chapter 5 - haloalkanes environmental effects

what is a CFC?

• a chlorofluorocarbon molecule

• they only contain carbon, chlorine and fluorine atoms

what is ozone?

• has the formula O3

  • it is less stable than O2

  • ozone is formed from oxygen and UV light

  • it is present in low concentrations

  • the highest concentrations are in the ozone layer of the earth’s atmosphere

what is photolysis?

• means broken down by light

• CFCs undergo photolysis in the atmosphere

what does photolysis produce?

• chlorine radicals

• chlorine radicals react with ozone

what type of mechanism is the destruction of ozone?

• free radical substitution mechanism

initiation, propagation and termination steps of the destruction of ozone?

overall mechanism reaction for destruction of ozone?

what are CFCs used in?

• aerosols

• as coolants in fridges

• solvents in the industry

  • however, scientists collected evidence in the 1970s that they were damaging the ozone layer

  • this led to bans, and the development of safer alternatives like hydrofluorocarbons (HFCs) and hydrocarbons

why are CFC’s good for their uses?

because they are:

• volatile

• low boiling

• non-toxic

• non-flammable

• unreactive

what forms the holes in the ozone layer?

• CFCs in the upper atmosphere absorb UV radiation and split to form chlorine free radicals

• these free radicals catalyse the destruction of ozone, destroying ozone molecules

  • then they are regenerated to destroy more ozone

how many ozone molecules can one atom of chlorine destroy before it forms a stable compound?

• 10,000

why are holes in the ozone layer bad?

• because they allow more harmful radiation to reach the earth

what else can destroy ozone?

• NO (nitrogen monoxide) free radicals from nitrogen oxides

what are nitrogen oxides produced from?

• car and aircraft engines

• thunderstorms

question: explain how CFCs cause a depletion in the ozone layer and include relevant equations in your answer (5)

• M1- chlorine free radicals are formed in the upper atmosphere

• M2- when energy from ultra-violet radiation causes the C-Cl bonds in CFCs to break

• M4- the chlorine free radical atoms catalyse the decomposition of ozone due to these reactions because they are regenerated

what is the benefit of the ozone layer?

• the frequency of uv light absorbed by the ozone layer is equal to the frequency of biologically damaging uv radiation

• ozone layer filters out harmful uv and allows life on earth to survive

• uv light can increase risk of skin cancer and increase crop mutation

what bond is more likely to break when CF3Cl is exposed to UV radiation?

• the C-Cl bond because it is the weaker bond

why is there still concern about ozone depletion?

• CFCs take many years to reach the ozone layer

• CFCs are still being used up

• there are other ozone depleting substances, such as NO and HFCs