Ozone Depletion

Halogenoalkanes Worksheet

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The Ozone Layer

1. The ozone layer is important in the protection of life on planet Earth. Explain why it is important.

Answer

Molecules of Ozone ($O_3$) in the upper atmosphere absorb high-energy UV-B radiation from the sun, preventing it from reaching the surface where it could cause skin cancer and genetic damage to organisms.

Chlorofluorocarbons (CFCs)

2. CF₂Cl₂ is a CFC.

a) What does CFC stand for?

Chlorofluorocarbons

b) What are CFCs used for?

They were commonly used as refrigerants, aerosol propellants, and blowing agents.

c) State the IUPAC name for CF₂Cl₂.

Dichlorodifluoromethane

d) Write an equation for the formation of a chlorine free radical from CF₂Cl₂.

CF₂Cl₂

→

•CF₂Cl + Cl•

Mechanism of Decomposition

3. Chlorine free radicals can cause the decomposition of ozone in the upper atmosphere.

a) Write a series of equations for the mechanism which results in the decomposition of ozone.

Step 1:

Cl• + O₃

→

•ClO + O₂

Step 2:

•ClO + O₃

→

Cl• + 2O₂

b) Write the overall equation for the reaction.

2O₃

→

3O₂

c) Explain why one molecule of CFC can result in many molecules of O₃ decomposing.

The chlorine radical (Cl•) is regenerated at the end of the mechanism (it acts as a catalyst). Therefore, a single radical can go on to react with many more ozone molecules in a chain reaction.

Alternatives to CFCs

4. CFCs have been replaced in many of their uses. Suggest an alternative molecule and explain why that molecule is safer.

Answer

Hydrofluorocarbons (HFCs) such as Difluoromethane (CH₂F₂) are suitable alternatives.

These molecules are safer because they do not contain any C-Cl bonds (no chlorine atoms). The C-F bond is extremely strong and cannot be broken by UV light in the stratosphere, so no ozone-depleting radicals are formed.