Introduction

The alkanes are saturated compounds since they contain single covalent bonds only. Each carbon atom is bonded to four other atoms; these can only be either carbon or hydrogen atoms

They have a general formula of CnH2n+2.

They are all naturally occurring hydrocarbons formed from crude oil. They are actually formed by the conversion of animal and vegetable matter, which existed under pressure for millions of years.

Student Activity: recognising functional groups from a molecule’s name.Identify the following molecules’ functional group name e.g. alkane.

Molecule Functional Group Name Molecule Functional Group NamePropanol Propanoic acidPropene 2-chloropropanePropanal PropanePropanone

Structure and Physical Properties of the alkanes

The alkanes are all saturated hydrocarbons and therefore have no double or triple bonds. There are three distinctive types of alkanes.

(a) Straight Chained Alkanes e.g. butane (C4H10). The displayed formula is: The skeletal formula is:

(b) Branched Chained Alkanes e.g. 2-methylpropane (C4H10). The displayed formula is: The skeletal formula is:

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(c) Ring or Cyclo Alkanes e.g. cyclobutane (C4H8). The displayed formula is: The skeletal formula is:

Student Activity: draw the stated formula for the following:

The effect of structure on physical properties

In a sample of an alkane there are many molecules. Between successive molecules there are weak attractive forces. These are Van der Waals forces of attraction. The attractive forces between branched-chained molecules are less than the forces between straight-chained molecules. This is because the branched-chained molecules cannot get as close together as the straight-chained ones so the forces are weaker and easier to break. The surface contact of branched chain isomers is less than that between straight-chain isomers.

The boiling point, melting point and density of the straight-chained alkane are therefore higher than the corresponding branched-chained isomer.

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a) Displayed formula of 2-methylhexane:

b) Displayed formula of 2,3-dimethylhexan-2-ol:

c) Skeletal formula of cyclopropane: d) Skeletal formula of 2,3-dimethylheptane:

Diagram

Student activity

1. Complete the names of the alkanes in the table below.Number of

carbon atoms present

Name Boiling point / K

Melting point / K

1 112 902 184 1013 propane 231 854 273 1385 309 1436 hexane 342 1787 371 1828 399 2169 nonane 424 219

10 447 24311 undecane 469 24712 dodecane 489 26315 pentadecane 544 28320 eicosane 617 310

2. What trend do you see in the values of the boiling points with increasing numbers of carbon atoms?

The boiling points of the alkanes ………………………… with increasing numbers of carbon atoms.

3. Predict the boiling point of C14H30 Answer……………………………..

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4. How can the trend in boiling points with increasing carbon atoms be accounted for?

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5. In each of the following pairs, underline which of the following alkanes has the highest boiling point. In each case explain your answer.

a) Hexane or 3-methylpentane

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b) 2-methyloctane or 2,2-dimethylheptane

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c) The straight chained molecules C27H56 or C29H60

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Chemical Reactions of the Alkanes

Alkanes show little reaction towards all the common reagents and so there are only a few reactions to consider. The reason for this unreactivity is because:

They have strong single bonds (both C-H and C-C have high bond enthalpies); They have no polar bonds.

A polar bond is formed when two elements of differing electronegativities are joined together. Carbon and hydrogen have similar electronegativities and so the bond is only very slightly polar.

Consequently common chemical reagents do not readily attack alkanes.

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(a) Combustion

In a plentiful supply of oxygen, alkanes burn completely to produce carbon dioxide and water.

CH4 (g) + 2O2(g) → CO2 (g) + 2H2O(g)

Because the process is exothermic, heat is given out. Oxidation of alkanes is the basis for their use as fuels. Natural gas is used for heating purposes and diesel is used for power.

They burn by a free radical reaction in the gas phase. They have to be vaporised before they will burn.

In a limited supply of oxygen alkanes burn to form carbon monoxide and water.

CH4 (g) + 1½ O2 (g) → CO (g) + 2H2O (g)

Student Activity:

Balance the following equations:

1. C2H6 (g) + O2(g) → CO2 (g) + H2O(g)

2. C5H12 (g) + O2(g) → CO2 (g) + H2O(g)

3. C6H14 (g) + O2(g) → CO (g) + H2O(g)

4. C9H20 (g) + O2(g) → CO (g) + H2O(g)

Student activity:

1. a) Write a fully balanced equation for the complete combustion of butane (C4H10) in a plentiful supply of oxygen.

