15.2 Organohalogen compounds in the environment

Question 1

What are organohalogen compounds

Answer 1

Molecules that contain at least one halogen atom joined to a carbon chain

Question 2

What are organohalogen compounds used in

Answer 2

Many pesticides

Question 3

Why are organohalogens concerning

Answer 3

Rarely found in nature and are not broken down naturally

Question 4

Where is the ozone layer

Answer 4

Found at the outer edge of the stratosphere at height that varies between 10-40km above Earth’s surface

Question 5

What is the ozone layer

Answer 5

Only small fraction is ozone

But enough to absorb most biologically damaging UV-B radiation from sun’s rays

Question 6

What is UV-B

Answer 6

Radiaion most commonly linked to sunburn

Increased genetic damage and greater risk of skin cancer

Question 7

Ozone in the stratosphere

Answer 7

Ozone continually formed and broken down by action of UV radiation
Initially very high energy UV braks oxygen molecules into radicals
O2->2O
Steady state set up involving O2 and radicals. Ozone forms and breaks down
Rate of formation same as rate at which it is broken down
O2+OO3

Question 8

Why are CFCs stable

Answer 8

Due to the strength of their carbon-halogen bonds within their molecules

Question 9

What is the impact of CFCs on the Earth’s atmosphere

Answer 9

Concluded that they remain stable until they reach the stratosphere, where they break down forming chlorine radicals which are thought to catalyse the breakdown of the ozone layer

Question 10

Why are CFCs stable until they reach the stratosphere

Answer 10

Can take many years to reach stratosphere
UV provides sufficient energy to break carbon-halogen bond by homolytic fission to form radicals
This is photodissociation

Question 11

Why is the breaking down of the carbon-halogen bond in CFCs photodissociation

Answer 11

As radiation initiates the breakdown

Question 12

How do CFCs deplete the ozone layer

Answer 12

UV enough energy to break carbon-halogen bond
Chlorine radical forms, very reactive intermediate
Reacts with ozone molecule, breaks down ozone into oxygen
Repeat in cycle

Question 13

Nitrogen oxide radicals and the ozone

Answer 13

Formed naturally during lightning strikes and aircraft travel in stratosphere
Breakdown of ozone in similar way to chlorine

Question 14

What is the overall equation of the process of depleting the ozone for chlorine radicals

Answer 14

O3+O->2O2

Question 15

What is the overall equation of the process of depleting the ozone for nitrogen oxide radicals

Answer 15

O3+O->2O2

Question 16

What was the Montreal Protocol

Answer 16

Signed in 1987
Introduced steps for the complete removal of CFCs in all but limited number of products where no suitable alternative can be found