1. Much of the work of chemists involves monitoring the reactants and products of reactions and managing reaction conditions

Outline the role of a chemist employed in a named industry or enterprise, identifying the branch of chemistry undertaken by the chemist and explaining a chemical principle that the chemist uses

Burhan Gemikonakli is a plant chemist at Qenos, a major Australian chemical manufacturing company that makes ethylene from ethane, then polymerises it to polyethylene. Qenos is located at Britany, NSW. Burhan’s job at Qenos has several components:

Monitoring the quality of the ethylene and the propylene products from the plant to ensure they meet the requirements for the next stage

Monitor waste water from the Qenos complex to ensure that it meets environmental requirements

Collaborating with process engineers at the cracking furnace to adjust operating conditions in order to optimise production yields

Branch of chemistry

The branch of chemistry Burhan mainly works is in analytical chemistry; the part of chemistry concerned with determining what substances (and how much of each) are present in materials.

Many of Burhan’s analyses use gas chromatography. This is a technique in which a mixture is vaporised into a stream of helium. A device at the end of the column detects each substance as it passes out of the column and measures it quantitatively.

Identify the need for collaboration between chemists as they collect and analyse data

Chemists tend to specialise within a particular branch of chemistry (i.e. they have a lot of detailed knowledge about a very specific area). However, in real life situations, chemical problems often require expertise and in depth knowledge from a wide range of chemical branches.

The collaboration between chemists is essential in the development of new products, or the solving of chemical issues. For example, an industrial process would require collaboration between physical chemists (equilibrium/rate considerations), organic chemists (how reaction occurs/ how to increase yield) and analytical chemists (monitoring products).

Therefore, it is essential that chemists work collaboratively and regularly communicate as they collect and analyse information.

Describe an example of a chemical reaction such as combustion, where reactants form different products under different conditions and thus would need monitoring

Combustion reactions are a type of reaction where reactants form different products under different conditions.

For example, the combustion of propane:

In an environment with plentiful oxygen, propane combusts completely, forming carbon dioxide and water:

- C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (g)

In an environment with insufficient oxygen, propane combusts incompletely, forming a range of products such as soot (carbon), carbon monoxide, carbon dioxide and water- C3H8 (g) + 3O2 (g) C (s) + 2CO (g) + 4H2O (g)

In certain situations (such as that in car engines), complete combustion would be more desirable; thus conditions must be monitored to ensure sufficient oxygen is supplied for the desired reaction to occur.

Gather, process and present information from secondary sources about the work of practising scientists identifying: the variety of chemical occupations, a specific chemical occupation for a more detailed study

The variety of chemical occupations

Analytical chemist Bio-molecular chemist Environmental chemist Industrial chemist Organic chemist Metallurgical chemist

A specific chemical occupation

The job of an environmental chemist includes:

Reviewing operation of effluent water treatment systems and ensuring compliance with government environmental regulations

Reviewing industry’s compliance with government environmental noise standards Asserting levels of potential contamination in wastes intended for landfill disposal and

classifying them in accordance with government guidelines Managing disposal of contaminated wastes Investigating reports of contamination in soil or groundwater to determine source and then

arranging to correct it Determining whether gas stack emissions contain unacceptable levels of regulated

materials; advising engineers and managers of corrective actions if needed. Answering public or professional enquires or complaints regarding environmental

performance

2. Chemical processes in industry require monitoring and management to maximise production

Identify and describe the industrial uses of ammonia

Industrial product derived from ammonia Use of productUrea, ammonium sulfate, ammonium nitrate FertiliserNitric acid Production of explosives (ammonium nitrate)

Production of plasticsProduction of fertiliser (ammonium nitrate)

Acrylonitrile Acrylic plasticsDiaminoalkanes Nylon plastics

Identify that ammonia can be synthesised from its component gases, nitrogen and hydrogen

From its molecular formula (NH3), we can see that ammonia is composed of nitrogen and hydrogen atoms. Hence it can be synthesised from these component gases

Describe that synthesis of ammonia occurs as a reversible reaction that will reach equilibrium

Identify the reaction of hydrogen with nitrogen as exothermic

In 1908, the German scientist Fritz Haber developed a method of synthesising ammonia from its elements, though it was not until 1914 that Carl Bosch successfully converted it into an industrial process. Haber manufactured ammonia from its component gaseous elements:

Production of ammonia through this process involves the exothermic reaction between nitrogen and hydrogen (as above). This is an equilibrium reaction, which at normal conditions (atmospheric pressure and room temperature) lies well to the left. However, in the Haber process, conditions are altered to yield more product (ammonia) and make the process viable to produce ammonia on a commercial scale.

Explain why the rate of reaction is increased with higher temperatures

The rate of reaction describes how fast a reaction occurs

There are two reasons why higher temperatures result in a higher rate of reaction:

As temperature is increased, energy is delivered into the reaction vessel as thermal energy, which is then converted into kinetic energy. The particles begin to move faster – causing more collisions between particles and hence more reactions occur

Also, higher temperatures increase the strength of the collisions between particles. Therefore, there is a higher chance that colliding particles will have the necessary activation energy for the reaction to take place.

