topic 3 – chemical monitoring and management - acehsc.net€¦ · web viewthey study organic...

Topic 3 – Chemical Monitoring and Management

By: Raymond Chen


11 – THE INDUSTRIAL CHEMIST

11.1 – THE WORK OF AN INDUSTRIAL CHEMIST

THE ROLE OF A WORKING CHEMIST Chemists employed in the industrial sector have many roles including:

o Development Chemist - Designing chemical processes for the manufacture of a chemical product to ensure the rate of the reaction and yield product are optimised.

o Production Chemist - Working with chemical engineers to designing the equipment to carry out the industrial process

o Research Chemist - Undertaking ongoing research to improve the product or process or to develop new products.

Teamwork , collaboration and communication skills are important for chemists. A company has many chemists that are skilled in different areas. There are a variety of chemists, including:

o Environmental chemist : Employed by a wide variety of organisations, including mining. Developed expertise in analytical chemistry. They collect, analyse and assess environmental data from the air, water

and soil.o Metallurgical chemist :

They have a high understanding of metals, alloys and ores and their reactions.

They specialise in all aspects of the use and development of metals and alloys in society.

They design and monitor methods of extracting metals from ores.o Biochemists :

They help determine the chemical structure and functions of molecules in living things.

They study organic chemistry and biochemistry They can be employed in areas including pharmaceutical laboratories,

hospitals and in the food and agricultural industries.

MONITORING COMBUSTION REACTIONS The combustion of alkanes obtained from petroleum is a major source of heat and

power. Though they are stable in oxygen, they are combustible when ignited - the products

from this are carbon dioxide and water. In a space where is a plentiful supply of oxygen the reaction is:

2C8H18(g )+25O2 (g)→16CO2(g )+18H2O(l)

However when there is a lack of oxygen - like in a car engine - then incomplete combustion may occur:

2C8H18(g )+20O2 (g)→8CO2 (g )+6CO(g)+2C(s)+18H 2O(l)

CATALYTIC CONVERTERS AND EMISSION CONTROL1 By: Raymond Chen


Industrial chemists have developed catalysts that help to reduce carbon monoxide, nitrogen oxide and unburnt hydrocarbon emissions from vehicle exhausts.

Catalytic converters are made from alloys of rhodium and platinum - they speed up reactions that convert pollutant gases to materials which are present in the air naturally.

The aim of this is to convert otherwise dangers NO and CO to N2 and CO2 respectively and also unburnt hydrocarbons into water.

11.2 – THE HABER PROCESS

THE USES AND IMPORTANCE OF AMMONIA Ammonia is a gas that produces an alkaline solution when dissolved in water. Solutions of ammonia in water are used domestically are cleaning agents and also as

refrigerant gas. Ammonia is the feedstock for a large variety of industrial chemicals. Fertilisers account for over 80% of worldwide use of ammonia.

Industrial Product Derived from Ammonia

Use of Product

Urea Ammonium Sulfate Ammonium Nitrate Ammonium Hydrogen Phosphate

Fertilisers

Nitric Acid Production of explosives Nitrate salts Strong laboratory acids

Acrylonitrile Acrylic plastics Diaminoalkanes Nylon plastics Cyanides Extraction of gold from gold veins Hydrazine Rocket propellant Sulfonamides Antibiotic drugs Aniline Derivatives Dyes Alkylammonium Hydrocarbons Cationic detergents

INDUSTRIAL MANUFACTURE OF AMMONIA The production involves balancing the conditions of the reaction so that the products

are produced at a fast rate and the quantity is of the product is maximised. Ammonia is manufactured by a process developed by Fritz Haber in early 20th century.

o It was manufactured from its component gas elements - the reaction is exothermic.

N 2 (g)+3H 2(g )⇌ 2NH 3(g )∆=−92kJ /mol

At the standard conditions of temperature and pressure lies to the left - Haber process changes this.

o Conditions in the Haber process make the manufacturing of ammonia viable.

FEEDSTOCKS FOR THE HABER PROCESS The ammonia industry requires nitrogen and hydrogen.

2 By: Raymond Chen


o They are derived from air, water and natural gas. The ammonia produced is not only used to make solid fertiliser, but is also directly

applied to the soil in anhydrous gaseous form.