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b) Calculate the number of moles of butane in 2.9g of the liquid.

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c) Calculate the volume of oxygen required to ensure the complete combustion of the butane (assume 1 mole of gas occupies 24dm3 at r.t.p)

2. The principal component of petrol is an isomer octane (C8H18)

a) Write equations for the combustion of octane in

i) A limited supply of oxygen

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ii) A supply of oxygen, which ensures complete combustion to carbon dioxide and water.

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b) Using your equations from part (a) calculate the additional number of moles of oxygen required to prevent the formation of carbon monoxide when one mole of octane is burned.

c) Using your equations from part (a) and (b), calculate the additional volume of air required to burn the same amount of octane without the formation of carbon monoxide ( assume that the air contains 20% oxygen)

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(b) Halogenation

Alkanes do not react with halogens in the dark but in the presence of ultra-violet light they undergo a series of substitution reactions. This is an example of a photo chemical reaction since it only occurs in the presence of light.

For the chemical reaction to take place, the covalent bonds have to be broken homolytically to form free radicals. In these reactions a hydrogen atom in the alkane is substituted by a halogen atom.

The reaction is an example of free radical substitution or halogenation. Your teacher will show you this happening between hexane and bromine in a plastic bag.

Example: Methane and Chlorine

Formation of Chloromethane:

CH4 (g) + Cl2 (g) → CH3Cl(g) + HCl(g) Chloromethane

If more chloromethane reacts with the chlorine

CH3Cl(g) + Cl2(g)→ CH2Cl2(g) + HCl(g)

and more

CH2Cl2(g) + Cl2(g)→ CHCl3(g) + HCl(g)

and more

CHCl3(g) + Cl2(g)→ CCl4(g) + HCl(g)

The reaction continues until all the hydrogens have been replaced with chlorineThe Mechanism for the free radical substitution of alkanes

A mechanism is a sequence of steps leading to the formation of a product.

Reagents: Methane and ChlorineConditions: Ultra-Violet light or 300oC

The mechanism occurs in three steps: -

Step 1: Initiation

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The Chlorine molecule is broken to form two highly reactive intermediates known as free radicals (this is homolytic fission). Once initiation has occurred the reaction can continue without the need of more energy. In other words it is self-propagating and continues by a number of propagation steps.

The absorption of light causing a bond to break in a molecule is a photochemical reaction.

Step 2: propagation

The Chlorine radicals are very reactive and a chain reaction is started, no more energy is now required to break the bonds, as the reaction is self-sustaining.

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The above reactions are rapid and no energy is required to carry out these steps. The reaction continues until supplies of the reagents are depleted.

Step 3: termination

This occurs when the two radicals combine. The energy evolved is dissipated to another molecule or to the side of the reaction vessel.

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The chain reaction ends with two free radicals colliding and combining. The reactions are highly exothermic. In reality all three reactions occur at the same time. This is not a good method for making a pure product as many side reactions occur over the course of the reaction.

As stated previously the reaction can form a mixture of products. This occurs when the product of the first reaction undergoes further reactions with the chlorine radical present in the system. In this case additional propagation steps will occur.

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Second set

CH3Cl + Cl∙ → ∙CH2Cl + HCl

∙CH2Cl + Cl2 → CH2Cl2  + Cl∙ Third set

CH2Cl2 + …… → ∙CHCl2 + ……..

∙CHCl2 + Cl2 → ……….  + Cl∙

Fourth Set

CHCl3 + Cl∙ → ………. + HCl

∙CCl3 + ………. → CCl4  + ………

Student Activity:

Propane reacts with bromine in the presence of UV light. Two of the products formed are 1-bromopropane and hydrogen bromide.

a) Write a balanced equation to show the reaction of propane with bromine to form 1-bromopropane and hydrogen bromide.

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b) Name one other bromine containing product that can be formed in this reaction.

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c) Name the mechanism for this reaction.

…………………………………………………………………………………………….d) Write an equation for the initiation step.

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e) Write equations to show the propagation steps.

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f) Explain, with the aid of an equation how hexane can be formed in this reaction.

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g) Why is this not a suitable method for making 1-bromopropane.

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Extension: Explain with the aid of a propagation step how dibromopropane can be produced.

