Haloalkanes and ozone

Question 1

What is bond fission? Describe two types of bond fission.

Answer 1

Bond fission involves breaking a covalent bond. There are two types:
Heterolytic fission - two different substances are formed (a positively charged cation and a negatively charged anion).
Homolytic fission - two electrically uncharged ‘radicals’ are formed. Radicals are particles that have an unpaired electron.

Question 2

Write an equation for the initiation step of the photochemical reaction between chlorine and methane.

Answer 2

CH₄ + Cl₂ (+UV) → Ch₃Cl + HCl

Question 3

What are the benefits to humans of the ozone layer?

Answer 3

The ozone layer protects us from most of the harmful effects of the Sun’s ultraviolet (UV) radiation. When ozone breaks down, it absorbs high energy UV radiation.

Question 4

How is ozone formed in the troposphere?

Answer 4

Ozone (O₃) occurs in the troposphere due to the effect of sunlight on mixtures of nitrogen dioxide and hydrocarbons.

Question 5

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.
Write an overall equation to show the formation of bromomethane from methane and bromine.
[1 mark]

Answer 5

CH₄ + Br₂ (+UV) → CH₃Br + HBr

Question 6

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.
Write down the two equations in the propagation step for the formation of CH₃Br.
[2 marks]

Answer 6

Br• + CH₄ → HBr + •CH₃

•CH₃ + Br₂ → CH₃Br + Br•

Question 7

When irradiated with UV light, methane gas will react with bromine to form a mixture of several organic compounds.
Explain why a tiny amount of ethane is found in the product mixture. Write the equation for the formation of ethane in this reaction.
[2 marks]

Answer 7

• Two methyl radicals bond together to form an ethane molecule.
• •Ch₃ + •Ch₃ → CH₃CH₃

Question 8

The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface.
Explain how ozone forms in this part of the atmosphere.
[3 marks]

Answer 8

• Ozone is formed by the effect of UV radiation from the Sun on oxygen molecules.
• The oxygen molecules split to form oxygen free radicals…
• …which react with more oxygen molecules to form ozone.

Question 9

The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface.
How does the ozone layer absorb harmful radiation without being permanently destroyed?
[2 marks]

Answer 9

• The ozone molecules absorb harmful UV radiation, breaking down to form an oxygen molecule and a free oxygen radical (O₃ +hν → O₂ + O).
• The radical produced forms more ozone with an O₂ molecule (O₂ + O → O₃) so ozone is continually destroyed and remade.

Question 10

The ‘ozone layer’ lies mostly between 15 and 30 km above the Earth’s surface.
Write equations to show how the CFC CCl₄ could catalyse the destruction of ozone in the atmosphere.
[3 marks]

Answer 10

CCl₄ + hυ → •CCl₃ + Cl•
Cl• + O₃ → ClO• + O₂ and
ClO• + O₃ → Cl• + 2O₂
Overall: 2O₃ → 3O₂

Question 11

Explain why large amounts of ground-level ozone can be a problem.
Why are these problems most frequently encountered in heavily industrialised areas?
[2 marks]

Answer 11

It causes photochemical smog which can cause…

• …respiratory problems.
• …harm to animals/plants/materials.

Ground-level ozone forms as a result of sunlight acting on mixtures of nitrogen dioxide and hydrocarbons, which are emitted by power stations/vehicles.