NITROGEN Filtered air is the source of nitrogen for the Haber process. Air contains ~78% nitrogen by volume.

o An expensive method for nitrogen extraction is to fractionally distil liquefied air. Nitrogen is more commonly extracted from air using chemical reactions

involving natural gas or methane.

HYDROGEN Hydrogen can be obtained via

the electrolysis of salt water - but it is too expensive.

Hydrogen is derived from the steam reforming of natural gas - CH4.

Extraction of hydrogen is as follows:

o Natural gas is purified to remove sulfur compounds through a cobalt/nickel/alumina catalyst.

o Hydrogen is extracted by reacting natural gas with steam at about 750°C with nickel catalyst - primary steam reforming - 90% of methane is consumed.

o The introduction of air produces steam with nitrogen remaining unreacted - high temperatures ~1000°C ensures combustion of almost all methane.

o Carbon monoxide is removed by passing it over two different catalysts - iron oxide (~400°C) and cooper (~200°C)

CO must be removed as it is poisonous - 0.2% remaining Reaction is exothermic - heat is recovered for further use.

o CO2 is removed by neutralisation with potassium carbonate under pressure - the decomposed potassium hydrogen carbonate is stored for further use.

There must not be any oxygen as it is explosive with hydrogen under high pressure and temperature.

The final gaseous mixture contains nitrogen and hydrogen in a ratio of 1:3 - very small amounts of methane and argon are present.

HABER PROCESS The Haber Process is as follows:

o Reactants pass through the catalytic reactors

o The mixture is cooled to condense out the ammonia formed

3 By: Raymond Chen


o The ammonia is drained out as required with the unreacted gas fed back into the catalyst chamber with incoming reactants

o None of the reactant mixture is wasted .

HABER PROCESS The conditions present in the Haber process is a compromise between temperature

and pressure and also kinetic factors.

N2 (g)+3H2(g )⇌ 2NH 3(g )∆=−92kJ /mol

EQUILIBRIUM FACTORS Temperature:

o The Haber process is an exothermic reaction - high temperature, low yield.

o According to L.C.P. - Lower the temperature the higher the yield.

Pressure:o Stoichiometric equation

shows that there are 4 moles of reactants for 2 moles of products - increased pressure, high yield.

o According to L.C.P. - Higher the pressure, the higher the yield .

KINETIC FACTORS Kinetic factors relate the speed at which reactions occur and how rapidly the

ammonia is formed. High temperatures increase kinetic energies of molecules therefore increased

productivity. High pressures increase frequency of collisions therefore increased productivity. The presence of a catalyst increases the reaction rate.

ECONOMIC FACTORS Constructing strong pipes and maintaining a high-pressure reactor vessel is very

expensive - therefore selected pressure should not be too high. Ammonia plants should be located locally with natural gas. Heat is not wasted as they are recycled in heat exchangers. Carbon dioxide is not wasted as it is used to manufacture urea and sold to brewers

and soft drink manufacturers.

COMPROMISE CONDITIONS The conditions in the manufacturing of ammonia vary but these are some of the typical

ranges:o Reactants : N2 and H2 (ratio 1:3) can be shifted to the left by increased

concentration - stoichiometric ratio must be maintained.o Pressure : 15-35MPa - although it should be as high as possible, but

economic and safety concerns require the pressure to be lower.

4 By: Raymond Chen


o Temperature : 400°C-550°C - equilibrium and kinetic factors are a problem; a compromise has to be struck so that the activation energy level can be reached.

o Catalyst : Magnetite (Fe3O4) - fused with K2O, Al2O3, and CaO - it is then reduced to porous iron. By grinding the iron catalyst to produce maximum surface area - this allows low temperatures and low pressures to be used.

MONITORING AND MANAGEMENT The Haber process must be monitored and managed for productivity maximisation

and safety concerns. Reasons include:

o Feedstock must be pure and free contaminants - they interfere with yield and can damage the catalyst.

Oxygen must not be present as it is explosive .o Ratio of nitrogen and hydrogen must be kept at 1:3 for optimum

production.o Temperature and pressure should be maintained - temperature too high can

damage the catalyst, pressure too high may cause the vessels to rupture.o Overtime, minor gases in the atmosphere such as argon and inert gases

accumulate - they need to be removed when it reaches 5%.o Remove ammonia at regular intervals to ensure no impurity contaminationo Structural integrity of reaction vessel must be maintained.

