Ozone Depletion

Question 1

Where is ozone naturally formed? Why is it beneficial?

Answer 1

• Ozone is formed, naturally, in the upper atmosphere.
• Benificial because it absorbs harmdul UV radiation (that causes skin cancer.)

() - extra info for clarification

Question 2

What are chlorofluorocarbons (CFC’s?) Properties?

Answer 2

• Hydrocarbons that have all their hydrogens replaced by chlorine/ fluorine (in any ratio.)
• Properties: stable, unreactive, non-toxic, non-flammable.

Question 3

What did chlorofluorocarbons used to be used in?

Answer 3

• Were used as refrigerants and as solvents (ie. the gas) in deodrants/ aerosols.

Question 4

What were chlorofluorocarbons in fridges and aerosols/ solvents replaced with and why?

Answer 4

• Replaced with safer alternatives: hydrofluorocarbons hydrocarbons as they are chlorine-free compounds. (the chlorine radical is what catalyses the decomposition of ozone. )
• Different groups in scientific community provided evidence that CFC’s were damaging ozone layer: causing CFC’s to be banned as refrigerants/ solvents.

() - extra info for clarification.

Question 5

How are Cl atoms (ie. radicals) formed in the upper atmosphere? What do these chlorine atoms/ radicals do?

Answer 5

• Formed when UV radiation causes C-Cl bond in CFC’s to break.
• Chlorine atoms catalyse the decomposition of ozone and
  contribute to the hole in the ozone layer.

Question 6

Why are the C-Cl bonds broken in CFC’s in the upper atmosphere by UV radiation, rather than the C-F bonds?

Answer 6

• Because C-Cl bonds are weaker because they have lower bond enthalpy than C-F bonds.
• A C-F bond is less likely to be broken because it is stronger.

Question 7

What are chlorofluorocarbons abbreviated as?

Answer 7

• CFC’s

Question 8

What happens in the initiation step of ozone depletion using example of chlorofluorocarbon - CCl₃F?

Answer 8

• UV light breks C-Cl bond in the molecule, producing two radicals, the Clο will react with ozone molecules.
  -CCl₃F –> Clο + CCl₂F (with dot on top of the C on CCl₂F.)
• Clο will always be formed as intermediate.

Question 9

What is the formula for one ozone molecule?

Answer 9

• O3

Question 10

What happens in the propogation steps of ozone depletion using example of chlorofluorocarbon - CCl₃F?

Answer 10

1.) Clο + O₃ –> O₂ + ClOο
ClOο = intermediate.
2.) ClOο + O₃ –> 2O₂ +Clο

Question 11

Why is Clο said to be a catalyst in the reaction of ozone depletion using Clο radicals?

Answer 11

• Because the Clο is reformed (ie. it’s formed in the second propogation step), it isn’t used up in reaction.

Question 12

Give an example of a termination step for ozone depletion reaction.

Answer 12

2Clο –> Cl₂

Question 13

What is the overall equation for ozone depletion using chlorine atoms from chlorofluorocarbons?

Answer 13

2O₃ –> 3O₂
(refer to propgation steps to see why this is the case - everything on both sides of equation is included in overall equation other than the free radicals!!)

Question 14

Structure of hydrofluorocarbons?

Answer 14

• Have F AND H’s bonded to C atoms, in any ratio.

Question 15

What is an intermediate?

Answer 15

• An intermediate is a species generated in one stage of the mechanism, that is used up in another stage.