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9.4 Chemical monitoring and management: 4. Human activity and the atmosphere

Syllabus reference (October 2002 version)
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	Students learn to: describe the composition and layered structure of the atmosphere identify the main pollutants found in the lower atmosphere and their sources describe ozone as a molecule able to act both as an upper atmosphere UV radiation shield and a lower atmosphere pollutant describe the formation of a coordinate covalent bond demonstrate the formation of coordinate covalent bonds using Lewis electron dot structures compare the properties of the oxygen allotropes O2 and O3 and account for them on the basis of molecular structure and bonding  compare the properties of the gaseous forms of oxygen and the oxygen free radical identify the origins of chlorofluorocarbons (CFCs) and halons in the atmosphere  identify and name examples of isomers (excluding geometrical and optical) of haloalkanes up to eight carbon atoms discuss the problems associated with the use of CFCs and assess the effectiveness of steps taken to alleviate these problems analyse the information available that indicates changes in atmospheric ozone concentrations, describe the changes observed and explain how this information was obtained	Students: present information from secondary sources to write the equations to show the reactions involving CFCs and ozone to demonstrate the removal of ozone from the atmosphere gather, process and present information from secondary sources including simulations, molecular model kits or pictorial representations to model isomers of haloalkanes present information from secondary sources to identify alternative chemicals used to replace CFCs and evaluate the effectiveness of their use as a replacement for CFCs

Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 24Jun08]

Prior learning: Preliminary modules 8.2 (8.2.1, 8.2.2, 8.2.3, 8.2.5), 8.4 (8.4.2, 8.4.4), 8.5 (8.5.2, 8.5.3, 8.5.4, 8.5.5);

HSC modules 9.2 (9.2.2), 9.3 (9.3.2, 9.3.3).

You should be able to:

• manipulate data to change the units expressed into other units
• use the mole concept and associated calculations
• recall the prefixes for the first eight alkanes
• identify and apply cause and effect relationships.

Background: The chemical composition of the atmosphere depends on the gases released and absorbed at the surface of the Earth and the rates at which chemical reactions take place.

The many uses of chlorofluorocarbons (CFCs), initially thought to be a harmless group of substances, have been found to cause long-term problems for our atmosphere, humans and all ecosystems.

The discovery of the ozone hole has triggered more research, monitoring and problem solving amongst atmospheric scientists and mathematical modellers. International agreements, such as the Montreal protocol on substances that deplete the ozone layer (1987), helped to manage the situation by reducing the production and use of CFCs. Alternative chemicals have been used in our homes and industry, in order to maintain the quality of life enjoyed by many people.

describe the composition and layered structure of the atmosphere.

• The atmosphere is made up of two main layers: the troposphere and the stratosphere. It is within these layers that ozone exists. The troposphere extends from the Earth’s surface to 15 kilometres above sea level. Over 90% of Earth's gases are in the troposphere. Temperatures drop with altitude in the troposphere. At the top there is a region where the temperature remains reasonably stable. This is called the tropopause and it reduces the mixing of gases. The stratosphere (upper atmosphere) is above the tropopause. In the stratosphere temperatures rise with increasing altitude.
   
• The atmosphere is predominantly composed of nitrogen (78% by volume), oxygen (21%), and argon (0.93%), with the other gases in very small concentrations. Despite these small concentrations, many of the other gases (e.g. ozone) cause concern because of the reactions they can undergo.

Composition of the atmosphere CFC StarTec LLC, USA.

identify the main pollutants found in the lower atmosphere and their sources.

• Main pollutants and sources are listed in the following table.

  Main pollutants	Main sources
  carbon monoxide	incomplete combustion in stoves, cars, fires and cigarettes
  nitrogen oxides	combustion at high temperatures in vehicles and power stations
  volatile organic compounds, such as hydrocarbons	solvents and unburnt fuels
  sulfur dioxide	some metal extraction processes and the burning of fossil fuels
  lead	leaded fuels, metal extraction, renovating old houses containing leaded paints and electrical wire coverings
  particulates	incomplete combustion, earthmoving dust storms and some agricultural and industrial practices

describe ozone as a molecule able to act both as an upper atmosphere UV radiation shield and a lower atmosphere pollutant.

• The ozone molecules in the stratosphere form a very thin layer that protects us from harmful UV radiation.
   
• In contrast, the ozone in the troposphere is a pollutant, even at the very low concentrations compared with the other gases. Ozone is a very reactive molecule capable of oxidising many substances.
  Fortunately most of the ozone occurs in the stratosphere.

describe the formation of a coordinate covalent bond.