Monitoring devices are connected to critical parts of the containment vessels. Electronic devices sound alarms when values fall outside acceptable limits.

12 – THE ANALYTICAL CHEMIST

12.1 – IDENTIFICATION OF IONS

ANION ANALYSES Chemists analyse materials

for the presence of specific cations and anions.

Anions can be identified and distinguished using a variety of simple qualitative tests involving the formation of gasses or precipitates.

There is a series of elimination tests conducted in strict order.

Then there are additional confirmation tests.

Solubility rules include:o Nitrate salts are

soluble - no precipitation of cations

o Group 1 salts are soluble - no precipitation

5 By: Raymond Chen


Anion Soluble Slightly Soluble InsolubleCO3

2−¿ ¿ Na+¿ , K+¿, NH4+¿¿¿ ¿ - Most

Cl−¿¿ Most Pb2+¿ ¿ Ag+¿¿

OH−¿¿Na+¿, K+¿, NH4

+¿ , Ba2+¿¿¿¿ ¿ Ca2+¿ ¿ MostPO4

3−¿¿ Na+¿ , K+¿, NH4+¿¿¿ ¿ - Most

SO42−¿ ¿ Most Ca2+¿ , Ag+¿¿¿ Ba2+¿ , Pb2+¿¿ ¿

ANION ELIMINATION TESTS To test for unknown solutions, there is a listed sequence. It is known as the elimination sequence - must be done in order to prevent invalid

conclusions.

Anion

Procedure Observation/Conclusion

CO32−¿ ¿ Add 2mol/L nitric acid Effervescence of colourless gas

(CO2) indicates a carbonateUse limewater to confirm

CO32−¿+2 H +¿→CO2(g)+H 2O( l) ¿¿

Confirmation Test: Test the original solution with pH paper

If solution is alkaline then the results are true.CO3

2−¿ ¿ + H 2O(l)⇌HCO3−¿+OH−¿ ¿¿

SO42−¿ ¿ Acidify the unknown solution with nitric acid

and add drops of dilute barium nitrateA white precipitate of barium sulfate indicates sulfate ions are present

SO42−¿+Ba2+¿→BaSO 4(s )¿ ¿

Confirmation Test: Add drops of lead nitrate solution

A white lead (II) sulfate precipitate forms

SO42−¿+Pb2+¿→PbSO 4(s ) ¿¿

Cl−¿¿ Acidify the unknown solution with nitric acid and add silver nitrate solution

A white precipitate of silver chloride

Cl−¿+Ag+¿→AgCl (s ) ¿¿

Confirmation Test: Add ammonia solution then heat in water bath

White precipitate should dissolvedAgCl(s)+2NH 3→ Ag¿¿

PO43−¿¿ Add drops of ammonia solution then solution

of barium nitrateWhite precipitates forms

2 PO43−¿+3Ba2+¿→Ba3¿ ¿¿¿

Confirmation Tests:Add ammonium molybdate and warm the mixtureAcidify the solution with sulfuric acid, then add ammonium molybdate and ascorbic acid

A yellow precipitate of ammonium phosphomolybdate forms

A blue complex forms

CATION ANALYSISCOLOUR OF SOLUTION

In aqueous solution many cations are colourless - but some are distinctive in colour

Hydrated Cation Solution ColourFe3+¿¿ Yellow-orange to pale yellow

6 By: Raymond Chen


Fe2+¿¿ Pale green to colourlessCu2+¿ ¿ Blue to green-blue

FLAME TESTS Many metal ions produce characteristic colours when their salts are heated - flame

test Some metal ions produce

characteristic flame colours. Chloride salts of various

cations work best. As an atom is heated the

electrons in the atom moves to a higher energy level but it is unstable hence they fall back.

According to the Law of Conservation of Energy the energy in the electron is emitted in the form of a frequency in the electromagnetic spectrum - coloured photons.

There are two ways to perform the flame test:

o Dip a platinum wire into concentrated HCl to clean it.

o Heat the wire to remove impuritieso Dip the wire into acid and then into powdered salt so that the salt stickso Heat using Bunsen burner and colour is displayed in the flame

Dissolve the chloride salt in water and spray the resulting solution into the blue Bunsen flame using an atomiser.

o Sodium may sometimes mask the colour of the unknown metal – it has a strong yellow colour.

