Ch. 4 - Atmosphere

Question 0

Sources and effects of oxides of nitrogen

Answer 0

High temperature combustion, e.g. vehicles and power stations
Acid rain, photochemical smog, irritant to respiratory system and eyes

Question 1

Identify main pollutants found in the lower atmosphere

Answer 1

Oxides of nitrogen, sulfur dioxide, carbon monoxide, carbon dioxide, volatile organic compounds and CFCs.

Question 2

Sources and effects of sulfur dioxide

Answer 2

Combustion of fuels containing sulfur impurities, metal smelters, power stations
Acid rain and smog, irritant to respiratory system

Question 3

Sources and effects of CO

Answer 3

Incomplete combustion of fossil fuels, motor vehicles, bushfires
Toxic, reduces oxygen in blood (via haemoglobin), fatigue

Question 4

Sources and effects of CO2

Answer 4

Combustion of fossil fuels in motor vehicles and electricity production
Greenhouse gas, contributes to global warming

Question 5

Sources and effects of volatile organic compounds

Answer 5

Exhaust gases of vehicles, homes (from paints and solvents)
Some are carcinogens and can irritate the lungs

Question 6

Sources and effects of chlorofluorocarbons (CFCs)

Answer 6

Formerly in refrigeration, air conditioning and foam plastics
Ozone depletion

Question 7

What is ozone?

Answer 7

• Ozone is a molecule able to act both as an upper atmosphere UV radiation shield and a lower atmosphere pollutant
• About 90% of atmospheric ozone is found in the stratosphere, where it protects life on Earth by absorbing short-wavelength UV radiation and acting as an upper atmosphere UV radiation shield.
  > O3 (g) –UV radiation–> O2 (g) + O· (g)
• This shield is vital since UV radiation has sufficient energy to damage proteins and DNA in cells
• The other 10% of atmospheric ozone is in the lower atmosphere, where it acts as a pollutant.
• It is a component and indicator of photochemical smog, and irritates the eyes and respiratory system.
• It is formed when sunlight splits off an oxygen atom from the NO2 molecule, and this O atom combines with O2 to form ozone
  1) NO2 (g) –UV radiation–> NO (g) + O· (g)
  2) O· (g) + O2 (g) —-> O3 (g)

Question 8

Describe the formation of a coordinate covalent bond

Answer 8

• A coordinate bond is a covalent bond in which both of the shared electrons come from the one atom
• For example, ozone is formed when one of the lone pairs on an oxygen atom in an oxygen molecule forms a new covalent bond with a third oxygen atom.
• During coordinate bond formation, there is a partial transfer of charge. The
  acceptor gains a half share of two electrons, so it gains a charge of -1.
• Other common molecules which have coordinate bonding include
  NH4+, H3O+, CO, and complex ions such as Ag(NH3)2+

Question 9

Compare the properties of the oxygen allotropes O2 and O3

Answer 9

MP and BP
O2: B.P. = -183oC, M.P. = -219oC
O3: B.P. = -111oC M.P. = -193oC

Solubility
O2: Slightly soluble
O3: More soluble than O2

Reactivity and stability
O2: Moderately reactive, moderately strong oxidising agent
O3: Highly reactive, very strong oxidising agent

Density
O2: Around same
as air
O3: Around 1.5 times of air

Question 10

Account for the differences between O2 and O3 in MP and BP

Answer 10

The intermolecular forces in O2 are weak dispersion forces, with no dipole-dipole forces due to an even sharing of electrons and linear molecular shape.
OTOH, the intermolecular forces in O3 include dipole-dipole forces due to its bent shape and uneven sharing of electrons (one double covalent, one coordinate covalent bond). Also larger molar mass hence more dispersion forces -> stronger intermolecular forces.
Hence O3 has stronger intermolecular forces and a higher melting/boiling point

Question 11

Account for the differences in solubility between O2 and O3.

Answer 11

Non-polar O2 does not form strong intermolecular forces in the polar water hence is not very soluble.
OTOH, ozone has bent structure, which creates slight polarity in molecule allowing it to have intermolecular interactions with H2O. The donation of the electron pair from the central oxygen atom leaves a small positive charge on the central oxygen atom.

Question 12

Account for the differences in density between O2 and O3.

Answer 12

Ozone has a higher density than oxygen because it has a higher molecular mass. Thus ozone takes up more mass in a given volume than oxygen resulting in a higher density.

Question 13

Account for the differences in reactivity between O2 and O3.

Answer 13

Ozone is highly reactive, readily decomposed by medium energy UV light. This is because its intramolecular forces aremuch more unstable. OTOH, oxygen is stable to UV light because it has a strong double bond.

Question 14

Account for differences in oxidation ability between O2 and O3.

Answer 14

Oxygen is a less powerful oxidant because oxygen contains
stable double covalent bond. Oxidises hot metal to form oxides
- Magnesium + oxygen –> Magnesium oxide.

OTOH O3 is a more powerful oxidant - comes from weakness of single coordinate bond; it easily releases oxygen whichcan then oxidise a compound.
- Magnesium + Ozone –> Magnesium Oxide + Oxygen

Question 15

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

Answer 15

Oxygen radicals (O)
– These radicals (which have 6 electrons in
the valence shell and 2 of these are unpaired electrons) are
very unstable and reactive due to the presence of unpaired electrons, making it more reactive than oxygen and ozone in the gaseous forms
- Hence the oxygen free radical exists only briefly before reacting, unless in the thermosphere where there are less particles.
- The oxygen free radical is also toxic, since it reacts with organic molecules in living cells, causing tissue damage

Oxygen molecules (O2) – this is a very stable form of oxygen 
that is necessary for respiration in all organisms.  

