2014 CHEMISTRY HSC ENRICHMENT LECTURE -...

No 1Stuart Terry, Department of Chemistry and Biomolecular Sciences

2014 CHEMISTRY HSC ENRICHMENT LECTURE

Chemistry and the Atmosphere

and Monitoring Water Quality

For these lecture notes See www.cbms.mq.edu.au/hsc_enrichment

See HSC Booklet from page 29

No 2

Are those clickers working???How’s your day going?

1. OK but I’m saturated!

2. Loving every minute!

3. Can I go home now?

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No 3

Structure of the Atmosphere

Troposphere – ~ 11 km above earth’s surface. Temperature decreases with altitude. Good convectional mixing of air. Substances at ground level get mixed thoroughly in the troposphere

Stratosphere - 15-50 km above earth’s surface. Temperature increases with altitude. Very little mixing of air.Stable air composition.

Substances do not easily transfer from troposphere to stratosphere.

No 4

Composition of the Dry AtmosphereGas Formula %Volume

Permanent Gases – Concentrations do not vary significantly

nitrogen N2 78.1 oxygen O2 20.95 argon Ar 0.93 neon Ne 0.0018 (18 ppm)

helium He 0.00052 (5.2 ppm) krypton Kr 0.0001 (1 ppm)

hydrogen H 0.00005 (0.5 ppm) xenon Xe 0.0000008 (8 ppb)

Variable Gases – Concentrations vary

water H2O 0 - 4 carbon dioxide CO2 0.04 (380 ppm)

methane CH4 0.00018 (1.8 ppm) nitrous oxide

(laughing gas) N2O 0.0000315 (315 ppb)

•Essentially constant, with regard to N2, O2and noble gases, up to 90 km, just becomes less dense with altitude.

No 5

Can Humans Affect the Composition of the Atmosphere?

If the Earth were the size of an apple, the apple’s skin and the atmosphere would be approximately the same thickness.

Since the industrial revolution, the concentration of CO2 in the atmosphere has increased from 280 ppm to over 400 ppm in many areas.

No 6

Main Atmospheric Pollutants and Sources

POLLUTANT SOURCE

Carbon monoxide (CO)

Combustion (cars, cigarettes, fires, slow combustion stoves)

Ozone (O3) Photochemical smog, lightning

Oxides of nitrogen -NOx (NO, NO2)

Combustion (cars, power stations)

Hydrocarbons and volatile organic compounds (VOCs)

Motor vehicles, factories, commerce, homes

SO2 Combustion, metal extraction, chemical manufacturing

CFCs Before 1996 from fire extinguishers, air conditioning, refrigerators, foam plastics

No 7

Ozone in The Atmosphere

• Ozone, O3, is naturally present in the atmosphere.• Near ground level in clean air it is about 0.02 ppm.• In the stratosphere it can reach up to 12 ppm. • In the stratosphere it absorbs most of the

biologically damaging UV-B radiation from the sun.• At ground level it is poisonous to humans and other

living organisms.

• Human activity is producing ozone at ground level where we do not want it and destroying it in the stratosphere where we do want it.

No 8

Ozone Structure and Properties

Molecular oxygen

Ozone O3

O2 O O O O

O OO

OO O

Coordinate covalent bond

Allotropes – different forms of elements in the same state/phase

No 9

Differences in Reactivity of Ozone and Molecular Oxygen

O2(g) 2O(g) H = +498 kJ/mol

However:O3(g) O2(g) + O(g) H = +106 kJ/mol

It requires less energy to produce an oxygen atom (free radical) from ozone hence ozone is much more reactive.

O3(g) is dangerous to living organisms as it triggers a complex series of reactions that result in the breakdown of important biological molecules.

No 10

Q18 (2008) – 5 marks (see p32)

• Required drawing the Lewis dot structures for both O2and O3 (2 marks) and account for the differences in properties of O2 and O3 on the basis of molecular structure and bonding (3 marks).

• O2 is a linear molecule whereas O3 is bent. Due to the larger molecular weight and surface area of ozone its intermolecular forces (dispersion forces) are stronger resulting in higher melting and boiling points. O3 is much more chemically reactive as it requires less energy to decompose an ozone molecule than an O2 molecule.