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Hydrocarbons: Fuels

Crude OilPolythene and PVC are both manufactured from crude oil, which is the primary source of alkanes available to industry today. Crude oil is a mixture of over 150 different hydrocarbons; most of them straight chained alkanes. Crude oil is not very easy to ignite and in itself isn’t very useful. However some of its components are very valuable such as petrol, lubricating oils, heating oils and power station fuel.

The composition of crude oil differs depending on where it is found. The main series of hydrocarbons present are the arenes (based on the ring structure of benzene), the cycloalkanes and the alkanes.Student activity Discuss the variety of uses of compounds derived from crude oil.

Crude oil is the source of many raw materials e.g. for making: -

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Separating the hydrocarbons in crude oil

The hydrocarbons in crude oil must be separated before they can be used and this is done by fractional distillation.

Crude oil is refined so that use can be made of the different hydrocarbons present. Chemists separate mixtures of similar liquids by distillation. The technique relies upon differences in the boiling points of the liquids in the sample.

As crude oil is a complex mixture, it is first broken down into fractions. Each fraction consists of a mixture of hydrocarbons with a much narrower range of boiling points than in crude oil.

The whole process is carried out in a fractionating column. These can be 100m in height and contain a number of steel trays. The crude oil is vaporised at about 650K and enters the fractionating column at the bottom. Vapour passes up the column through the trays via holes. (See diagram below)

Each tray is like a sieve. Liquid flows from one layer to another over a weir.

The bottom of the column is about 340OC and the temperature falls further up the column and is coolest at the top. Any crude oil that is not gaseous at 340OC falls to the bottom of the column and is removed as residue. The other components rise up the column. At the point where a particular component liquefies it is collected in trays. The products collected at different points are called fractions.

Diagram of a fractionating column for the distillation of crude oil

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You won’t have to draw this in an examination.

The fractions available in crude oil

Fraction Boiling point/ OC

Length of chain

Uses

Refinery Gas 20 1-4 Gas (methane) / bottled gasLight Petroleum 20-60 5-6 Solvents/fuelLight Naphtha 60-100 6-7 Feedstock/solvents/fuelPetrol 40-205 5-11 CarsKerosene 175-325 12-18 Fuel/heatingDiesel 275-400 18-25 Engines/cars/fuelLubricating Oil non-volatile 24-34 Lubricants/enginesParaffin Wax non-volatile 25-40 Candles/lightingBitumen non-volatile 30+ Tar/roofing felt

Student Activity:

1. Give the name of the process which separates compounds based on boiling points

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2. What does the term ‘hydrocarbon’ mean?

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3. What is the name given to a group of hydrocarbons with similar boiling points/similar length carbon chains?

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4. Why are long chain hydrocarbon molecules e.g. 30+ carbons in length broken up to make smaller chain molecules?

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Cracking

Petrol is the component of crude oil that is in the most demand. With more and more vehicles coming onto the roads each year, the demand for petrol continues to rise. This demand is greater than the available petrol fraction found in crude oil but scientists have found that short-chained alkanes can be made from longer chained compounds.

Cracking involves heating the oil fraction with a catalyst. The catalyst is often a mixture of Al2O3 and SiO2. Under these conditions long-chained molecules are broken down to give low molecular mass alkanes and alkenes.

In the process C-H and C-C bonds are broken in a random fashion. A variety of products including hydrogen can be produced and branched chained alkanes and alkenes can also be formed. For example:-

C10H22 → C4H8 + C6H14

Cracking is also used to produce alkenes in particular ethene, which is an important raw material in the production of plastic such as polythene.

Student activity

a) Write a balanced equation for the possible products formed when C18H38 is cracked.

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b) Give the structural formulae for the two products formed

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c) The same alkane is cracked and one of the products isolated is 2-methylpentane, draw a full displayed structure for this product and identify a possible alkene formed.

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Isomerisation

This process involves the heating of the straight-chained isomers in the presence of a platinum catalyst. The resulting mixture of straight and branched are then separated using a zeolite molecular sieve.

Example:

Student activityDraw skeletal isomers of C7H16

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Reforming

This involves the conversion of alkanes into cycloalkanes or arenes. Reforming reactions are catalysed by bimetallic catalysts usually platinum and rhodium.