Cation Flame ColourCalcium Brick redBarium Yellow-greenCopper GreenSodium YellowStrontium Scarlet-RedCATION ELIMINATION TESTS

Like with anions, a series of elimination tests are carried out. These elimination tests are based on the formation of precipitates in solutions of

varying pH. The cation solutions should have a minimum concentration of 0.1 molar.

Cations Procedure Observation/ConclusionPb2+ Add hydrochloric acid White precipitate indicates lead ions

Pb2+¿+2Cl−¿→PbCl2( s) ¿ ¿

Lead chloride is soluble in hot waterConfirmation Test: Add drops of sodium iodide to original solution

Yellow precipitate formsPb2+¿+2 I−¿→PbI2( s) ¿ ¿

7 By: Raymond Chen


Ba2+, Ca2+

Add sulfuric acid White precipitate indicates either barium or calcium ions

Ca2+¿+SO 42−¿→CaSO4(s )¿ ¿

Ba2+¿+SO42−¿→BaSO4( s) ¿¿

Confirmation Test:Add solution of sodium fluorideConduct flame test

White precipitate confirms calcium - no precipitate confirms bariumBrick red - calcium

Ca2+¿+2F−¿→CaF2( s) ¿¿

Yellow-green - bariumCu2+ Add sodium hydroxide then

add ammonia solutionBlue precipitate forms from an original blue-green solution - precipitate dissolves in ammonia to form deep blue solution

Cu2+¿+2OH−¿→Cu(OH )2( s) ¿ ¿

Cu(OH )2 (s)+4NH 3→Cu ¿¿Confirmation Test: Conduct flame test

Green flame

Fe2+, Fe3+

Add same of sodium hydroxide Brown precipitate indicates Fe3+

Fe3+¿+3OH−¿→Fe (OH )3(s )¿ ¿

Greenish precipitate indicates Fe2+ - rapidly turns brown

Fe2+¿+ 2OH−¿→Fe (OH )2(s) ¿ ¿

Confirmation Test: Add HClAdd potassium hexacyanferrate reagentAdd potassium thiocyanate reagent

Dark blue indicates Fe2+

3 Fe2+¿+2 Fe(CN )63−¿→Fe3¿ ¿¿

Deep blood red indicates Fe3+

Fe3+¿+SCN−¿→FeSCN2+ ¿¿¿¿

QUANTITATIVE ANALYSIS There are a variety of techniques to determine the amount or concentration of an

element, ion or compound in the sample. These techniques include:

o Gravimetric Analysis - involves weighing materials and determining the percentage composition of elements

o Volumetric analysis - involves measuring the volume of solutions that react with other solutions

o Instrumental analysis - involves the use of special instruments that can determine the concentration or amount of material by measuring a property of the material.

12.2 – INSTRUMENTAL ANALYSIS

ATOMIC ABSORPTION SPECTROSCOPY (AAS) Atomic vapours selectively absorb and emit various frequencies of light. When a sample of an element is vapourised in a hot flame, electrons are promoted

from the ground state into unstable or excited energy levels. As the electrons fall back to more stable levels they emit light through

characteristic frequencies.8 By: Raymond Chen


If white light is passed through an atomic vapour at a suitable low temperature, some wavelengths are selectively absorbed and dark lines appear the in the spectrum produced.

The dark lines correspond to the exact bright line wavelengths in atomic emission spectra.

The AAS was developed by CSIRO scientist Alan Walsh - AAS uses the exact principles as above.

This technique is very sensitive - it can detect concentrations in part per million and parts per billion.

HOLLOW-CATHODE LAMP SELECTION The light source in the AAS is usually a hollow-cathode lamp of the element. Specific wavelengths of light characteristic of the elements being analysed are

generated from this lamp.

STANDARD SOLUTION PREPARATION A standard solution of the metal being analysed is prepared using standard volumetric

techniques.

ASPIRATING THE SOLUTIONS The dilution standards and the unknown solution are sprayed or aspirated into the

flame or graphite furnace. The flame in the AAS is about 1000C to increase absorbance of light. The graphite furnace is about 3000C - it is more efficient.