• [Multiple choice Q3 - comparison between oxygen, O2, and ozone, O3, required identifying them as allotropes.]

No 11

What property of O3 makes it more soluble in water than O2 in water? (HSC

2011)1. O3 is a polar molecule.2. O3 has a resonance

structure.3. O3 is a highly reactive

molecule.4. O3 has a coordinate

covalent bond.

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No 12

Ozone in Troposphere – close to us

• Formed in photochemical smog• Formed from the chemical

reactions in the atmosphere between primarily exhaust gases from motor vehicles, sunlight and oxygen.

• The reactions depend on high motor vehicle numbers, weather conditions and sunlight.

• Ozone concentration is an accepted indicator of smog level

No 13

Reactions in the Troposphere

• NO2 + sunlight NO + O

• O + O2 O3 (increased ozone levels in troposphere)

• NO + O3 NO2 + O2 BUT….

• NO + O2 + VOCs + sunlight NO2 (lead to

more O3)

• NO2 + VOCs peroxyacyl nitrates (PANs) (major irritants)

No 14

Q18 2009 (5 marks), see p32“There has been an increase in the concentration of the oxides of nitrogen in the atmosphere as a result

of combustion.Assess both the evidence to support this statement

and the need to monitor these oxides.”

No 15

• CSIRO and NSW EPA take direct measurements of NOx• Major sources of NOx are combustion of coal in power

stations and fuel in motor vehicles.• N2(g) + O2(g) → 2NO(g) (combustion)• 2NO(g) + O2(g) → 2NO2(g)(oxidation of NO in lower atmosphere in presence of sunlight)

• NO2(g) is involved in the production of ozone, a pollutant, in the lower atmosphere in the presence of sunlight.

• NO2(g) + UV → NO(g) + O(g)• O(g) + O2(g) → O3(g)

• Catalytic converters on cars reduce the emissions of NOx by reducing NOx to N2.

• NOx also contributes to acid rain which adversely affects both living organisms and man-made structures.

• 2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)

No 16

No 17

Ozone in the Stratosphere

• Maximum concentration (~12 ppm) of ozone occurs at about 25 km above the earth’s surface (stratosphere).

• Ozone in the stratosphere absorbs most of the solar UV-B (320-280 nm) radiation, which, if it reaches ground level, causes skin cancer, eye cataracts, decreased immune response, plant damage and severe sunburn.

• O2 + UV light 2O• O + O2 O3 (exothermic)• O + O3 2 O2 (exothermic)• O3 + UV light O + O2

No 18

The Ozone Hole

• In 1985 Dr Joe Farman (British Antarctic Survey) discovered that the concentration of ozone in the stratosphere over the Antarctic had been decreasing since the early 1970s by up to 50%. The ‘hole’ appears in spring ie September – October, each year.

• An ozone hole has also been observed over the Arctic since the mid 1990s.

No 19

Ozone Hole over the Antarctichttp://science.nasa.gov/headlines/y2000/ast02oct_1.

htm

Blue denotes low levels of ozoneYellow denotes high levels

No 20

What Causes the Ozone Hole?

• Synthetic chlorinated fluorocarbons (CFCs) are extremely stable in the troposphere and eventually find their way up to the stratosphere where they react with high energy UV light.

• ExamplesCFC-11 = trichlorofluoromethane, CCl3FCFC-12 = dichlorodifluoromethane, CCl2F2

No 21

Reactions in stratosphere:• CCl3F + UV light Cl + CCl2F• Cl + O3 ClO + O2 (ClO is very reactive)• ClO + O Cl + O2

• Overall: O3 + O 2O2

Ozone has been converted to oxygen gas and Cl atom (radical) has not been used up, it is still available for many more ozone destroying reactions. 1 Cl atom can destroy up to 100,000 ozone molecules. Cl is a catalyst and the reaction is a chain reaction.

• Cl is eventually removed by reactions:• Cl + CH4 HCl + CH3

• ClO + NO2 ClONO2

No 22

Which of the following gases can cause major depletion of the ozone

layer? (HSC 2011)

1. 2. 3. 4.

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1. O2

2. NO2

3. CO2

4. CCl3F

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No 23

Main Atmospheric Pollutants and SourcesQ29, 2012 exam (see p32)Draw a diagram to show the layered structure of the atmosphere. In your diagram include: the names of TWO atmospheric pollutants, positioned in the layers where the detrimental impact occurs and the names of the sources of the two pollutants identified.