Ozone molecules (O3) – ozone is poisonous and a powerful 
oxidant. It exists only in low concentrations in the troposphere. It readily dissociates into oxygen and energised oxygen atoms.

Question 16

Identify the origins of chlorofluorocarbons (CFCs) in the atmosphere.

Answer 16

• 1930s - Introduced under the name Freons, replacement for ammonia for refrigeration
  
  • Odourless, non-flammable, non-toxic and very inert, which
    made them attractive
• Post-WW2: used as aerosol spray cans and foaming agents in the manufacture of polystyrene
• 1960s - with technological advancements in electronics - cleaning for circuit boards
• In these last three uses CFCs were directly released into the
  atmosphere
  
  • Not considered to be a concern since they were inert and non-toxic
    –> However this was the problem as they gradually accumulated in the troposphere

Question 17

Identify the origins of halons in the atmosphere

Answer 17

Halons are compounds of carbon, bromine and other halogens.They include bromine atoms in addition to chlorine and/or
fluorine atoms.

Most commonly used one was bromochlorodiflouromethane CBrClF2
- Was used in the BCF fire extinguishers (BCF = Br,Cl, F)
Another halon in widespread use bromotrifluoromethane, CBrF3, used in automatic fire extinguishing systems

Halons do not support combustion and thus as they were used onto fires, gaseous halons from this application were emitted into the atmosphere.

Question 18

Discuss the problems associated with the use of CFCs

Answer 18

1) Ozone depletion
- CFCs remain in the troposphere for decades, slowly diffusing.throughout the stratosphere.
- There they are decomposed by short wavelength UV rays (not removed by ozone yet), releasing chlorine free radicals
- These free radicals ultimately causes a reaction in which ozone molecule + an oxygen atom is converted into two oxygen molecules and the chlorine atom still remains, acting as catalyst to repeat this process in a chain reaction / cycle
- Hence just one molecule of CFC can destroy a large amount of ozone

• On a global year-round basis, CFCs have caused a 3-8% reduction in the amount of ozone in the stratosphere
• In recent years they have caused a 50-90% decrease over Antarctica during spring.

Ozone depletion is a problem as it causes
- Increased incidence of sunburn, skin cancer and eye cataracts
- Increased risk of disease and illness; lowers people’s immune response
- Reduced plant growth due to UV interference with their
mechanisms for photosynthesis (rice is particularly vulnerable)
- Increased damage to many synthetic materials

2) Enhanced greenhouse effect
- CFCs contribute to the enhanced greenhouse effect
- The Earth receives energy from incoming sunlight and radiates energy back into space, some of which is trapped in the atmosphere by gaseous molecules such as CFCs.
- This causes the temperature of the Earth to increase, having a huge effect on aquatic ecosystems

Question 19

Assess the steps taken to alleviate these problems.

Answer 19

The two main steps taken to alleviate ozone depletion and global
warming include

1) International treaties such as the Montreal Protocol
- A long term solution
- Has successfully phased out CFCs and other ozone depleting substances. Many of these substances are also greenhouse gases (has reduced greenhouse gases by 11 gigatons of CO2 from 1990 to 2010)
- Hence an extremely effective environmental treaty despite not being able to control damage caused by CFC released in the past

2) The replacement of CFCs with hydrofluorocarbons and hydrochlorofluorocarbons.
HCFCs
- More easily broken down in the troposphere due to the more reactive C-H bond
- Contain chlorine atoms thus can produce chlorine free radicals which can contribute to ozone depletion
- Also contribute to advanced greenhouse effect
- Very high stratospheric lifetime
- Only partially effective, as they contribute to some ozone depletion and the advanced greenhouse effect

HFCs

• now widely used as replacements for CFCs
• they also contain relatively reactive C-H bonds, allowing for greater decomposition in the troposphere
• however they are less efficient and more expensive than CFCs

Question 20

Analyse the information available that indicates changes in atmospheric ozone concentrations, and describe the changes observed

Answer 20

• Significant decreases in atmospheric ozone concentrations were first recorded during Antarctica’s springin 1976 by the British Antarctic Survey.
  
  • Used ground based Dobson UV spectrometers and air
    samples collected by high-altitude balloons and aircrafts.
• Since then, satellites with UV detectors and spectrometers such as TOMS (Total Ozone Mapping Spectrometer) have given more accurate and global information about stratospheric ozone depletion.
• TOMS instruments orbited the Earth and measured backscattered UV light from Earth to provide global measurements of total column ozone on a daily basis.
  
  • Has led to the current use of the Ozone Monitoring Instrument (OMI).
• These instruments observed the formation of Antarctica’s ozone hole, which twenty years ago spread over large areas including southern Australia.
  
  • Further significant ozone depletions were recorded in the southern springs of the 1990s, while even the Arctic had experienced small decreases.
• Other modern methods include UV lasers and spectroscopes, and electrochemical techniques
• The information confirms that atmospheric ozone concentrations have significantly decreased over the last 40 years, but fluctuate both seasonally (lowest in Antarctic springs) and annually.
• In the last decade, the rate of decrease in atmospheric ozone concentrations has been reducing, and there have been signs that ozone levels may be increasing.