The rate of reaction is given by:
k[O2][O]
k is the rate constant for the reaction and [O2] and [O] are the concentrations of oxygen molecules and oxygen atoms (Gwen & Mike Pilling ‘Do CFCs destroy the ozone layer?’ from Chemistry Review, March 1993). Reference tables have been drawn up giving rate constants for hundreds of possible reactions, and showing how the rate constants vary with temperature and pressure (Gwen & Mike Pilling ‘Do CFCs destroy the ozone layer?’ from Chemistry Review, March 1993).
Meteorology involves knowing the conditions in the stratosphere and how the gases circulate and mix.
Computers may be used. Information from all the sources is fed into computers to produce an overall simulation of what happens in the stratosphere (Gwen & Mike Pilling ‘Do CFCs destroy the ozone layer?’ from Chemistry Review, March 1993).
To investigate the link between ozone depletion and CFCs scientists flew into the ozone hole and measured the concentrations of ClO radicals and ozone (Gwen & Mike Pilling ‘Do CFCs destroy the ozone layer?’ from Chemistry Review, March 1993).
Section 2: Summarise the evidence that led scientists to link ozone depletion with the use of CFCs
The link between ozone depletion and CFCs was established in 1987 by an international team of scientists based at Punta Arenas (Gwen & Mike Pilling ‘Do CFCs destroy the ozone layer?’ from Chemistry Review, March 1993). Scientists flew into the ozone hole and measured the concentrations of ClO radicals and ozone. The concentration of ozone fell dramatically at the point where the concentration of ClO radicals soared. This was undeniable evidence that a catalytic cycle involving Cl radicals is involved in ozone depletion.
Section 3: Describe the chemical reactions by which ozone is depleted in the stratosphere and explain why ozone depletion is most severe over Antarctica in the southern spring
Some gases such as nitrogen dioxide (NO2) and methane (CH4) can react with Cl and ClO radicals and interrupt the catalytic cycle:
Cl + CH4 HCl + CH3
ClO + NO2 ClONO2
(Equations from Gwen & Mike Pilling ‘Do CFCs destroy the ozone layer?’ from Chemistry Review, March 1993).
The chlorine atoms become bound up in the stable reservoir molecules, HCl and ClONO2 and remain chemically inactive until released once more when these molecules eventually break down. The formation of such reservoir molecules helps to reduce the loss of ozone at most latitudes.
Ozone depletion is dramatic in Antarctic spring due to special weather conditions whereby the sun disappears for 6 months in winter (Salters Chemical Storylines, Second Edition, p75). A vortex of cold air forms over the South Pole in June. As the air cools and temperature falls, clouds begin to form in the cloudless stratosphere. Water vapour condenses as ice on the surface of the small nuclei of sulphuric acid. The solid particles provide a surface for chemical reactions to take place. Nitric acid stays in the cloud and chlorine molecules are released into the stratosphere. When sunlight returns to the vortex in September, it splits the chlorine molecules into chlorine atoms causing ozone concentrations to fall.
Section 4: Explain why CFCs have been used so widely and discuss the advantages and disadvantages of their replacements
CFCs were discovered in 1930 by Thomas Midgley (Salters Chemical Storylines, Second Edition, p75). They contain chlorine, fluorine and carbon, all of which have very different boiling points. This is a useful property as there were CFCs with different boiling points to suit different applications. They are also unreactive, and have low flammability and low toxicity. Some uses include as propellants for aerosols, as refrigerants, as cleaning solvents and as blowing agents for making expanded plastics (Salters Chemical Storylines, Second Edition, p76).
Hydrofluorocarbons (HFCs) have been accepted by industry and manufacturers as the preferred replacements for CFCs and HCFCs (Dr Dick Powell, ‘The rise and fall of CFCs’ from Chemistry Review, September 1996). They have been accepted because they offer the safety the safety advantages for which CFCs and HCFCs were chosen originally, but don’t have the same effect on the ozone layer (Dr Dick Powell, ‘The rise and fall of CFCs’ from Chemistry Review, September 1996). Appropriately chosen formulations combine the non-flammability and low toxicity of the CFCs and HCFCs while avoiding their damaging effect on the stratospheric ozone.
In Northern Europe a return to hydrocarbons has occurred. Their claimed advantage is not only do they not lead to destruction of stratospheric ozone, but they also have lower global warming potentials than the HFCs. However, the acceptability of hydrocarbons depends upon the importance of their flammability hazard, the pollution from emissions (photochemical smog) and the energy consumption of the systems (Dr Dick Powell, ‘The rise and fall of CFCs’ from Chemistry Review, September 1996).
After many years of investigation and testing, two refrigerants have emerged as the current industry choice for replacement of CFC-12. They are HFC-134a and MP-39, which is a blend of HCFC-22, HFC-152a and HCFC-124 (http://www.edis.ifas.ufl.edu/EH313). Although less harmful than CFC-12, the HCFCs in MP-39 do have a measurable effect on. HFCs, which contain no chlorine, are more promising. Unfortunately, the ability of HFCs to absorb infrared radiation as a greenhouse gas makes their future acceptability uncertain. HFC-134a, HFC-152a, and HCFC-22 have a Global Warming Potential (GWP) of less than one, as compared with a GWP of 3.0 for CFC-12 (http://www.edis.ifas.ufl.edu/EH313). While HFC-134a appears to be the most environmentally friendly alternative for newly manufactured equipment, MP-39 is more attractive as a replacement for CFC-12 in existing systems. A direct switch to HFC-134a in a system designed for CFC-12 will result in about a 10 percent decrease in refrigeration capacity. This will cause an increase in energy consumption due to the longer run time that will be needed to satisfy the cooling load.
References
- Dr Dick Powell, ‘The rise and fall of CFCs’ from Chemistry Review, September 1996.
- Salters Chemical Storylines, Second Edition, p75 & 76.
- Gwen & Mike Pilling ‘Do CFCs destroy the ozone layer?’ from Chemistry Review, March 1993.
- http://www.edis.ifas.ufl.edu/EH313
(959 total word count excluding referencing, headings and sub-titles).
Abstract
- Techniques of data collection include monitoring, laboratory measurements, meteorology and computers.
- Evidence that CFCs causes ozone depletion was the concentration of ozone fell dramatically at the point where the concentration of ClO radicals soared in the ozone hole.
- Replacements for CFCs include HFCs and hydrocarbons as they reduce GWP potential.
