4.2.2 Organohalogen compounds in the environment

Question 1

list some of the uses of haloalkanes

Answer 1

• solvents
• polymers
• refridgerants

Question 2

What is PVC

Answer 2

PVC is a haloalkane polymer with a C-Cl bond

Question 3

What is PTFE

Answer 3

PTFE is a haloalkane polymer with 4 C-F bonds on 2 carbons

tetrafluoroethene

Question 4

what does CFC stand for

Answer 4

chlorofluorocarbons

Question 5

What are CFC’s

Answer 5

they are haloalkanes that were used for refridgeration and as propellants till the 1980’s

Question 6

why did the use of CFC’s depeat

Answer 6

in the 1980s we realised that CFC’s were destroying the ozone layer.

Question 7

What are the chemical characteristics of CFC’s

Answer 7

• chemically unreactive
• non-toxic
• volatile

volatile = shorter carbon chains so can vapourise easier

Question 8

breifly outline the ozone layer

Answer 8

• it is found in the outer edge of the stratosphere.
• only a tiny fraction of the gases making up the ozone layer is ozone but it is enough to absorb most of the biologically damaging ultraviolet radiation (UV-B) from the suns rays allowing only a small amount to reach earth’s surface

Question 9

How does ozone prevent UV-B light reaching earths surface

Answer 9

Ozone is continually being formed and broken down down by the action of UV radiation.
An equilibrium exists between ozone and oxygen.
O₃ ⇌ O₂ + O
The forward reaction is initiated by UV light but it is not given out when the reverse reaction happens.
The equilibrium is disturbed by radicals. One radical can disturb up to 100,000 O₃ molecules.

Question 10

How do CFC’s damage the ozone layer

Answer 10

CFC’s produce radicals in the presence of UV light.
e.g.
CF₃Cl (UV)→ CF₃⦁ + Cl⦁ (initiation)

Question 11

What is the mechanism for Cl⦁ destroying ozone

Answer 11

O₃ + Cl⦁ ➞ O₂ + ClO⦁
ClO⦁ + O ➞ O₂ + Cl⦁
overall = O₃ + O → 2O₂

propergation

Question 12

give another molecule (other than CFC) that damage the ozone layer

Answer 12

NO⦁

Question 13

Where does NO⦁ come from

Answer 13

when N₂ and O₂ react at high temperatures in aircraft engines

Question 14

How does NO⦁ damage the ozone layer

Answer 14

through the same mechanism as CFC’s
O₃ + NO⦁ → O₂ + NO₂⦁
NO₂⦁ + O → O₂ + NO⦁
overall = O₃ + O → 2O₂

(propergation)

Question 15

Why do CFCs have such a large impact

Answer 15

• Due to the strength of their carbon-halogen bonds CFCs have a long residence time in the troposphere it may take years to reach the stratosphere
• once there the UV radiation provides sufficient energy to break the carbon-halogen bond by homolytic fission to form radicals.
• The C-Cl bond has the lowest enthalpy so is the bond to break.
• The radiation initiates the the breakdown in a process called photodissociation:
  CF₂Cl₂ → CF₂Cl⦁ + Cl⦁