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•Carbon dioxide (CO2), methane (CH4) andwater vapour (H2O) are all greenhouse gases.(They trap the Earth’s radiated infra red energy

in the atmosphere).

•Water is the main greenhouse gas (but isnatural), followed by carbon dioxide andmethane.

The Earth is thought to be gettingwarmer, and many scientists

believe it is due to increasingamounts of greenhouse gases inthe atmosphere.

Concentrations of Carbon dioxide in the atmosphere have risensignificantly in recent years due to increasing burning of fossil fuels.

Carbon dioxide is a particularly effective greenhouse gas and itsincrease is thought to be largely responsible for global warming.

Carbon Capture and Storage, CCSCarbon dioxide could potentially be stored in anumber of different ways• removal of waste carbon dioxide as a liquid

injected deep in the oceans or on the sea-

bed.• storage in geological formations or under

the sea-bed by the reaction of carbondioxide with metal oxides to form stablesolid carbonates, which can be stored

Chemists can also develop• alternative energy sources such as developing fuel

cells or developing solar power or fuels that do notproduce CO2

• Develop more efficient engines for transport or lean

burn engines• Develop uses for carbon dioxide e.g. dry cleaning or

making decaffeinated coffee OR blowing agent inpolymer or making fizzy drinks.

2.4 Module 4: Resources

2.4.1 Chemistry of the Air

The ‘Greenhouse Effect’

Infrared radiation is absorbed byC=O, O–H and C–H bonds in H2O, CO2

and CH4.

These absorptions contribute toglobal warming

The ‘Greenhouse Effect’ of a given gas isdependent both on its atmospheric concentrationand its ability to absorb infrared radiation andalso its residence time. (Time it stays inatmosphere)

Role of chemists in minimising climate change resulting from global warming

Chemists provide scientific evidence to governments toverify that global warming is taking place such asmonitoring atmospheric changes

Chemists are investigating solutions to environmentalproblems, such as developing carbon capture andstorage, CCS.

Chemists can monitor progress against initiativessuch as the Kyoto protocol

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The Ozone Layer

The naturally occurring ozone (O3) layer in the upperatmosphere is beneficial as it filters out much of the sun’sharmful UV radiation

Ozone in the lower atmosphereis a pollutant and contributestowards the formation of smog

Ozone is continuously being formed and broken downin the stratosphere by the action of ultraviolet radiation

Ozone formationUV light causes an O2 molecule to split into free radicals

O2 + UV-light → O + OWhen the free radical hits another O2 molecule ozoneforms

O + O2  → O3

Ozone depletionThis is the reverse of the formation reaction..The energy is supplied by ultraviolet light

O3 + ultraviolet light O2 + O

There is a continuous cycle of formationand depletion of ozone

rate of ozone formation = rate of ozone removal

So there is a constant amount of ozone inthe atmosphere

The frequency of ultra-violet light absorbed equals the

frequency of biologically damaging ultra-violetradiation. These reactions therefore filter out harmfulUV and allow life to survive on earth.UV light can increase risk of skin cancer and increasecrop mutation.

Radicals from CFCs, and NOx from thunderstorms oraircraft, may catalyse the breakdown of ozone

O + O2 O3

Destruction of Ozone Layer Chlorine radicals are formed in the upperatmosphere when energy from ultra-violetradiation causes C–Cl bonds inchlorofluorocarbons (CFCs) to break

CF2Cl2  → CF2Cl + Cl

Cl. + O3   ClO. + O2

ClO.

+ O. O2 + Cl

.

Overall equation

O3 + O. 2 O2

The chlorine free radical atoms catalyse thedecomposition of ozone due to these reactionsbecause they are regenerated. (They provide analternative route with a lower activation energy)

They contributed to the formation of a hole in theozone layer.

The regenerated Cl radical meansthat one Cl radical could destroymany thousands of ozone moleculesNO + O3   NO2 + O2

NO2 + O

.

O2 + NOOverall equation

O3 + O. 2 O2

Legislation to ban the use of CFCs was supported bychemists and that they have now developedalternative chlorine-free compounds

HFCs (Hydro fluoro carbons) e.g..CH2FCF3 are now used for refrigeratorsand air-conditioners. These are safer asthey do not contain the C-Cl bond

CFC’s still concern us because CFCs are still entering the atmosphere from disused items and are still usedfor some purposes and by some countries.