For the Haber process, at higher temperatures, the rate of reaction of both the forward and reverse reactions are increased – hence equilibrium is reached faster

Explain why the yield of product in the Haber process is reduced at higher temperatures using LCP

The Haber process is an exothermic reaction, and thus, if we increase the temperature of the system, it will shift itself to counteract this imposed change. The system will want to oppose the heating, and the backwards reaction (which is endothermic) will be encouraged.

As a result, the equilibrium shifts towards the left, and the yield of product is reduced.

Analyse the impact of increased pressure on the system involved in the Haber process

According to LCP, increasing the pressure will shift the equilibrium towards the side that has the least moles of gas (thus reducing pressure).

In the Haber process, there are less moles of gas in the product side, and hence increasing the pressure will shift equilibrium towards the right (increasing yield)

In addition, higher pressure will also increase the reaction rate as the gas molecules are closer and at higher concentrations

Explain why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium

In order for the Haber process to be economically viable, we must consider yield of products, rate of reaction and costs.

Hence a compromise/balancing act of the above must be made:

Temperature – higher temperatures will lead to an increase in the rate of reaction (producing ammonia faster), but lower temperatures will lead to an increase in yield (producing more ammonia). Hence, a moderate temperature of about 500oC is used

Pressure – increased pressure will increase the yield (producing more ammonia) and the reaction rate (producing it quicker). However, due to both economic (high-pressure equipment is expensive to build and maintain) and safety considerations, a relatively low pressure of 25MPa is used.

Catalyst – a catalyst of magnetite (Fe3O4), fused with metal oxides is reduced to a fine powder (increasing SA) and acts as a catalyst. It reduces activation energy – increasing the rate of reaction whilst allowing lower temperatures/pressures to be used.

Ratio of reactants – a molar ratio of 1:3 (N2:H2) should be maintained. If this stoichiometric is not maintained, an excess of either nitrogen or hydrogen will exist and will not react. As a result, this would decrease batch efficiency.

Liquefaction of ammonia – Removing gaseous ammonia from the reaction tank via liquefaction will shift the equilibrium in favour of ammonia formation (by LCP as we have reduced the concentration of the products).

Explain why monitoring of the reaction vessel used in the Haber process is crucial and discuss the monitoring required

Monitoring of the reaction vessel used in the Haber process is crucial to ensure quality control:

Temperature must be monitored to ensure it is kept within an acceptable range (approx. 500oC) If temperatures are too high, yield is reduced; if temperatures are too low, rate of reaction is too slow

The pressure of the high-pressure reaction vessel must be constantly monitored to ensure it is within working range of 25MPa – if pressure is too low, there is a loss of yield; if pressure is too high it poses safety hazard

Ratio of reactants should be monitored to keep it at a stoichiometric 1:3 (N2:H2). If one reactant is in excess, batch efficiency is decreased

Activity of the catalyst – the pore size of the catalyst needs to be monitored to ensure the SA is consistently large, otherwise reaction rate decreases.

Ammonia liquefaction process – the temperature of the condensers need to be monitored to ensure they are cold enough to liquefy the ammonia so it doesn’t recirculate and decrease yield of the next cycle.

Concentrations of gases – A build-up of methane and argon needs to be prevented as it will lower the efficiency of conversion. Concentrations of CO and CO2 must be sufficiently low to prevent poisoning of the catalyst. Oxygen must be completely absent as it would create an explosive mixture with H2 in the Haber process.

Gather and process information from secondary sources to describe the conditions under which Haber developed the industrial synthesis of ammonia and evaluate its significance at that time in world history

Fritz Haber was a German scientist who developed the method of producing ammonia from hydrogen and nitrogen in 1905. He produced small yields of ammonia when nitrogen and hydrogen gases were combined at 1000oC over an iron catalyst.

Further investigations showed that increased gas pressure and decreased temperature could help achieve better yields; and by 1909, he was able to synthesis about 100g of ammonia using his modified procedures. However, it was Carl Bosch (another German scientist), who was able to scale the process up to industrial levels. He showed that the best yields were achieved at pressures around 15-20MPa, and temperatures around 500oC using an iron-based catalyst.

At the time when Haber/Bosch developed the industrial synthesis of ammonia, WWI was occurring, and Britain and its allies had blockaded Germany from importing saltpetre (containing nitrates needed for fertiliser and explosives). Haber’s discovery thus allowed the synthetic production of ammonia which could be used to produce fertiliser to feed the German population/troops as well to produce nitric acid (which could in turn be used to produce explosives for the war effort).

3. Manufactured products, including food, drugs and household chemicals, are analysed to determine or ensure their chemical composition

Describe the use of atomic absorption spectroscopy (AAS) in detecting concentrations of metal ions in solutions and assess its impact on scientific understanding of the effects of trace elements

When we observe the ‘colour’ of an object, we are actually detecting the wavelengths of visible light that have not been absorbed by the object.