• A coordinate covalent bond forms when one atom in a species (a molecule or ion containing non-metallic atoms) provides both electrons in the covalent bond.
   
• Once formed this coordinate bond is indistinguishable from other covalent bonds.

demonstrate the formation of coordinate covalent bonds using Lewis electron dot structures.

• Ions, such as the hydronium H₃O⁺ and the ammonium NH₄⁺, contain a coordinate covalent bond. In the formation of the hydronium ion, one of the non-bonding electron pairs on the oxygen atom is used to form a covalent bond between the hydrogen ion H⁺ (which has no electrons) and the oxygen atom.

  Formation of a coordinate covalent bond in the hydronium ion

  

  Formation of a coordinate covalent bond in the ammonium ion

  

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

• The chemical and physical properties differ, namely:
   

  Properties	gaseous oxygen	gaseous ozone	Explanation
  colour	colourless	blue	-
  boiling point	–183°C	–111°C	The boiling point of diatomic oxygen is lower than that of the ozone as diatomic oxygen has a lower molecular mass requiring less energy in the boiling process.
  solubility in water	sparingly soluble	more soluble than oxygen	Non-polar O2 does not form strong intermolecular forces in the polar water. Ozone has a bent structure, which provides for some polarity of the molecule in its interaction with water.
  chemical stability	far more stable than the ozone molecule	far less stable than the oxygen molecule	Ozone is easily decomposed into oxygen molecules: #More detailed information below.
  oxidation ability	less powerful oxidant	more powerful oxidant	e.g. reaction with metals: oxygen forms the oxide as the only product whereas ozone reacts more readily producing the metallic oxide and an oxygen molecule.

  ^#More detailed information regarding chemical stability:

  The oxygen molecule contains one double covalent bond O=O.

  Lewis electron dot structure for oxygen

  

  The ozone molecule can be represented as containing a covalent double bond and a coordinate covalent single bond.  The coordinate covalent bond can be represented by an arrow.

  Lewis electron dot structure for ozone

  

  Measurements show that the bonds between the oxygen atoms in ozone are of equal length and strength and can be represented so:

  

  The two identical oxygen to oxygen bonds in ozone consist of a single bond and a partial bond. This results in lower stability of the ozone molecule, compared with the diatomic oxygen molecule.

compare the properties of the gaseous forms of oxygen and the oxygen free radical.

• The oxygen atom in its ground state (electrons in the lowest possible energy levels) has 3 pairs of electrons in its valence shell.
   
• When UV energy splits an oxygen molecule, two oxygen atom radicals are formed, i.e. they each have two electron pairs and two unpaired electrons. The energy absorbed in the splitting and the unpaired electrons make the free radical very reactive.
  
  Lewis electron dot structure of gaseous forms of oxygen

  

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

identify the origins of chlorofluorocarbons (CFCs) and halons in the atmosphere.

• CFCs were developed to replace ammonia as a refrigerant in the 1930s. At the time, their properties were found to be ‘safer’ than the ammonia.
   
• Their properties, such as inertness and low boiling point (near room temperature), also led them to be used more widely as solvents, propellants and blowing agents in foams.
   
• As CFC products were used, the gases were released to the atmosphere.
   
• It was discovered that the CFCs were so inert they did not react in the troposphere. They gradually make their way to the stratosphere where UV energy breaks C–Cl bonds releasing Cl free radicals. Chlorine and related free radicals deplete the ozone layer.
   
• Halons are fluorocarbons containing bromine.  They used to be used extensively in fire extinguishers for electrical fires or to protect computer systems.  Fortunately they were never used as extensively as CFCs were.
   
• Halon use has been drastically reduced because bromine atoms are even more effective than chlorine atoms in the chain reactions that lead to the depletion of the ozone layer.
   
• The concentration of the chlorine radicals that react with ozone is increased in springtime in the Antarctic with the return to longer periods of sunlight. The rate is reduced by summer when the chlorine is basically used up after the accumulation over winter.

gather, process and present information from secondary sources including simulations, molecular model kits or pictorial representations to model isomers of haloalkanes.

• Gather the names and general formulas of some haloalkanes up to eight carbons.
   
• Determine the possible isomers for these haloalkanes by making models of them or by drawing representations of them. Present your isomers as structural formulae. Write the systematic names for each isomer using the IUPAC (International Union of Pure and Applied Chemistry) convention.
   
• Process your representations of isomers and evaluate their validity by comparing them with examples from other secondary sources.