Sample Answer right and below:

O3 – photo-oxidation or industrial production CO – incomplete combustion, motor vehicles, other combustionNOx – internal combustion engineSOx – fossil fuels combustionCO2 – fossil fuels CH4 – agriculture, anaerobic processes

TroposphereO3, NOx, SOx,CO, CO2, CH4

Stratospherechlorofluorocarbons

Mesosphere

Thermosphere

No 24

Part of the Solution• CFCs have been replaced by hydrochlorofluorocarbons,

HCFCs (contain H and C as well as F and Cl) and hydrofluorocarbons, HFCs (contain no chlorine).

• HCFCs and HFCs contain C-H bonds that are readily attacked and these molecules are decomposed in the troposphere.

• Additionally, HFCs have no C-Cl bonds and therefore have no ozone depleting effects. (Q27, 2004 examined this)

No 25

Choose the incorrect statement about ozone

1. 2. 3. 4.

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1. Ozone is a measure of pollution in troposphere.

2. Higher levels of ozone in the stratosphere are normal.

3. O3 and O2 are allotropes.4. Ozone is an ionic

substance.

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No 26

Past HSC Exam Questions on Chemistry and the Atmosphere

• Identifying atmosphere layers, common atmospheric pollutants and role of ozone in troposphere and stratosphere – questions on ozone structure and properties also commonly asked

• Photochemical smog – what it consists of, effects, role and source of O3 and NO2 (with equations)

• Ozone hole – role of CFCs (with equations), effect of HCFCs and HFCs, naming haloalkanes

No 27

Sample Past HSC Exam Questions & Answers

• See p32-33• Include atmosphere levels and pollution (2004,

2007, 2012) and questions about structural features of ozone

• Damaging and protective effect of ozone (1998, 2001, 2006, 2011), NOx (1998, 2009) and CFCs (1998, 2002, 2004, 2007, 2008) needed with relevant equations and why it is important to monitor

No 28

MONITORING WATER QUALITY (p33)

No 29

Our Water Quality

• Water is one of our most important natural resources. Humans require clean, safe water for purposes such as:– Drinking – Agriculture– Industry– Recreation

• Our water quality is at risk because of urban and agricultural expansion into catchment areas due to population increases.

• Water needs to be monitored, managed and protected.

No 30

Our Water Quality

Water quality indicators:• Biological: bacteria, algae, viruses and parasites• Physical: temperature, turbidity, colour, salinity,

suspended solids, dissolved solids• Chemical: pH, dissolved oxygen, biochemical oxygen

demand (BOD), P and N, organic (eg pesticides and herbicides) and inorganic (eg metal ions) compounds

• Aesthetic: odours, colour, floating matter• Radioactive: , and radiation emitters

No 31

Tests for Common Ions• Precipitation (gravimetric analysis) – useful as a

qualitative test for many ions and for quantitative analysis where concentrations are high

• eg Ag+ to precipitate Cl- , Ba2+ to precipitate SO42-.

Image of AgCl precipitation fromhttp://www.dartmouth.edu/~chemlab/chem3-5/qual_cat/graphics/procedure/proc6.gif

No 32

HSC Exam Q27 (2007)

This question had a diagram of AgNO3 being added to the water sample and the precipitate being collected. It required a description on how this could be used and a balanced equation.

Best answers indicated that AgNO3 had to be added in excess and the ppt collected (AgCl) had to be dried to constant mass.

AgNO3(aq) + Cl-(aq) -> AgCl(s) + NO3-(aq)

No 33

Tests for Common Ions

• Atomic Absorption Spectrophotometry (AAS) for metal ions

• AAS was developed in the 1950s by an Australian CSIRO scientist, Alan Walsh

• It uses the unique light absorption spectrum of each metal atom to measure the concentration of the metal in the water sample and is a useful method for ppm concentrations

No 34

Tests for Common Ions

• Ion Selective Electrodes (ISEs) for low ion concentrations eg fluoride (F-), chloride (Cl-), nitrate (NO3