Electrons in atoms exist in discrete energy levels. When we vaporise an element in a hot flame, some of the electrons are promoted to higher energy levels to reach an excited state. However, after a short time, these electrons relax back to ground state – emitting energy in the process (as light). When atoms interact with visible light in this way, the element appears coloured.

It has also been found that elements absorb/emit light of particular wavelengths characteristic to itself (i.e. different elements emit discrete wavelengths). This is called the emission spectrum of the element. (Example on the right)

Using this principle of selective light absorption by metal ions, the Australian CSIRO scientist Alan Walsh and his team developed the technique of Atomic Absorption Spectroscopy (AAS). Using the AAS system allowed for the ppm and even ppb concentrations of elements.

Atomic absorption spectroscopy

The solution containing the sample is fed into a flame which vaporises it, converting molecules and ions into atoms (forming atomic vapour). A light beam (from a hollow-cathode lamp) is passed through the flame. The lamp emits light of a particular wavelength which is known to be absorbed by the element to be measured. (Note that the lamp is actually producing the emission spectrum of the element so there is an exact match of emitted and absorbed wavelengths). The light passes through the flame into a monochromator. The monochromator helps select only the wavelength of light to be analysed and focuses it on a photomultiplier. This detector measures the intensity of light reaching itself (and thus how much light has been absorbed by the element), and uses it to calculate absorbance.

The

amount of light absorbed by a series of standard solutions of the element is plotted against the concentrations of the element – allowing a calibration curve to be constructed. By measuring the absorption of light by a sample of unknown concentration, we can determine the concentration of the element within the sample (using the calibration curve).

Impact on understanding of trace elements

Trace elements are those that exist in very low abundance in the environment and in living organisms (e.g. copper). Many trace elements are essential for the proper functioning of plants and animals.

Before AAS, the only way to detect/analyse metal ions was through ‘wet’ techniques (volumetric analysis, precipitation reactions). However, these techniques were not sensitive or specific enough to identify and quantify trace elements in complex systems. Thus, the existence of such trace elements was not known until sensitive analytical methods such as AAS were developed.

Through AAS, scientists were able to quickly and reliably establish the role of which trace elements had in metabolism. Deficiencies of these trace elements (e.g. copper or zinc) in soil or animal diets often lead to severe health issues. As well, if concentrations of heavy metals such as mercury were too high, there could be disastrous consequences on the local ecosystem.

Perform first-hand investigations to carry out a range of tests, including flame tests, to identify the following ions: phosphate, sulphate, carbonate, chloride, barium, calcium, lead, copper, iron

Anions

Carbonate, chloride, phosphate, sulphate

Anions can be identified and distinguished using simple qualitative tests involving the formation of gases or precipitates. Normally, a series of elimination tests is conducted in a strict order, known as an elimination sequence. This is important since with some reagents; different ions will produce similar results.

We start with the most unambiguous test and eliminate the obvious ions first.

*add in solubility rules table?

1_________________________________________________________________________________

Carbonate – Carbonates form precipitates with numerous cations, so we need to avoid using a precipitation reaction to test for carbonate. Instead, we use a common-acid base reaction (all carbonates react with dilute acids to form carbon dioxide and water)

We add 2mol/L of nitric acid drop-wise into the original solution. Fizzing of the solution indicates release of gas – use limewater test to confirm release of CO2.

Confirmation test: confirm that the original test is basic

2________________________________________________________________________________

Sulfate – in dilute acid solution, barium ions produce a precipitate with sulphate (but not phosphate)

Acidify the unknown solution with nitric acid (to remove any carbonate) Then add drops of dilute solution of barium nitrate. A white precipitate of barium sulphate

indicates sulphate ions are present. Confirmation test: add lead nitrate Pb(NO3)2 to the solution and a white precipitate of lead

sulphate PbSO4 should form.

3________________________________________________________________________________

Phosphate – in ammonia solution, however, barium phosphate can precipitate. According to the equilibrium reaction from which phosphate is derived from, the reduction of concentration of hydronium ions (in ammonia solution) shifts the equilibrium well to the right:

In the step before, there was insufficient phosphate for barium phosphate to precipitate out.

Now, when barium nitrate is added, barium phosphate precipitates:

Add drops of ammonia to the solution (increase pH to ~10)

Add drop wise a solution of barium nitrate. A white precipitate should form, indicating the presence of phosphate ions.

Confirmation test: Add nitric acid followed by ammonium molybdite solution and warm gently. This should produce a yellow precipitate.

4_________________________________________________________________________________

Chloride – adding silver (nitrate) will produce a white precipitate of silver chloride

Ag+ + Cl- AgCl(s)

Acidify the original solution with 5 drops of dilute nitric acid (not hydrochloric, as it is a source of chloride).

Add drops of silver nitrate solution. A white precipitate should form, indicating the presence of chloride ions

Confirmation test: add a few drops of ammonia solution. The white precipitate should dissolve:

AgCl(s) + 2NH3 [Ag(NH3)2]+ + Cl-

Cation

analysis

Similar to anions, cations can be identified and distinguished using simple qualitative tests. However, with cations, we can also conduct hydration and flame tests.