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

• Isomers are molecules with the same molecular formula but different structural formulas (arrangements of atoms). For example, there are a number of isomers for C₅H₁₀BrCl. Some are shown below.

  Isomers of haloalkanes

  

  How many isomers are there for C₂H₃Cl₂F?

• The longer the carbon chain, the more possible isomers there will be.

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

• Scientists identified that a dramatic decline in springtime ozone occurred from the late 1970s over the entire Antarctic. The decline reached approximately 30% by 1985. In some places, the ozone layer had been completely destroyed. The ozone decline over Antarctica during springtime now exceeds 50%.
   
• Measurements of ozone levels in the atmosphere can be taken using UV spectrophotometers. Measurements were taken by British scientists in the Antarctic using UV spectrophotometers directed vertically up into the atmosphere.
   
• UV spectrophotometers could also be directed vertically down. Helium filled balloons were used to carry UV spectrophotometers. Satellites were used to carry a device called a total ozone mapping spectrophotometer (TOMS), which proved very efficient in recording changes in ozone levels.
   
• Scientists recorded the total ozone per unit area as a function of time and noted the changes by season and since their recordings began (a dramatic increase due to increasing use of CFCs).
   
• Data from NASA satellites can now be seen via the Internet at Total Ozone Mapping Spectrometer (TOMS)   the official web site for information, data, and images from the Total Ozone Mapping Spectrometer (TOMS) instruments, NASA, USA.

Changes in the ozone layer and climate Roderick Jones, Queens' College Cambridge, UK.

present information from secondary sources to write the equations to show the reactions involving CFCs and ozone to demonstrate the removal of ozone from the atmosphere.

• CFCs can undergo photodissociation (reactions using the energy of light to break bonds) to form reactive chlorine atom radicals. The chlorine atom radical then rapidly reacts with an ozone molecule to produce the chlorine oxide molecule, ClO. The chlorine oxide molecule can react with a free oxygen atom (which could have formed O₃ by reaction with O₂) regenerating a Cl atom. This information can be presented effectively by the use of a series of chemical equations.

  The reactions below represent depletion of the ozone layer in the stratosphere. All species are gases. The CF₃, Cl, ClO and O are freee radicals with unpaired electrons and thus are very reactive.

  

  Ozone depletion is more frequent in winter and spring due to more ice particles. These provide a surface to act as a catalyst.

present information from secondary sources to identify alternative chemicals used to replace CFCs and evaluatethe effectiveness of their use as a replacement for CFCs.

• After gathering appropriate information, present a short report to summarise and evaluate the alternatives to CFCs. A sample report is provided below.

HCFCs and HFCs CFC Replacements, Maxfields Freeserve, Bristol, UK

Ozone depletion glossary US Environmental Protection Agency, USA

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

• The problems associated with CFCs include:
  • depletion of the ozone layer
  • more UV radiation reaching Earth, which increases the chances of cancer in living things (including humans)
  • an increase in the enhanced greenhouse effect.

  The enhanced greenhouse effect

  The enhanced greenhouse effect is caused by gases, released by human activity, absorbing heat rays that come from the Earth's surface, then emitting the heat rays. Many of these heat rays come back to the Earth's surface. This raises the temperature of the atmosphere.

  These problems are caused by CFCs moving to the stratosphere where the UV radiation photodissociates the CFCs by breaking C-X bonds. When these bonds break, halogen radicals (X) are formed, which react with an ozone molecule to form new compounds, such as ClO or BrO. A chain reaction occurs and this decreases the ozone concentration allowing more UV to penetrate to the surface of the Earth. The CFCs have a long lifetime and can last up to 150 years.

  Steps to reduce the formation of the ozone hole have been taken since its cause was identified. These steps include:

  • the Montreal Protocol, an international treaty designed to gain cooperation for the global reduction in the production of CFCs and halons (bromine containing carbon compounds).
  • the identification and introduction of alternative chemicals, such as hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs).
  • assistance to less developed countries to phase out the use of CFCs.

  The effectiveness of these steps is dependent on a number of actions. 

  • Global control of production and use of CFCs is needed. If governments do not adhere to the Montreal Protocol it may be difficult to ensure that CFC use is halted and that CFC levels in the atmosphere are actively reduced.

  We cannot remove the CFCs already in the stratosphere at this stage of technological development. So some measures are needed to reduce the effects of the problems caused by CFCs, such as high levels of UV radiation. These include:

  • people using new sunscreens, as advised by organisations like the Cancer Council.
  • the use of UV stabilizers in polymers that are exposed to sunlight to reduce breakdown by UV radiation.