-), sulfate (SO42-) ion concentrations

Image from http://www.rmprocesscontrol.co.uk/images/direct.jpg

No 35

Tests for Common Ions

Colorimetric – used commonly for measuring low phosphate (PO4

3-) and nitrate concentrations• Ammonium molybdate, (NH4)2MoO4 added to

phosphate sample, followed by ascorbic acid (vitamin C) => intense blue colour (molybdenum blue), which can be compared to standard phosphate solutions treated identically

• Nitrate (NO3-) reduced to nitrite (NO2

-) and then colour-producing reagent added to form pink-purple azo-dye

No 36

Total Dissolved Solids (TDS)

• Measurement of mass of residue by evaporation of filtered water from a known volume of water sample. Unit usually mg/L

• <500 mg/L good quality drinking water• >1000 mg/L poor quality• As the majority of the solids are typically salts (ionic),

electrical conductivity of the water is often used to measure TDS. Units are in Siemens cm-1 of mS cm-1.

No 37

Test for Hardness

• ‘Hard’ describes water that will not lather with soap but instead forms a ‘scum’

• Soap ‘scum’ is the insoluble salt of the long chain (~C16-C18) alkanoic acid of the soap

eg Ca2+(aq) + C17H35COO-(aq) -> Ca(C17H35COO)2(s)

• The ion causing hardness is predominantly calcium but magnesium and aluminium also contribute

No 38

Test for Hardness

• Shaking a water sample with soap is a method of determining whether the water is ‘hard’ or ‘soft’. If the water is hard then a precipitate or scum will form. (Q26, 2002 exam (a))

• Hardness is measured by an EDTA (ethylenediamine tetraacetic acid) titration, which binds the cation

No 39

HSC Exam Q26 (2002) part b - 5 marks

• A sample of hard water contains 610-4 mol L-1 of magnesium carbonate. Calculate the mass, in mg, of magnesium carbonate in 150 mL of this sample.

• moles MgCO3 in 150 mL = molarity x volume (L)= 6 x 10-4 mol/L x 0.150 L= 0.00009 mol

mass MgCO3 = mol x molar mass= 0.00009 mol x 84.3 g/mol= 0.007587 g= 7.587 mg= 8 mg (1 significant figure)

No 40

Dissolved Oxygen and Biochemical Oxygen Demand (BOD)

• Small amounts O2 gas dissolve in water (~9 ppm at 20oC)• Essential for aquatic life and breakdown of organic matter• If dissolved O2 <5 ppm aquatic life suffers• Biochemical oxygen demand (BOD) is a measure of the

dissolved oxygen required for the complete breakdown of the organic matter in the water by aerobic bacteria

• In natural and unpolluted water ~5 ppm. Raw sewage and stormwater runoff from urban areas have large BOD (100 ppm for sewage and urban 100 – 500 ppm)

No 41

Measuring Dissolved O2

• dissolved oxygen in water measured by:– Titration (Winkler method) – dissolved O2 oxidises

Mn2+ to Mn4+ (in alkaline solution), which oxidises I- to I2, which is then titrated with thiosulfate, S2O3

2- in the presence of starch – forms intense blue with I2 which disappears at end point

– Oxygen-sensitive electrode

No 42

N:P ratio• N - present mainly as nitrate, nitrite, ammonia, organic N• P - present mainly as phosphate, organic P• Sources of N and P: fertilisers, sewage, detergents• A ratio of 10:1 (approximately) is recommended for N:P.

A lower ratio indicates conditions favourable for eutrophication (the enrichment of a water body by nutrients such as fertilisers and/or sewage) and hence high algal growth

• Algae use nutrients, deplete oxygen – major damage

No 43

Algae Problem in Beijing (2008)

Image from http://wwwimage.cbsnews.com/images/2008/06/30/image4219827.jpg

No 44

2010 HSC Exam, Q10What is the consequence of having large concentrations of

Mg2+ and Ca2+ ions in waterways?

1. 2. 3. 4.