Flame tests

Many metal ions produce characteristic colours when their salts are volatised in the blue (heating) Bunsen flame. There are two main methods to conduct the flame test

Platinum wire method –- Clean the platinum wire with hydrochloric or nitric acid, then rinse with distilled water- Test the wire in the flame to make sure there are no colour changes (i.e. it is clean)- Dip the wire in either a powder or solution of the ionic (metal) salt- Place the wire in the flame and observe colour

Spray method – - Pour the solution into an atomiser- Spray the atomiser with the solution into the Bunsen flame- The colour of the flame should indicate ions present

You need to memorise the colours of certain cations:

- Copper (blue green) - Barium (apple green)- Calcium (brick red)

Lead should not be flame tested as it is poisonous

As with anion analysis, a series of elimination tests is conducted to identify cations.

1_________________________________________________________________________________

Lead – adding chloride (hydrochloric acid) to the solution will produce a white precipitate of lead chloride

Pb2++ 2Cl-PbCl2 (s)

Add 5 drops of dilute hydrochloric acid to the unknown solution. A faint, white precipitate should form, indicating the presence of lead ions Confirmation test: To the original solution, add sodium iodide. A yellow precipitate should

form- Pb2++2I-PbI2(s)

2_________________________________________________________________________________

Barium, calcium –adding sulphate (sulphuric acid) to a fresh sample of the solution should form a white precipitate

Ca2+ + SO4- CaSO4

Ba2+ + SO4- BaSO4

To a fresh sample of the solution, add 5 drops of 1M sulphuric acid solution A white precipitate indicates wither calcium or barium ions are present Confirmation tests

- Add sodium fluoride to the solution. If a white precipitate forms, calcium ions are present, it not, barium ions are present. Ca2+ + 2F- CaF2

- Flame test: calcium produces a brick red colour, barium produces an apple green colour

3_________________________________________________________________________________

Copper – adding OH- (NaOH) forms a blue precipitate of copper hydroxide. This precipitate dissolves in excess ammonia to form a deep blue solution

Cu2+ + 2OH- Cu(OH)2

To a fresh sample of the solution, add 5 drops of 1M NaOH

A blue precipitate should form from an original blue or green solution. Confirmation tests:

- Add ammonia solution. The precipitate should dissolve to form a deeper blue solution- Flame test: copper produces a blue green flame

4_________________________________________________________________________________

Iron(III), Iron(II)– adding OH- produces a brown precipitate for Iron(III), and a white/green precipitate for Iron(II) which may subsequently turn brown.

Fe2+ +2OH- Fe(OH)2

Fe3+ + 3OH- Fe(OH)3

To a fresh sample of the original solution add 5 drops of 1M NaOH Fe(III) should form a brown precipitate, Fe(II) should form a white/green precipitate Confirmation tests:- Add drops of potassium permanganate solution to your acidified solution. Fe2+ should

decolourise the permanganate- Add a drop of potassium thiocyanate to the solution. The presence of Fe3+ should form a

blood red colour

Gather, process and present information to describe and explain evidence for the need to monitor levels of one of the above ions in substances used in society

The level of lead in substances used in society needs to be monitored due to that fact that it is a severe neurotoxin. It retards intellectual development in young children, causes brain damage and can lead to neurological disorders.

Until recently, lead was widely used in petrol and so was released to the atmosphere in vehicle exhausts and deposited out on soil near busy highways. Lead also used to be a constituent in many paints, and is often released to air, water and soil when old houses are renovated.

Monitoring lead concentrations in soil near highways, waterways and in the atmosphere in urban areas is essential to ensure that people are not exposed to harmful levels of toxic lead ions.

Identify data, plan, select equipment and perform first hand investigations to measure the sulfate content of lawn fertiliser and explain the chemistry involved

Aim: To use gravimetric analysis to determine the sulfate content of a lawn fertiliser (a mixed fertiliser containing ammonium sulfate)

Materials:

Solid, soluble fertiliser Concentrated hydrochloric acid in dropper bottle Barium chloride solution (7% w/v) Electronic balance 250mL beaker Burette Hotplate Glass rod Watch glass Quantitative filter paper

Method:

1. Weigh out approximately 0.60g of the powdered lawn fertiliser into a clean 250mL beaker. Record the exact mass of the fertiliser. Add 25mL of warm water from a measuring cylinder. Stir to dissolve the crystals

2. Add 10 drops of concentrated hydrochloric acid to the solution.

3. Use a hotplate to heat the mixture until it is just boiling4. Add from the burette the 7% barium chloride solution slowly to the hot sulfate solution until

no further white precipitate forms.5. Gently re-boil the mixture for a further 5 minutes to coagulate the precipitate. Let it stand

on a fibreboard for 5-10 minutes to cool6. Meanwhile, weigh a circle of quantitative filter paper on an electric balance. Weigh a clock

glass and record this weight7. Fold the filter paper into a cone and set up the apparatus for filtration8. Pour the sulfate/precipitate solution slowly through the filter paper. Ensure that all the solid

is transferred into the filter by using small amounts of warm washing water9. When filtration is complete, carefully transfer the opened filter paper to the weighed clock

glass and allow to dry in a low temperature oven10. When dry, weight the clock glass and filter. Calculate the mass of barium sulfate collected

