F332 The atmosphere

  • explain why some properties of CFCs made them such useful compounds and
  • discuss the relative advantages disadvantages of replacement compounds for CFCs: hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and hydrocarbons;
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What is a CFC?

A CFC is a covalently bonded relatively small molecule of carbon, chlorine and fluorine atoms (chlorofluorocarbon).

  • If enough energy is supplied by heat or by visible/uv electromagnetic radiation, or the is weak enough, a covalent bond can break in two ways (heterolytically or homolytically)
  • CFCs are very useful low boiling organic liquids or gases
  • until recently, extensively used in refrigerators and aerosol sprays
  • They are relatively unreactive, non-toxic and have low flammability
  • It is their chemical stability in the environment that eventually causes the ozone problem (depletion)
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Ozone formation

Ozone is formed in the stratosphere by free radical reactions.

-Ordinary' stable oxygen O2 (dioxygen) is split (dissociates) into two

-By high energy ultraviolet electromagnetic radiation from the Sun

- It splits in to two oxygen atoms which are radicals.

- a 'free' oxygen atom combines with an oxygen molecule (dioxygen) to form ozone (trioxygen).

O2 + uv ==> O.  + O.

O. + O2 ==> O3 

  • The ozone is a highly reactive and unstable molecule and decomposes into dioxygen when hit by other uv light photons

O3 + uv ==> O2 + O.

  • This last reaction is the main uv screening effect of the upper atmosphere and the ozone absorbs a lot of the harmful incoming uv radiation from the Sun 
  • If the ozone levels are reduced more harmful uv radiation reaches the Earth's surface

  • can lead to medical problems: 

  • increased risk of sunburn 

  • skin cancer

  • accelerates skin aging processes.

 

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Back to CFCs

The chemically very stable CFCs diffuse up into the stratosphere

  • Decompose when hit by ultraviolet light (uv)
  • Produce free radicals, including free chlorine atoms, which themselves are highly reactive free radicals.

 CCl2CF2 ==> CClF2. + Cl.

  • The formation of chlorine atom radicals is the root of the problem
  • They readily react with ozone and change it back to much more stable ordinary oxygen.

O3 + Cl. ==> O2 + ClO. and then:

ClO + O ==> Cl + O2 -'destructive' Cl atom free radical is still around!

  • The two reactions above involving chlorine atoms are known as a catalytic cycle because the chlorine atoms from CFC's etc. act as a catalyst in the destruction of ozone.
  • Therefore many countries are banning the use of CFCs,
  • but not all despite the fact that scientists predict it will take many years for the depleted ozone layer to return to its 'original' O3 concentration
  • alternatives to CFC's are already being marketed.

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Alternatives to CFCs

The idea is to use replacement compounds that are less harmful to the ozone layer.

  • Hydrochlorofluorohydrocarbons
  • Hydrofluorocarbons
  • Alkanes

However, all of these molecules are greenhouse gases and will contribute to global warming!

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