Example:

Alkanes as fuels

Many substances burn in oxygen and transfer energy to the surroundings. Only those used on a large scale however are properly described as fuels. Oxidation of a fuel such as coal, gas and petroleum provides over 90% of the energy used in the developed countries.

A good fuel must burn efficiently with the liberation of energy, which can do work. Alkanes burn effectively and find use as fuels in industry, the home and in transport.

Student activity Discuss whether it is likely that coal or hydrogen could be used to replace petrol as the principal fuel used in personal transport (cars).

Characteristics of fuels

A good fuel should have some or preferably all of the following characteristics:-

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Petrol as motor fuel

Petrol burns most efficiently if the air petrol mixture is compressed in the cylinder before it is ignited. Unfortunately this compression can lead to premature ignition (Auto ignition). The result of this is a sudden increase in pressure, which is heard as a knocking sound in the engine.

Branched chain alkanes and cycloalkanes produced by cracking or reforming cause less knocking than straight chained alkanes. Because of this branched chained and cycloalkanes are used in petrol to promote efficient combustion.

Alcohol as fuel

In 1975 Brazil set up the world’s biggest programme for the production of an alternative fuel to petroleum. Brazil does not have large deposits of oil for self-sufficiency but it does have a climate ideal for growing sugar cane. Today an alcohol, ethanol produced from sugar cane is used to power 33% of Brazil’s 12 million cars.

Ethanol fuel can be used in spark ignition engines and in diesel engines. The engines have to be modified so that they can stand ethanol because the fuel would corrode traditional engines. The fuel tank has to be coated in tin and the fuel lines with nickel.

Methanol can be used to power racing cars. Its main advantages are that it burns cleanly and completely and little carbon monoxide is produced. It has some disadvantages in that it is more toxic than ethanol and it provide less energy per litre than petrol.

Student Activity:

Balance the following equations:

1. CH3OH + O2 → CO2 + H2O

2. C4H9OH + O2 → CO2 + H2O

3. C3H7OH + O2 → CO + H2O

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Student Activity: Fuels and sustainability

1. For each of the following:- Write a balanced equation for the reaction;- Delete as appropriate from the statements about renewability.

i) Burning ethanol as a fuel.

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Ethanol is renewable/non-renewable when made from sugar.

Ethanol is renewable/non-renewable when made from ethene.

ii) Burning hydrogen as a fuel.

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Hydrogen can be produced by the electrolysis/ cracking of water.

Hydrogen produced from cracking alkanes would be renewable/non-renewable.

Hydrogen is renewable/non-renewable when made from water.

2. Suggest a reason (other than renewability of resources) why hydrogen may be a better fuel than octane the main component of petrol) or ethanol.

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The Greenhouse Effect

Greenhouse gases, which include water, ozone, carbon dioxide, methane, and nitrous oxide, are able to absorb and re-emit infrared radiation, while not blocking visible light. The greenhouse effect occurs when solar radiation is absorbed by the Earth’s surface, converted into heat, and subsequently emitted into the atmosphere as infrared radiation.

Some of this infrared radiation escapes into outer space; however, some is absorbed and re-emitted back towards the earth’s surface by greenhouse gases. These returning waves of infrared radiation warm the troposphere and the earth’s surface once again.

The greenhouse effect creates an equilibrium. The Earth’s surface and atmospheric gases absorbs energy at the same rate as it radiates energy, thus maintaining a steady temperature.

Radiation from the sun is absorbed by the Earth as radiant visible light. Eventually, the heat from the Earth is re-emitted into the atmosphere as infra-red radiation (IR). Certain gases in the atmosphere have the property of absorbing IR. Oxygen and nitrogen, the major gases in the atmosphere, do not absorb IR.

IR strikes a molecule such as carbon dioxide and causes the bonds to bend and vibrate (this is the absorption of IR energy). IR is absorbed by the bending and stretching or bonds. The molecule gains kinetic energy by this absorption of IR radiation and this extra kinetic energy may then be transmitted to other molecules such as O2 and N2 and causes a general heating of the atmosphere.

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IR is absorbed by C=O, O-H and C-H bonds in water, carbon dioxide and methane. It is these three absorptions that mainly contribute to global warming. Minor contributions are also made from NOx. The effect of each of these greenhouse gases is dependent on their atmospheric concentrations, their residence time (how long they stay in the atmosphere) and their ability to absorb IR.