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1. Turbidity 2. Hardness 3. Eutrophication 4. Heavy metal

contamination

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No 45

Factors Affecting Levels of Ions and Other Substances in Natural Water

Ways

• Rainwater dissolves CO2 (also SO2 and NOx) from the atmosphere, cations eg Ca2+, Mg2+, Fe3+, Mn2+, Cu2+, Zn2+, and anions eg SO4

2-, Cl-, CO32-, from rocks and

soils• Agricultural activities and land clearing can result in

fertilisers, pesticides, herbicides and inorganic salts being added to natural water

• Industrial activities and mining – organics from petrochemical industries, also heavy metals such as Cu, Hg, Cd, Cr, Pb, Zn, As, Au

No 46

• Sewage and stormwater runoff – microorganisms, organic substances

• Leaching from rubbish dumps and mine tailings egCd, Hg, Pb from batteries, Zn, nitrate, phosphate, from decaying organic waste

No 47

Purifying and Sanitising Mass Drinking Water Supplies

• Catchment should be protected with people and activity excluded from the areas immediately surrounding water storage facilities

• Water is treated. Ferric chloride (Sydney) or aluminium salts are used as a

flocculating agent to cause small precipitate particles to come together to form large particles which can then settle out

water is filtered through sand and gravel water is chlorinated (by hypochlorite ions, OCl-) to kill bacteria

and some viruses. Fluoride, in the form of NaF, is sometimes added (Sydney) to strengthen tooth enamel in growing children

Water quality is monitored at every stage. (Q27, 2002)

No 48

• Membrane filters are widely used for filtering very small particles (> 0.2 m) from drinking water, treated sewage and water for industry (esp beverages including bottled water).

• Dirty water in at top, clean water out the bottom.

No 49

FeCl3 and Al2(SO4)3 can be used in the purification of town water supplies. What is the

role of these chemicals? (HSC 2011)

1. To disinfect water by removing bacteria.

2. To remove particulate material by flocculation.

3. To control the concentration of total dissolved solids.

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No 50

Past HSC Exam Questions

• Testing for common ions especially phosphates and halides

• Defining and testing for hardness, biochemical (biological) oxygen demand

• Sources of common ions and other contaminants• Purification and sanitisation of water systems• Eutrophication and tests to measure• Some ppm, Molarity and mass calculations

No 51

Sample Past HSC Exam Questions and Answers pp35-37

•Questions on defining main water quality measurements and how to determine them (especially biochemical oxygen demand (BOD), hardness, P and N levels and inorganic (eg metal ions with equations) compounds (1996, 1998, 2002, 2004, 2005, 2007, 2009, 2010, 2011, 2012)

•Questions on explaining quality of water in various locations very common (1998, 2005, 2008, 2011)

•Eutrophication process (2003, 2009, Q25 2012) and questions on sanitisation (2002, 2006, 2009) also turn up

No 52

Q25 (2009) – 7 marksStudents given absorbance measurements of

phosphate concentrations in 3 streams and asked to analyse the phosphate concentrations in terms

of sources and discuss why phosphate concentration in water is a quality issue.

No 53

• One of the streams contained very high levels of phosphate. Possible sources are natural (from leaching of soils and minerals locally) and man-made (runoff from fertilised farm land; runoff and stormwater containing animal waste; effluent from sewerage treatment works).

• High levels of phosphate in water can cause eutrophication and algal blooms. With algal blooms oxygen levels in water become reduced affecting the life of all organisms living in the water and less sunlight penetrates to lower levels of the stream further reducing organism growth and oxygen generation. Some algae are toxic causing illness in animals drinking the water.

No 54

Q26 (2005)- 7 marks

• You are given a diagram of a water catchment area for a town and asked to describe 2 possible sources of contamination and assess purification methods that could be used for the water before it reaches the town. Some sources of contamination could be:– farmyard – animal excrement, fertilisers– vegetable patch – fertilisers and pesticides/herbicides– logged native forest – increased runoff taking decaying organic

material, from both plant and animal origins, soil particles– undisturbed pine forest – possible use of fertilisers and

pesticides/herbicides?– human recreational lake activity – fuel pollution, discarded

packaging eg empty drink bottles, gladwrap, cigarette butts etc.

No 55

In a water treatment plant the correct order of processes is

1. 2. 3.

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1. Flocculation, chlorination, filtration

2. Filtration, flocculation, chlorination

3. Flocculation, filtration chlorination

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No 56

Clickers and Surveys

Lecture notes will be made available soon at www.cbms.mq.edu.au/hsc_enrichment

THANK YOU AND GOOD LUCK