Chemistry involved:

Analyse information to evaluate the reliability of the results of the above investigation and to propose solutions to problems encountered in the procedure

Considerable care is needed in this experiment, with sources of error including:

Passage of some barium sulfate through the filter because the precipitate had formed as very small particles; a solution to this would be to ‘digest’ the particles for longer

There was incomplete drying of the precipitate so that it still contains what when weighed- a solution to this is drying to constant mass (dry, cool, weight, then dry, cool, weigh again until a constant mass is obtained) or to allow a longer period of heating

Not all the sulfate has precipitated out as barium sulfate; a solution to this would be adding a slight excess of barium chloride just to make sure

*note that you can’t write that barium sulfate is slightly soluble; conquering is wrong

Gather, process and present information to interpret secondary data from AAS measurements and evaluate the effectiveness of this in pollution control

The government monitors our air and water to ensure that pollution is kept under control. People living in industrialised areas or mining areas are particularly at risk from air and water pollution. AAS

Ba2+ (aq) + SO4

2- (s) BaSO4 (s)

CALCULATING SULFATE CONTENT:

mass of BaSO4 formed = 5.92 g percentage of sulfate in BaSO4 = molar mass sulfate / molar mass BaSO4

= (32.1 + 4×16.0) / (137.3 + 32.1 + 4×16.0)

= 41.2 % mass of sulfate in BaSO4 = 5.92 × 41.2 %

= 2.44 g percentage of sulfate in fertilizer = mass of SO4 / mass of fertilizer × 100

= 2.44 / 5.0 × 100= 48.9 %

is a technique that can be used to analyse for the presence of metal ions in situations where gravimetric/volumetric techniques are not suitable as they are insufficiently sensitive to detect low levels of metal pollutants.

Effectiveness

AAS is effective in monitoring/measuring pollution, as it is:

Relatively cost effective Scientifically valid Highly sensitive Easy to use and readily available

However, AAS has different sensitivities (detection limits) for different metals. It is important to account for these detection limits when measuring pollution in order make a valid judgement as to whether an area is too polluted or not.

AAS data can help environmental chemists to offer appropriate advice to government agencies whether or not action is required to prevent further contamination or pollution within an area.

4. Human activity has caused changes in the composition and the structure of the atmosphere. Chemists monitor these changes so that further damage can be limited

Describe the composition and layered structure of the atmosphere

The composition of the atmosphere is relatively constant at all altitudes/layers, although the concentration of total gas particles (pressure) drops with increasing altitude.

Element Percentage by volume Nitrogen 78.1%Oxygen 20.9%Argon 0.9%Carbon Dioxide 0.03%Other Less than 1%

The atmosphere is mainly composed of nitrogen, oxygen, argon and small amounts of:

Carbon dioxide Neon Methane Helium Krypton Hydrogen Xenon

The atmosphere is a thin gaseous layer that extends about 600km above the Earth’s surface. The main layers are:

Troposphere (0-10km) Stratosphere (10-30km) Mesosphere (30-50km) Thermosphere (50-400km) Exosphere (400km +)

As you increase altitude, the air pressure decreases.

Troposphere (0-10km)

The troposphere is the layer closest to the ground. Approximately 75% of the mass of the atmosphere is concentrated in the troposphere, where the air pressure is highest. The troposphere cools with increasing altitude until reaching the tropopause.

Stratosphere (15-50km)

The first 9km of the stratosphere has a fairly uniform temperature; above that, however, temperature increases. Within the stratosphere is the main part of the ozone layer. The air in the stratosphere is dry and stable; therefore the ozone in this region will not migrate into the troposphere.

Mesosphere

The temperature in the mesosphere decreases with altitude – few molecules in this zone absorb radiation, so the zone is very cold. This is also the layer in which most meteors burn up while entering Earth’s atmosphere

Thermosphere

The temperature in the thermosphere increases with altitude due to the absorption of high frequency radiation. The thermosphere contains the ionosphere – the region in which atoms and molecules are converted into gaseous ions (e.g. O+, O2

+)

Identify the main pollutants found in the lower atmosphere and their sources

The atmosphere becomes polluted by both natural processes and the activities of humans. Lower atmospheric pollutants often include:

Natural pollutants Industrial pollutants Soot (from bushfires) CO2 (from bushfires and volcanoes) SO2 (from volcanoes) NO (from lightning)

NO and NO2 (from car exhausts) SO2 (from smelting of metals, burning of

coal) CO (from incomplete combustion in car

exhausts) *note that CO does not build up in atmosphere, as it is rapidly removed by soil organisms

VOC’s volatile organic compounds (from

car exhausts)Describe ozone as a molecule able to act as both an upper atmosphere UV radiation shield and a lower atmosphere pollutant

The four main types of UV radiation are:

UV-A –Most of the UV-A reaches the earth’s surface. This is important as UV-A promotes the production of vitamin-D in our skin.