MEASURING LIGHT ABSORPTION As the light beam passes through the vapourised sample, some of the light is absorbed. A second reference beam passes through a monochromator which contains a

diffraction grating and focussing mirrors. The light then passes through a narrow slight to select only one of the wavelength

bands - the light is now monochromatic. Photomultiplier tubes are used the measure the light intensity and convert it into an

electrical signal.

CALIBRATION Concentration measurements are determined from a calibration curve created with

the standard solutions. A control blank is also run - it should indicate zero.

MONITORING TRACE ELEMENTS AND POLLUTANTS IN THE ENVIRONMENTESSENTIAL TRACE ELEMENTS

There are many elements that are needed in small quantities by plants and animals for the proper function or their physiological processes.

There trace elements include copper, zinc, cobalt and molybdenum. The advent of AAS has allowed for a deeper understanding of trace elements and its

composition in organisms and the environment.

Metal Function9 By: Raymond Chen


Copper Haemoglobin formation and enzyme actionZinc Enzyme action, metabolism of amino acids and insulin synthesisSelenium Enzyme actionManganese

Enzyme action, blood clotting, carbohydrate and fat metabolism

Cobalt Red blood cell formationChromium

Required for carbohydrate, fat and nucleic acid metabolism

Iodine Proper functioning of the thyroid glandUSES OF AAS

AAS is capable of detecting the presence of well over sixty metals in minute concentrations.

It can be used for:o To test the purity of metallic samples in the mining industryo Monitor pollution levels in waste waters - especially heavy metalso Detect harmful levels of metals in organismso Monitor dangerous air-borne metallic particleso Quality control of alloyso Detect minute contaminants in food

WHY MONITOR CATIONS AND ANIONS Phosphate occurs in waterways at low concentrations and essential for normal aquatic

plant growth. At high concentrations can lead to:

o Algal bloomo Covers surface of lakeo Prevents penetration of light - hence plants and fish dieo Algae dies when phosphate is used upo Decay uses oxygen in water

Zinc and copper:o Desirable in small concentrations in water bodieso High concentrations are harmful to humans and cause poisoningo Lead is poisonous - intellectually retards young children and causes brain

damage.o Was widely used in petrolo Was a constituent of house paint

CHAPTER 13 – ATMOSPHERIC CHEMISTRY

13.1 – CHEMISTRY OF ATMOSPHERIC POLLUTION AND OZONE DEPLETION

COMPOSITION AND STRUCTURE OF THE ATMOSPHERE The atmosphere is a thin gaseous layer that extends to a distance of about 600 km

above the Earth's surface.

TROPOSPHERE The troposphere is the layer closest to the ground. 75% of mass in concentrated in the troposphere. The air pressure is also the highest - 100kPa on the ground.

10 By: Raymond Chen


At 15km altitude the air pressure drops to 10kPa. Temperature decreases with

increase altitude. 15C at the bottom and -50C to

-60C at the tropopause. The transfer of gases of

pollutants across the tropopause is slow.

Water vapours freezes before reaching the stratosphere as to prevent water loss

The tropopause is at a higher altitude above the equator than at the poles due the expansion of air.

STRATOSPHERE Air pressure continues to decrease with altitude and it drops to about 0.1kPa at the

stratopause. In the first 9km temperature is uniform but increases thereafter. The ozone layer is situated within the stratosphere. The greatest concentration is about 25km altitude The higher layers of the ozone are warmer as they absorb UV-B and some UV-C rays. There is a very little of mixing gases as temperature increase with altitude and

prevents convection currents. Pollutants that enter remain for a long time 99.9% of Earth's atmosphere is present in the troposphere and stratosphere. The average temperature is about -2C to 0C.

MESOSPHERE AND THERMOSPHERE Air pressure continues to decrease from about 0.1kPa to 0.01kPa. Temperature decreased with altitude to about -90C at the mesopause at 85km. Above the mesosphere is the thermosphere. Temperature rises again due to high frequency radiation. The thermosphere is about 600km thick. The ionosphere is within this region. Temperatures can reach 1700C.

COMPOSITION OF THE ATMOSPHERE The concentration of total gas particles drop with increasing altitudes - proportions

remains constant The amount of water vapour in the atmosphere varies between 1-5%

Gas Concentration

Nitrogen 78%Oxygen 21%Argon 0.9%Carbon Dioxide

0.04%

Neon 0.002%Helium 0.0005%

11 By: Raymond Chen


For most gases the concentrations are best expressed in parts per million - 1ppm = 1mL/kL

LOWER ATMOSPHERE POLLUTANTS The atmosphere gets polluted by both natural processes and human activity. Volcanoes release toxic gases and lighting produces nitrogen oxides and ozone.

