carbon dioxide. Evidence has been found of a lake of liquid carbon dioxide in an

ocean trench near Taiwan. This finding has implications for the removal and storage of carbondioxide produced in thermal power stations. Natural volcanic lakes can sometimes explode,

releasing toxic mixtures of gases, the major component being carbon dioxide as happenedmany times in West Africa.

In recent geological time, the level of carbon dioxide was fairly constant at around 280 ppm.

Animals and plants converted carbohydrates into carbon dioxide (respiration) and plants alsoconverted carbon dioxide into carbohydrates (photosynthesis). The two processes balanced. In

earlier geological time, vegetation and marine organisms were submerged and changed into

coal and oil.The atmospheres of other planets often contain carbon dioxide. It is the major component of

the atmosphere of Venus, whose surface pressure is about 90 times that of Earth. The carbondioxide on Venus has raised the surface temperature to 450C, an effect described as a

runaway greenhouse effect.

From anthropogenic sources

When carbon-based fuels are burned, carbon dioxide is released. New coal-fired power

stations are being opened every month, especially in the industrialising Far East. The growth

in the use of petrol- and diesel-powered vehicles is also a major factor in adding carbondioxide to the atmosphere.

Figure 2 Amount of CO2 generated per capita, per country, 2006 (Source: Wikipedia)

After the industrial revolution, large and increasing amounts of these fossil fuels were burnt,and the carbon dioxide that had been trapped for billions of years was released. This has

caused the carbon dioxide level to rise. Currently air contains about 380 ppm of carbon

dioxide compared with 320 ppm 40 years ago.

The major source of carbon dioxide emissions caused by burning fossil fuels is electricity

production.


9/18



Figure 3 Sources of anthropogenic carbon dioxide emissions

Chemistry of CO2

In carbon dioxide the C=O bonds are polar, but the molecule is linear so the dipoles cancel andthe molecule has zero dipole moment. A change in dipole moment from zero happens with the

asymmetrical stretching of the bonds and the bending mode.

Figure 4 Infrared active bending and stretching vibrations of carbon dioxide

This means that carbon dioxide is able to absorb infrared radiation and so is a greenhouse gas.It is an undoubted fact that the percentage of carbon dioxide in the air is increasing and thatthis will cause more infrared radiation to be absorbed. The discussion is whether this man-

made increase in carbon dioxide levels is the major cause of climate change or one that is less

important than natural changes, such as variations in Sun activity or changes in the Earthsorbit around the Sun.

CFCs and HCFCs


CFCs and HCFCs are not found naturally.


When it was realised that CFCs in fridges and aerosols were depleting atmospheric ozone,alternatives were sought. CFCs are being replaced in refrigerants by similar compounds that

contain a hydrogen atom. These are given the generic name of HCFCs. One such is HCFC-22which has the formula CHClF2. The presence of the hydrogen atom causes the molecule to beoxidised at lower altitudes. Thus it does not diffuse into the stratosphere, where it would

cause ozone depletion.

industry25%

cars and lorries30%

electricity35%

homeand

officeheating

10%


10/18



CFCs and HCFCs are also powerful greenhouse gases and they contribute significantly to the

warming of the atmosphere. One problem, the destruction of atmospheric ozone, has been

replaced by another. An interesting side effect is that ozone also absorbs radiation, and so thedepletion of ozone in the stratosphere by CFCs causes a cooling in that region.

The amount of waste HFCs in the atmosphere is increasing by 20 per cent each year. There isa certain irony in that the use of hydrocarbons, ammonia or carbon dioxide as refrigerants or

in air conditioning would eliminate this particular problem with HFCs. The impacts on thegreenhouse effect would be much less than continuing to vent HFCs.

Figure 5 The hole in the ozone layer, over Antarctica,

as imaged in 2006 at its largest size (Source: Wikipedia)

Chemistry of CFCs and HCFCs

Chlorofluorocarbons (CFCs) have been widely used as refrigerants, aerosol propellants,

solvents and blowing agents for making expanded plastics. Radiation breaks down CFCs in theupper atmosphere to produce chlorine radicals, and these radicals cause ozone breakdown and

prevent ozone formation by several different reactions. This is explained in detail in theChemistry of ozone section.

Chemists have made it a priority to find alternatives to CFCs so that ozone depletion does not

continue.

Methane


Methane is also a significant greenhouse gas and has many potential sources. The bacterialdecay of cellulose in vegetation in marshy conditions releases marsh gas, which is mostly

methane and carbon dioxide. Methane is also vented naturally from oil seepages such as at

Kirkuk in the Middle East. Methane is found as a crystalline hydrate where ice crystals containabout 12 per cent of the gas. Enormous deposits of hydrates exist in the permafrost of Siberiaas well as in some ocean sediments. Any significant warming of the Earth will release large

amounts of methane as the ice crystals melt.


11/18




Agriculture is responsible for the addition of some greenhouse gases to the atmosphere.