UV-B – Harmful; absorption of UV-B can cause photochemical reactions in DNA, leading to skin cancer

UV-C – Harmful; absorption of UV-C can cause photochemical reactions in DNA, leading to skin cancer. Even worse than UV-B

Formation of ozone Decomposition of ozoneO2 + UV radiation 2O·

O· + O2 O3

O3 + UV radiation O· + O2

O· + O3 2O2

Upper atmosphere

In the stratosphere, ozone is present at 2-8ppm, concentrated mainly in the ozone layer, which is vital in sustaining life here on Earth. Ozone absorbs UV light and reacts with it:

O3 + UV radiation O· + O2

Through this reaction, it is able to absorb 97-99% of the Sun’s high frequency ultraviolet light – including UV-B and UV-C (which damages living tissue, increasing melanomas (skin cancer), eye cataracts and destroying crops)

Lower atmosphere

In the troposphere, ozone is a pollutant - where it is harmful to the biosphere and manmade structures. In clean air at ground level, ozone is present at about 0.02 ppm, and so health effects are minimal. However, at levels higher than 20ppm, it is poisonous:

It irritates the eyes and causes breathing difficulties It is toxic to plants It is a strong oxidising agent; readily attacks rubber, plastics and rusts metals.

Photochemical smog is a type of air pollution that is produced when sunlight acts on car exhaust gases to form ozone and other harmful substances.

Photochemical smog forms in the lower atmosphere when sunlight is intense and when concentrations of NO2 are particularly high (occurs when there is no breeze to disperse pollutants). The most harmful component of the smog is ozone (O3)

Sunlight splits off an oxygen radical from the NO2 molecule, which then reacts with molecular oxygen to form ozone:

NO2 NO + O·

O· + O2 O3

Describe the formation of a coordinate covalent bond

Coordinate bonding is a special type of covalent bonding. It is similar to a standard covalent bond, but both electrons in the bond originate from only one bonding partner. However, once formed, a coordinate covalent bond is identical to a normal covalent bond (only difference lies in how the bond is formed).

Demonstrate the formation of coordinate covalent bonds using Lewis electron dot structures

In the OZONE (O3) molecule, one of the oxygen atoms forms a coordinate covalent bond with an oxygen atom:

TIP – When asked to draw a Lewis diagram of ozone, check that all oxygen atoms have only 6 electrons of their own, but 8 electrons bonded altogether:

Left : 4 unbonded electrons + 4 electrons in covalent bond = 8 Middle : 2 unbonded + 4 in covalent bond + 2 in coordinate covalent = 8 Right : 6 unbonded + 2 in coordinate covalent = 8

Hydonium ion

Ammonium ion

Ozone

*Refer to EIM notes booklet 5 for more detail

Compare the properties of the oxygen allotropes O2 and O3 and account for them on the basis of molecular structure and bonding

Properties Gaseous oxygen Gaseous ozone ExplanationColour Colourless Pale blue Ozone interacts differently with

visible light due to different electronic structure

Boiling point -183oC -111oC O2 is non-polar, whilst ozone is a slightly polar molecule (undergoes dipole-dipole interactions and therefore has stronger dispersion forces)

Solubility in water

Sparingly soluble Slightly more soluble than O2

Ozone is polar and engages in dipole-dipole interactions with water

Chemical stability

Moderately stable (readily engages in combustion reactions with sufficient energy input)

Unstable (readily oxidises anything it touches)

Ozone readily transfers an oxygen radical to other molecules that it encounters. This makes it an excellent oxidising agent

Oxidation stability

Moderate Excellent Ozone is much more electron deficient than O2. It readily abstracts electrons to undergo oxidation (with two electrons)

Compare the properties of the gaseous forms of oxygen and oxygen free radicals

A free radical is a neutral species that has an unpaired electron; formed by the splitting of a molecule into two neutral fragments.

In the stratosphere, oxygen radicals are formed by the action of UV radiation on O2 and O3. These oxygen radicals each have two electron pairs and two unpaired electrons – as a result, they are highly reactive and unstable. In contrast, O2 and O3 are stable molecules due to the fact that the atoms in these molecules all have completed valence shells.

Diatomic oxygen reacts with organic compounds only at elevated temperatures (via combustion); ozone reacts with organic compounds only if they contain a double/triple bond; oxygen radicals react with most organic compounds at room temperature.

In order of reactivity, the diatomic molecule is less reactive than the ozone molecule, which is less reactive than the oxygen free radical.

Identify the origins of chlorofluorocarbons (CFC’s) and halons in the atmosphere

CFC’s and halons are examples of halo-alkanes.

Chlorofluorocarbons (CFCs) are compounds containing chlorine, fluorine and carbon only – i.e. they contain no hydrogen.

Origins of CFC’s in the atmosphere are the result of its use:

As replacements for ammonia as a refrigerant gas due to the fact that they were odourless, non-flammable, non-toxic and very inert

In aerosol spray cans As foaming agents in the manufacture of expanded polystyrene For cleaning electrical circuits

Halons are compounds that contain carbon, bromine and other halogens.

