13.4 - Halogenoalkanes and the Environment

Question 1

Alternative to CFCs (2):

Answer 1

• Hydrofluorocarbons - don’t contain chlorine - so no harm is done
• Hydrocarbons - decompose rapidly but are flammable and contribute to GG emission

Question 2

Chain reaction of chlorine radicals (propagation steps)

Answer 2

UV radiation forms chlorine radicals (Cl*) which react with ozone molecules:

Cl* + O3➔ ClO* + O2

The chlorine radical is regenerated:

ClO* + O3➔ Cl* + 2O2

The overall effect of these reactions is:

2O3➔ 3O2

Question 3

What is the role of ozone in the upper atmosphere? What does it prevent

Answer 3

acts as a protective barrier by absorbing harmful UV radiation from the sun
it prevents damage to humans and ecosystems

Question 4

How do CFCs reach the ozone layer? how does it break and what’s the formula

Answer 4

• they can survive long enough in the atmosphere to be carried up into the stratosphere
• high-energy UV radiation breaks the carbon-halogen which releases chlorine radicals
• CF2Cl2➔ CF2Cl* + Cl*

Question 5

What are the good properties of CFCs? (4)

Answer 5

• high stability
• low toxicity
• non-flammibility
• volatility

stability comes from the strong carbon-halogen bonds

Question 6

Key features of CFCs and what are 2 examples of them

Answer 6

• they contain no C-H bonds as all hydrogen atoms are subsituted by chlorine and fluorine
• examples include CCl3F and CClF3