Methane is produced in the digestive tract of ruminant animals such as cows and sheep, and isreleased from flooded paddy fields when rice is cultivated. When new land is cleared for

agriculture by the primitive slash-and-burn technique, the combustion of the forests bothreleases carbon dioxide and reduces its uptake by photosynthesis. The rotting of farm waste,

both animal waste and discarded plant material, also releases methane and carbon dioxide.This has been utilised on a small scale to prepare biogas for local use. It is a useful fuel but

the admixture of carbon dioxide reduces its calorific value. A similar gas mixture is producedand released from landfill sites filled with domestic waste.

The energy industries are major contributors to the release of greenhouse gases. Since the

beginning of the industrial revolution, large quantities of coal have been extracted for use asfuel. Methane is released naturally from coal mines where it was known as fire-damp, the

cause of serious explosions. Leakage of natural gas, mostly methane, from gas and petroleumwells is also a significant contributor to the greenhouse gases in the atmosphere.

Figure 6 Amount of methane on the Earths surface and in the stratosphere

(Source: Wikipedia)

Chemistry of methane

Methane, like carbon dioxide, is a non-polar molecule, but the bonds are polar, and so certainvibrations will produce a dipole moment enabling it to absorb infrared radiation.


12/18



Nitrogen oxides (NOX)


Lightning flashes release enough energy to combine nitrogen and oxygen, giving nitrogen

oxides. These oxides then generate ozone in the atmosphere.From anthropogenic sources

The oxides of nitrogen, N2O, NO and NO2 are also greenhouse gases. They are produced by the

reaction of nitrogen and oxygen in the high temperatures of vehicle engines and aircraft andin the breakdown of organic and inorganic fertilizers.

Oxides of nitrogen also act as catalysts in the oxidation of sulphur dioxide, and contribute to

acid rain formation.

Chemistry of NOX

The oxides of nitrogen play a significant role in the destruction of ozone. This is discussed in

the Chemistry of ozone section. This effect is less significant than the effect of CFCs.

Ozone


Ozone is a potent greenhouse gas produced directly from oxygen in the higher levels of theatmosphere. Ozone is also produced as a result of electrical storms. Lightning flashes releaseenough energy to combine nitrogen and oxygen, giving nitrogen oxides. Theses oxides in turn

generate ozone in the atmosphere.


At low levels in the atmosphere, ozone is a pollutant; it is produced when exhaust fumes

containing oxides of nitrogen are involved in a series of photo-catalysed reactions withunburnt hydrocarbons, oxygen and water vapour. The result is a photochemical smog.

Figure 7 Smog in New York (Source: Wikipedia)

Chemistry of ozone

Ozone is generally studied in relation to its depletion by CFCs in the atmosphere andconsequent increase in ultraviolet radiation at ground level. At high levels in the atmosphere,

ultraviolet (UV) radiation breaks down oxygen molecules and ozone forms.


13/18



The overall equation is:

3O2 2O3

Along with oxygen, this ozone is important in protecting us from the damaging effects of UVradiation. Oxygen is changed into ozone by action of UV light. Oxygen, O2, absorbs UV

radiation at wavelengths below 240 nm to produce oxygen atoms (free radicals).

These free radicals react with more O2 molecules to form ozone in an exothermic reaction

(which is why the stratosphere is hotter than the top of the troposphere).

O(g) + O2(g) O3(g) H = - 600 kJ mol-1(approx)

Ozone molecules absorb UV radiation of wavelengths between 200 to 300 nm and aredecomposed by it.

Under natural conditions an equilibrium is set up and organisms on Earth are protected from

much of the UV radiation, as both O2 and O3 molecules absorb these harmful rays.

The two main causes of anthropogenic depletion of ozone are CFCs and NOX, specifically nitricoxide, NO. Both work in complex chain reactions. This means that a single free radical from

CFCs or from NOX will destroy millions of ozone molecules.

Nitric oxide, NO, is produced at high altitudes by the jet engines in aircraft. The followingsequence of reactions takes place.

NO(g) + O3(g) NO2(g) + O2(g)

The NO2 molecules react with oxygen free radicals produced from the decomposition of ozone(or the photolysis of O2 molecules).

NO2(g) + O(g) NO(g) + O2(g)

The overall process, combining these three reactions, is:

2O3(g) 3O2(g)

where the NO is acting as a catalyst.

CFCs are decomposed by UV light of wavelength between 175 and 220 nm and form chlorineradicals, Cl.

O2(g)UV 2O(g)

O3(g)UV O2(g) + O(g)

O3(g) UV O2(g) + O(g)


14/18



A similar set of reactions then takes place:

Cl(g) + O3(g) ClO(g) + O2(g)

ClO(g) + O(g) Cl(g) + O2(g)

Overall:

2O3(g) 3O2(g)

Water vapour


Water vapour is part of the natural water cycle on Earth and is essential to life. It is naturallyproduced when bodies of water are heated by the Sun and evaporate. This water vapour then

forms clouds, which can be effected by gases and particles in the atmosphere. This can leadto effects such as global dimming or global brightening.


An increase in jet aircraft has led to an increase in contrails (condensation trails) produced bythem. These are formed when water vapour condenses at high altitudes.