The origins of halons in the atmosphere were the result of its use in fire-extinguishers (they are dense, non-flammable liquids) in airplanes and cars.

Identify and name examples of isomers (excluding geometrical and optical) of haloalkanes up to eight carbon atoms

Rules for naming haloalkanes

1. Select the longest unbranched hydrocarbon chain and count the number of carbons. This will decide the stem name of the alkane

2. Number the chain from the end that produces the lowest set of locants (for all functional groups)

3. In naming the haloalkanes, the functional groups are named alphabetically (disregarding di-, tri- etc.)

4. If previous rules have more than one possible name, the correct one has the lowest locant assigned to the functional group cited first

Isomers are compounds that have the same molecular formulae but different structural formulae.

Discuss the problems associated with the use of CFCs and assess the effectiveness of steps taken to alleviate these problems

Present information from secondary sources to show the reactions involving CFCs and ozone to demonstrate the removal of ozone from the atmosphere

CFCs are very inert and water insoluble, and as a result:

They do not get washed out by rain At low altitude are not destroyed by sunlight and oxygen like most compounds are.

Hence CFCs remain in the troposphere for years, and gradually diffuse into the stratosphere – where it becomes destructive to the ozone layer.

1. In the stratosphere, CFCs come into contact with short wavelength UV radiation (before it is filtered out by the ozone layer), which breaks off a chlorine atom (radical) off the molecule. For example, the breakdown of trichlorfluoromethane: CFCl3 (g) + UV radiation ·Cl (g) + ·CFCl2 (g)

2. The chlorine radical then reacts with ozone, forming oxygen and a chlorine monoxide radical:·Cl (g) + O3 (g) ·ClO (g) + O2 (g)

3. The chlorine monoxide radical reacts with oxygen free radicals to regenerate the chlorine radical and produce a molecule of oxygen·ClO (g) + ·O (g) ·Cl (g) + O2 (g)

The net result therefore is:

O3 (g) + ·O (g) 2O2 (g)

At the same time, the reactive chlorine radical has not been used up, and can attack another ozone molecule to repeat the process over and over again. This essentially becomes a chain reaction (where the reactive species is regenerated and can continually repeat the reaction). One CFC molecule can destroying thousands of ozone molecules, hence, small amounts of CFCs reaching the stratosphere can cause significant amounts of ozone depletion.

Certain natural reactions eventually remove these radicals from the stratosphere:

Reaction between chlorine radical with methane in the stratosphere:·Cl (g) + CH4 (g) HCl (g) + ·CH3 (g)

Reaction between chlorine monoxide radical and nitrogen dioxide in the stratosphere:·ClO (g) + NO2 (g) ClONO2 (g)

Serious ozone depletion is seasonal and localised (over the Antarctic and in spring). This serious depletion of the ozone layer is called the ozone hole. It is the result of the conditions of Antartica in winter, as well as spring:

Antarctic winters are perpetually dark and cold – allowing certain solid particulates in the air to catalyse the reaction between hydrogen chloride and chlorine nitrate:HCl (g) + ClONO2 (g) Cl2 (g) + HNO3 (g)

This has no effect upon ozone concentrations during winter However, in early spring, the situation changes dramatically when the sun begins to rise. The

UV light is able to separate the chlorine atoms:Cl2 (g) + UV radiation 2·Cl (g)

Hence, in spring there is an extra source of chlorine radicals to destroy more ozone – so the concentration of ozone is reduced dramatically, creating a hole.

Only a finite amount of molecular chlorine is formed during the winter, and is virtually all used up by early summer – thus the ozone layer regenerates.

Ozone depletion can pose itself to be extremely dangerous, as it results in more UV-radiation reaching Earth’s surface. This could cause:

Increased incidence of skin cancer Increased risk of eye cataracts Reduced plant and crop growth due to UV interference with processes of photosynthesis Reduction of phytoplankton populations (which are crucial for photosynthesis on Earth) due

to DNA damage from UV radiation Damage to many synthetic materials such as plastics like PVC

Montreal protocol

In 1985, The Vienna Convention examined ways of preventing the destruction of the ozone layer. As a result of this convention, 28 countries signed an international treaty called the Montreal Protocol. The purpose of the protocol was to establish a timeline for the phasing-out of CFC’s and halons to prevent future destruction of the ozone layer.

The agreement for developed countries was to:

Phase out halons by 1993 Phase out CFCs by 1995 Phase out HCFCs by the early 21st century

Developing countries were allowed a ‘period of grace’ where they could phase out later.

Effectiveness:

The Montreal Protocol has so far been a huge success, with most countries meeting the required targets. This has been largely due to the availability of acceptable alternative compounds like HCFC’s and HFC’s.

