3.3.3.3 ozone depletion

what are chlorofluorocarbons (CFCs)?

halogenoalkanes donating both chlorine + fluorine atoms but no hydrogen eg CCl₃F, trichlorofluoromethane

what were CFCs used for?

• the short chain ones are gases + were used, eg in refrigerators

• longer chains are used for dry cleaning + as degreasing solvents

under normal conditions, CFC gases are very _____

unreactive

but if CFC gases are unreactive, then why are they a problem?

they eventually end up in the atmosphere, where they are decomposed to give chlorine free radicals

what is ozone?

a molecule made from three oxygen atoms, O₃

why is the ozone layer important?

it absorbs the harmful UV radiation from the Sun that causes skin cancer by damaging DNA in skin cells

what do chlorine radicals do?

they decompose ozone in the stratosphere, causing a hole in the earth’s ozone layer

describe the steps to how chlorine radicals decompose ozone + give equations for each step

1. the C-Cl bond breaks in the presence of UV radiation to produce Cl•

   R-Cl → R• + Cl•

2. these Cl• then decompose ozone in the stratosphere

   Cl• + O₃ → ClO• + O₂

   the resulting free radicals, ClO•, attack more ozone molecules + regenerate Cl•

   ClO• + O₃ → 2O₂ + Cl•

what can we say the Cl• acts as in this process + why?

it acts as a catalyst in the breakdown of ozone to oxygen as it is not destroyed + is regenerated

give the overall equation to show the ozone breakdown

2O₃ → 3O₂

what has research by chemists provided evidence for?

for legislation to ban the use of CFCs as solvents + refrigerants in order to prevent the further breakdown of earth’s ozone layer

what alternatives to CFCs have chemists developed?

safer alternative chlorine-free compounds such as 1,1,1,2-tetrafluoroethane, which is used primarily in refrigeration + conditioning

why are chlorine free compounds safer?

they contain only C-H and C-F bonds + which therefore do not produce any chlorine radicals → these C-H and C-F bonds are stronger