Pollutant SourcesCarbon Dioxide Combustion of fossil fuels in industries and vehicles

DeforestationDecomposition of organic matter

Carbon Monoxide Incomplete combustionForest fires

Methane Anaerobic decomposition of organic matterNitrogen Oxides (NOx)

Combustion of organic matterInternal combustion enginesFactories

Sulfur Dioxide Internal combustion enginesSulfide ore smeltingSome chemical manufacturingVolcanic gases

Chlorofluorocarbons (CFCs)

Manufactured chemicals used in aerosols, refrigerants, foams and air conditioners

Particulates Dust - from industrial and domestic activitySoot from fires, burn-offs

Ozone Photochemical smog

COORDINATE BONDING Coordinate bonding is a special case of covalent bonding. It is also commonly known as coordinate covalent bonding A coordinate bond forms when one atoms provides both electrons for the shared pair

o Ammonium ions Forms when ammonia reacts with hydrogen ions Non-bonding pair of electrons on the nitrogen atom is shared with the

hydrogen iono Hydronium ions

Oxygen atom in water has two non-bonding electron pairs Hydronium ions form when these non-bonding pairs are shared with a

hydrogen ion.

o Carbon monoxide Both covalent and coordinate

bonds are present in a carbon monoxide molecule

o Ozone Ozone is a bent

molecule

OXYGEN AND OZONE Property Oxygen Ozone

12 By: Raymond Chen


Colour Colourless as a gas, but pale blue when liquid or solid

Pale blue as gas, deep blue when liquid and purple when solid

Odour Odourless Sharp, pungentMolecular Shape

Linear Bent

Melting and Boiling Point

MP = -219BP = -183

MP = -193BP = -111

Density 1.3g/L 2.0g/LWater Solubility

Sparingly soluble, about 4.9mL O2 per 100mL

More soluble than O2, dissolves readily into turpentine, cinnamon oil and many other organic liquids

Effects on Life

Essential for all living things Poisonous and harmful, reactive with chemicals in living tissue

Stability Very stable Easily decomposed into O2Preparation Photosynthesis

Fractional distillation of liquid airDecomposition of H2O2

Effect of UV light on O2Electric charge on O2

Uses Oxyacetylene weldingLiquid O2 as fuel for space shuttlesSteel makingMedical uses for patients

Germicidal actionsBleaching agent in paperPowerful oxidising agent

OZONE IN THE ATMOSPHERE Stratosphere:

Contains 90% of atmospheric ozone

Acts as primary UV radiation shield

Issues: Long-term

downwards trend Antarctic and Artic

ozone holes.o In the stratosphere, the ozone acts like a

UV shield.o UV radiation from the sun reacts with

oxygen gas forming oxygen atoms that bond with oxygen gas to form ozone.

O2( g)UV ,240nm→

2O(g)

O2( g)+O2→O3(g )

o Most ozone is made above the equator as sunlight is most direct.o Ozone can absorb harmful UV-B and UV-C radiation.

O3( g)UV ,200−300nm→

O2 (g)+O(g)

o Ozone can be decomposed using oxygen atoms13 By: Raymond Chen


O3( g)+O( g)→2O(2)

Troposphere: Contains 10% of atmospheric ozone Toxic effects on humans and vegetation Effects on humans include:

Irritation to eyes Increased respiratory conditions Compromised lung functions Increase susceptibility to infection

Issues: High surface ozone in urban and rural areas

o Formed during electrical discharge - such as sparks from photocopiers, overhead power lines and lightening

O2( g)→2O(g )

O2( g)+O( g)→O3( g)

o Ozone forms in the lower atmosphere where there are: Sunlight Nitrogen dioxide

o NO and NO2 are produced in high temperatures of internal combustion engines - hence catalytic converters are used.

NO2(g)UV , sunligh t→

NO(g)+O(g)

O2( g)+O(g )→O3(g )

o The oxygen radical is reactive and can readily bond with other molecules.o However, nitric oxide, NO, can destroy ozone.