Figure 8 Contrails from a jet aircraft (Source: Wikipedia)

These contrails affect cloud formation, but are not classified as air pollution. They are

important as any change in global cloud cover can contribute to long-term changes in theEarths climate.

CF2Cl2(g)UV CF2Cl(g) + Cl(g)

O3(g)UV O2(g) + O(g)


15/18



Power stations, and other chemical plants, are also a major contributor of water vapour to the

atmosphere. The steam used to drive the turbines and generate the electricity is cooled in

cooling towers. These release huge clouds of water vapour into the atmosphere, which willaffect the global cloud cover, and the Earths climate.

Figure 9 Water vapour being released from cooling towers at Drax power station,

North Yorkshire (Source: Wikipedia)

Chemistry of water vapour

The most important greenhouse gas is water vapour. Oxygen is more electronegative than

hydrogen and so the O-H bonds are polar. The H2O molecule is V-shaped and so the twodipoles do not cancel and the molecule is also polar. This means that it has a dipole moment.

Any vibration or bending of bonds in the molecule will cause a change in the magnitude of the

overall polarity (dipole moment), so water vapour will absorb infrared radiation.

Deserts are bitterly cold at night, because the air contains very little moisture and so most of

the heat radiated from the Earths surface is not absorbed. It is estimated that about 95 per

cent of the global greenhouse effect is caused by atmospheric water vapour. A water moleculewill absorb infrared radiation and the molecule is promoted to a higher vibrational energylevel. These excited molecules lose this extra energy either by collision, causing the air

temperature to rise, or by emitting radiation of the same frequency. Some of this radiation isgoes into space and some back to Earth.

Other gases

Other gases produced by industry pollute the atmosphere, but chemists work to reduce thispollution. A simple example is the sulphur dioxide produced in power stations, which can be

absorbed by calcium carbonate to make calcium sulphate. This can then be used to make

plasterboard for building.

There are some other chemicals whose impact on the atmosphere is enormously greater than

that of either methane or carbon dioxide, molecule for molecule. These are known as the

PIGGS: potent industrial greenhouse gases. They already account for 3 per cent of theenhanced greenhouse effect and this seems likely to rise. Sulfur hexafluoride (SF6) isestimated to have an effect that is 25 000 times more than an equivalent amount of carbon

dioxide. It is used in the manufacture of reactive magnesium metal.


16/18



Figure 10 Sulfur hexafluoride (SF6) (Source: Wikipedia)

Their effect on the environment

Carbon dioxide, methane and oxides of nitrogen (NOX) all contribute, to some extent, to

global warming. Methane molecules are more efficient at absorbing infrared radiation than

carbon dioxide. NOX and CFCs are even more powerful.

The greenhouse factor of a gas is a comparison with carbon dioxide, which has a greenhouse

factor of one. On this scale, the greenhouse factors of methane, nitrogen(I) oxide, andtrichlorofluoromethane (CCl3F) are 30, 160 and 21 000 respectively. This means that one

molecule of methane has the same effect as 30 molecules of carbon dioxide. Processes thatreduce greenhouse gas production are therefore important in reducing global warming.

The contribution of these four greenhouse gases to global warming is shown in the pie chart

below. The contribution of water vapour has been excluded, as its concentration remainsvirtually constant from year to year.

CO2

55%

NOX

10%

CH4

15%

HCFCs and

CFCs

20%

Figure 11 Contribution to global warming by various greenhouse gases

(excluding water vapour)

The relative effect of these gases to man-made global warming is determined by two factors.

One is their ability to absorb infrared radiation and the other is their half-life in theatmosphere. The combination is called the Global Warming Potential (GWP). The calculation

of a GWP is based on the radiative efficiency (infrared-absorbing ability) of the gas relative to

the radiative efficiency of the reference gas (carbon dioxide), as well as the removal process(or decay rate) for the gas relative to the reference gas over a specified time horizon (usually100 years). The table below shows the GWP for each of the major greenhouse gases.


17/18



Greenhouse gas GWP

Carbon dioxide, CO2 1

Methane, CH4 20

Nitrous oxide, N2O 300

CFC-12, CCl2F2 6500

HCFC-22, CHClF2 1300

Table 1 GWP for each of the major greenhouse gases

The IPCC (International Panel on Climate Change) gives the error on these values as 35 percent, which indicates just how unreliable the predictions are of long-term climatic changes.

Resources

Contrail Education http://asd-www.larc.nasa.gov/GLOBE/science.html

IPCC http://www.ipcc.ch

1781sb150708\LT\PD\Support\GCE Chemistry The Atmosphere Cxt study Issue 2.doc.1-18/1


18/18

July 2008

For more information on Edexcel and BTEC qualifications

please visit our website: www.edexcel.org.uk

Edexcel Limited. Registered in England and Wales No. 4496750

Registered Office: One90 High Holborn, London WC1V 7BH. VAT Reg No 780 0898 07
http://www.edexcel.org.uk/http://www.edexcel.org.uk/

gce chemistry the atomosphere context study edexcel

Documents