CFCs have a long lifetime in the atmosphere and it takes a long time for CFCs to reach upper atmosphere.

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Incomplete combustion producesless energy per mole thancomplete combustion

If there is a limited amount of oxygen then  incomplete  combustionoccurs, producing CO (which is very toxic) and/or C (producing a sootyflame)

CH4(g) + 3 / 2 O2(g) → CO(g) + 2 H2O(l)

CH4(g) + O2(g) → C(s) + 2 H2O(l)

Incomplete Combustion

Carbon (soot) can cause globaldimming- reflection of the sun’slight

Controlling air pollution

Nitrogen oxides form from the reaction between N2 and O2 inside the car engine.The high temperature and spark in the engine provides sufficient energy to break strong N2 bond

N2 + O2 2NO N2 + 2O2 2NO2

Nitrogen Oxides NOx

Pollutant Environmental consequence

Nitrogen oxides (formed when N2 in the airreacts at the high temperatures and spark inthe engine)

NOx are toxic and can form contribute toformation of photochemical smog and lowlevel ozone.

Carbon monoxide toxic

Carbon dioxide Contributes towards global warming

Unburnt hydrocarbons (not all fuel burns in theengine)

Contributes towards formation ofphotochemical smog

soot Global dimming and respiratory problems

Catalytic converters

These remove CO, NOx and unburned hydrocarbons (e.g. octane,C8H18) from the exhaust gases, turning them into ‘harmless’ CO2, N2

and H2O.

2 CO + 2 NO   → 2 CO2 + N2

C8H18 + 25 NO   → 8 CO2 + 12½ N2 + 9 H2O

Converters have a ceramichoneycomb coated with a thinlayer of catalyst metalsPlatinum, Palladium, Rhodium – to give a large surface area.

How Catalytic converter works• CO and NO are adsorbed onto surface of catalyst

• This weaken bonds in the CO and NO, therebylowering activation energy

• Reaction occurs• CO2 and N2 desorbs from the surface

Bonding to the catalyst surface

must be weak enough foradsorption and desorption totake place but strong enoughto weaken bonds and allowreaction to take place.

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2.4.2 Green Chemistry

Sustainability

Apparent benefits may be offset by unexpected and detrimental side effects. Note theadvantages and disadvantages of using biofuels.

The five principles of chemical sustainability:

(i) using industrial processes that reduce or eliminate

hazardous chemicals and which involve the use of fewerchemicals

(ii) designing processes with a high atom economy thatminimise the production of waste materials,

(iii) using renewable resources such asplant-based substances,

(iv) seeking alternative energy sources such as solarenergy, rather than consuming finite resources such asfossil fuels that will eventually be exhausted,

(v) ensuring that any waste products produced are non-toxic, and can be recycled or biodegraded by beingbroken down into harmless substances in theenvironment;

e.g. reduce the need to use finite rawmaterials such as crude oil

e.g. less toxic, corrosive waste products

less harmful or corrosive reactants

e.g. Develop ways of making polymers/fuels from plant-based substances

Using more readily available startingmaterials

e.g. Designing processes with highatom economy that minimiseproduction of waste products.

Develop biodegradable polymers

Recycle heat in a process to heat theincoming reactants

The importance of establishing international cooperationto promote the reduction of pollution levels;

Advantages of using BiofuelsReduction of use of fossil fuels which are finite resourcesbiofuels are renewableUse of biodiesel is more carbon-neutralAllows fossil fuels to be used as a feedstock for organiccompoundsNo risk of large scale pollution from exploitation of fossilfuels

Disadvantages of BiofuelsLess food crops may be grownLand not used to grow food cropsRain forests have to be cut down toprovide landShortage of fertile soils

• Pollution travels across country borders and allcountries contribute towards pollution.

• Cooperation means that scientists can share ideas andcan warn governments of risk.

• World-wide legislation can be introduced• Allows monitoring of pollution in different countries• Richer countries can help poorer countries introduce

pollution controls