Student Activity: True or False

Identify whether the following statements are true or false.

1. Carbon dioxide is the only greenhouse gas.

2. The greenhouse effect occurs when solar radiation is absorbed, converted into

heat and emitted as infra-red radiation.

3. Greenhouse gases absorb infra-red radiation.

4. The concentration of each gas in the atmosphere has no effect on the

greenhouse effect.

5. Oxygen and nitrogen also absorb infra-red radiation.

6. Greenhouse gases absorb visible light.

7. Infra-red radiation causes bonds to bend and vibrate.

8. Methane and water are greenhouse gases.

9. The transfer of kinetic energy produced by the absorption of IR radiation

causes the atmosphere to heat up.

10. Gases that have a greater ability to absorb infra-red radiation have more

of an effect as a greenhouse gas.

11. NOx gases are not greenhouse gases. They merely contribute to acid

rain levels.

12. A greenhouse gas with a short residence time is less problematic than

one with a long residence time.

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Discussion Activity:

1. Which of the alternative energy sources below are most likely to replace fossil fuels in the UK? (Underline the 3 most likely)nuclear fission/fusion, wave power, solar power, wind power, bio-fuel,Explain your choices.

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2. Why is research into alternative fuel sources for transport important?

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http://www.energyzone.net/ has some interesting information for you

Carbon Capture and Storage (CCS)

This process stores CO2 underground safely rather than it being released into the atmosphere.

Why is this good idea?

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Write an equation to show how gases like methane burn completely in oxygen.

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Storage

Porous rocks ( also called saline aquifers) under the ground can act as huge sponges to soak up unwanted carbon dioxide. Natural containers exist that once contained crude oil and gas and these could be filled with CO2. There are many of these already in the North Sea. In fact, it is estimated that in the North Sea we have space for about 20,000,000,000 tonnes of carbon dioxide. That’s a lot! About 40 years of emissions! Existing Oil Rigs (not Oxidation is loss, Reduction is Gain!) could be inexpensively converted to pump CO2 into these containers. Also the carbon dioxide could be stored deep in the oceans or on the sea bed.

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Decarbonised fuels can be made by combining (reforming) methane and water to make hydrogen and CO2. Equation:

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The CO2 produced could be stored underground and the hydrogen used as a “clean fuel”. Equation: for hydrogen combustion

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Explain why it is ‘clean’.

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Carbon dioxide could also be stored by making limestone (Mineral Storage). This involves combining CO2 with calcium oxide. Equation:

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This is a natural but very slow process. Efforts are being made to speed up this process, but so far it is very energy demanding.

Have a look at these websites. http://www.co2storage.org.uk/ and http://www.bbc.co.uk/news/uk-scotland-12712913

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Minimising Climate Change

The increase in atmospheric levels of greenhouse gases has led to an increase in global warming and scientists have to devise ways of minimising the climate change. They do this by:

- providing scientific evidence to governments to prove global warming is taking place;

- investigating solutions to environmental problems such as carbon capture and storage;

- monitoring the progress against initiatives such as the Kyoto protocol.

Exam Question: Jan 2010 Question 7

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Student Task:

If you have not studied ‘The Greenhouse Effect’ as a topic at GCSE or your memory is a little hazy then you need to make notes on the following. This is not a homework or class exercise – part of your study time should be used to complete this task.

http://www.nasa.gov/worldbook/global_warming_worldbook.html http://www.elmhurst.edu/~chm/vchembook/globalwarmA.htmlhttp://www.co2capture.org.uk/http://news.bbc.co.uk/2/hi/europe/2233897.stmhttp://en.wikipedia.org/wiki/Kyoto_Protocol

Using your text book or the websites given above as a starting point to produce a summary on Global Warming to include:

- sources of greenhouse gases;- possible local effects of the enhanced greenhouse effect;- possible global effects of the enhanced greenhouse effect;- evidence that global warming is taking place;- methods of controlling or reducing levels of CO2;- how the Kyoto Protocol can help;- the EU strategy for the future.

You should use each of the above as headings. Once completed, print them out and add them to your notes.

End note: You are required to include in your notes:

i) the value of fossil fuels in relation to needs for energy and raw materials;ii) the non-renewable nature of fossil fuels;iii) the need to develop renewable fuels, for example biofuels, which do not

further deplete finite resources.

You should also read about alternative forms of energy.

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