NO(g )+O3 (g)→NO2( g)+O2(g )

o Unburnt hydrocarbons mixed with ozone forms a toxic photochemical smog.o Ozone is the most harmful, by greenhouse gases readily react with other gases,

so ozone doesn't remain in the atmosphere for that long.

OXYGEN AND THE OXYGEN FREE RADICAL Oxygen atoms have six electrons in their outer shell. When oxygen is passed through electrical discharge or UV radiation, the oxygen

molecule splits, forming oxygen atoms. The atoms have two paired and unpaired electrons.

o This makes the atom unstable and reactiveo They are called oxygen free radicals

The unpaired electrons exist in higher energy states than the ground state. They exist only briefly in the lower layers of the atmosphere. In the thermosphere, oxygen free radicals are formed when far UV photons cause

photodissociation of oxygen molecules Oxygen free radicals are even more reactive than ozone.

HALOALKANES, CHLOROFLUOROCARBONS AND HALONS14 By: Raymond Chen


When alkanes react with halogens, they form new compounds that are known as haloalkanes.

Haloalkanes often exist in isomeric forms - the variable location of the halogen within the molecule leads to the formation of isomers

CHLOROFLUOROCARBONS Chlorofluorocarbons (CFCs) and halons are examples of haloalkanes. CFCs are alkanes containing only fluorine and carbon. They are alkanes that only have chlorine and fluorine atoms instead of hydrogen. Halons contain bromine atoms in addition to the chlorine and fluorine atoms in CFCs. CFCs are odourless, non-toxic, non-flammable, inert substances. CFCs were developed in the 1930's to replace ammonia in refrigerators. They were then extensively used as:

o Refrigerantso Solvents in dry cleaningo Propellant in spray canso Blowing agents for expanded plastic products

Halons are dense, non-flammable liquids. Halons are used in extinguishers especially for electrical fires.

STRATOSPHERIC OZONE DEPLETION CFCs are stable and insoluble;

hence they stay in the troposphere and eventually into the stratosphere.

In the stratosphere they come into contact with short wave UV radiation and undergo photodissociation.

o Photodissociation breaks a chlorine atom off the CFC molecule.

CCl3 FUV→Cl∎+∎CCl2 F

CCl2F2UV→Cl∎+∎CClF2

The free chlorine atom can now catalyse the reaction of ozone to oxygen.

Cl∎+O3→ClO∎+O2

ClO∎+O∎→Cl∎+O2

Hence the overall reaction is:

O+O3→2O2

The free chlorine atom is able to continue this process hundreds of times before it gets removed by some other species.

CH 4+Cl∎→CH 3∎+HCl

15 By: Raymond Chen


NO2+ClO∎→ClONO2

This chain reaction is important because one CFC molecule can destroy countless numbers of ozone molecules and can cause significant damage.

The dramatic depletion of stratospheric ozone has been observed only over the Antarctic and then only in spring.

STRATOSPHERIC OZONE DEPLETION In 1976, a British Antarctic Survey noted a 10% drop in ozone levels in the stratosphere

over Antarctica in the southern spring.o It was unusual as levels have remained the same since 1957.o Initially this data was treated as an outliero In 1983, ozone depletion was of mass concern as they observed record losses of

ozone that spring. Measurements:

o In 1985, measurements over Antarctica showed a 50% reduction in ozone concentrations over the past 10 years.

o The results were recorded by the total ozone mapping spectrometer (TOMS) and solar backscatter ultraviolet detector.

o Another technique is through the use of UV lasers - UV laser light is fired into the sky, the level of absorption determines the level of concentration.

THE "OZONE HOLE" The thinning of the ozone layer results in what is

known as an "ozone hole". During 1987, the ozone hole spread over southern

Australia and New Zealand. Further depletions were recorded in the 1990’s; the

worst was in 2003, 2000 and 1998. The ozone hole is not confined to the Antarctic, with

small decreases over the Artic too. By 1996, the thinning of ozone over the Arctic

reached 40%. Decreased concentrations of ozone are a problem

as:o More UV rays would reach the groundo Phytoplankton and zooplankton would be

affectedo The food chain in the end would be affected too.o Skin cancer rates have increased by 66% over 14 years in world's most

southernmost city.o Causes respiratory conditionso Damage and cause mutations of DNA.

16 By: Raymond Chen

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