However, there has not yet been a major reduction in measured atmospheric concentrations of CFC’s– largely due to the large amount of CFC’s already present in the troposphere (which takes a long time to diffuse into the stratosphere and then be destroyed). As well, CFC’s are a very resilient molecule which take a long time to decompose (due to the chain reactions involved in its destruction of ozone) - Furthermore, it has been suggested that continued CFC emission is the result of old

refrigerators and air conditioners been dumped and crushed at waste sites or during the recycling process

Analyse the information available that indicates changes in atmospheric ozone concentrations, describe the changes observed and explain how this information was obtained

Changes observed

Since 1956, measurements of the total amount of ozone have been recorded. In 1976, scientists identified a 10% drop in ozone levels in the stratosphere over Antarctica during the southern spring. This was unusual as levels had typically remained relatively constant since measurements began.

By 1985, atmospheric measurements over Antarctica showed a 50% reduction in ozone concentrations in the stratosphere over the previous decade.

After international agreement through the Montreal protocol, use of CFC’s has dramatically decreased. There has been minimal decrease in CFC concentration and there hasn’t been detectable improvement in the ozone hole. This is due to the fact that CFC’s are very resilient molecules which take a long time to decompose (due to the chain reactions involved in its destruction of ozone).

Instruments used

Ozone levels are measured in Dobson Units (DU).

The scientists made their measurements using ground-based instruments (Dobson UV spectrometers) as well as instruments in satellites (TOMS) and instruments in high-altitude balloons.

Ground-based instruments – UV spectrophotometers measure the intensity of light received from the sun at a wavelength which ozone absorbs and then wavelengths either side of this which ozone does not absorb. A comparison of these intensities gives a measure of the total ozone in the atmosphere per unit area at that location. Such spectrophotometers provide ozone measurements as a function (diagram to the right)f

Balloon based instruments – spectrophotometers are placed in high-altitude weather balloons rising above the stratosphere. The instruments are pointed downwards, measuring ozone from above. Early profiles of the type like the diagram in the right were produced this way.

Satellite based instruments – TOMS (Total ozone mapping spectrophotometry) involves placing a spectrophotometer on board satellites (e.g. the Nimbus-7 Satellite), where incoming solar radiation is compared to the backscattered UV radiation (which has not been absorbed by ozone) at identical wavelengths to determine the amount of ozone present. Because the satellites have been in orbit, these instruments have been able to measure ozone concentrations over different areas of the Earth’s surface and produce contour maps (diagram to the right)

Present information from secondary sources to identify alternate chemicals used to replace CFCs and evaluate the effectiveness of their use as a replacement for CFCs

Alternate chemicals used to replaced CFCs include: HCFC’s and HFC’s

HCFC’s (hydrochlorofluorocarbons)

Hydrochlorofluorocarbons are CFCs that contain hydrogen. They are commonly used in place of CFCs as they are broken down in the troposphere due to the higher reactivity of their C-H bonds. As a result, only a very small proportion will reach the stratosphere and cause ozone depletion (as a result HCFC’s have lower ODP’s). HCFCs have replaced CFCs in domestic refrigeration, as propellants in spray cans and as a foaming agent.

HCFCs are seen as only a temporary substitute for CFC’s, and are expected to be fully phased out by 2030. This is because although they are decomposed in the troposphere, small amounts of HCFCs do reach the stratosphere and cause ozone depletion (approx. 10% the Ozone Depletion Potential (ODP) of CFCs). Furthermore, HCFCs contribute to the greenhouse effect, and their long-term toxicity for humans is still unknown. Therefore, although HCFCs are effective as a short-term replacement for CFC’s, they are not environmentally sustainable to be used in the long term (and so their use needs to be discontinued in the near future)

HFC’s (hydrofluorocarbons)

Hydrofluorocarbons are a group of CFC replacements which contain no bromine or chlorine. Their ODP is zero (they do not cause ozone depletion at all) because they contain no C-Cl or C-Br bonds. Furthermore, the C-H bonds allow some decomposition in the troposphere. They are being used in place of CFCs in refrigeration and air-conditioning units.

As they have zero ODP, HFCs are a promising alternative for CFCs and HCFCs in the long term. However, currently their use is limited by the fact that they expensive to produce, less effective in application, and are strong contributors to the greenhouse effect. Therefore, although HFCs have the

potential to become effective replacements for CFCs in the long term, more research must be undertaken first.

5. Human activity also impacts on waterways. Chemical monitoring and management assists in providing safe water for human use and to protect the habitats of other organisms

Identify that water quality can be determined by considering: concentrations of common ions, total dissolved solids, hardness, turbidity, acidity, dissolved oxygen and biochemical oxygen demand

In scientific terms, the criteria used to assess water quality include: concentration of ions, total dissolved solids, hardness, turbidity, acidity, dissolved oxygen and biochemical oxygen demand:

Identify factors that affect the concentrations of a range of ions in solution in natural bodies of water such as rivers and oceans

Within a natural body of water, there are many factors that affect the ion concentrations within it:

The pH of the rainwater – the more acidic the water, the more ions will dissolve (with the exception of CO3

2-. Very acidic waters can leach toxic metals such as Al3+

Leaching of rocks and soils – ground water remaining in contact with rocks and soil for long periods of time will have a high concentration of ions (that are in these rocks/soil)

Water temperature – minerals dissolve faster at higher temperatures Heavy rain/flooding – sustained heavy rain/flooding will dilute a body of water, reducing ion

concentration