haloalkanes

Question 1

uses of organohalogen compounds

Answer 1

• organohalogens are molecules that contain at least one halogen attached to a carbon chain
• pesticides, solvents, flame retardants, general solvents, making polymers

Question 2

ozone layer

Answer 2

• found at the outer edge of the stratosphere, at a height that varies from about 20 to 40km above the earths surface. Only a tiny fraction of gases that make up the ozone layer is ozone, but this is enough to absorb the biologically damaging UV radiation (UV-B) from the suns rays
• troposphere then stratosphere then ozone layer
• feared that continue depletion of the ozone layer will allow more UV-B radiation to reach the earths surface
• UV-B radiation leads to increased genetic damage and greater risk of skin cancer in humans

Question 3

ozone breakdown

Answer 3

• in the stratosphere, ozone is continually being formed and brocken down by the action of UV. Initially very high energy UV breaks down oxygen molecules into oxygen radicals
  02 —> 20
  02+ 0 —-> 03- A REVERSABLE REACTION

Question 4

CFCs and the ozone layer

Answer 4

• until recently CFCs and HCFCs were most common compounds in air-conditioning units and aerosol propellants
• very stable until they reach the stratosphere. Here they begin to break down, forming chlorine radicals which catalyse the breakdown of the ozone layer
• stability of CFCs is due to the strength in their carbon- halogen bonds, this means CFCs have a long residence time in the troposphere. May take them many years to reach the stratosphere.

Question 5

how CFCs deplete the ozone layer

Answer 5

once in the stratosphere UV radiation provides sufficient energy to break the carbon- halogen bond in CFCs by homolytic fission to form radicals. The C-Cl bond has the lowest bond enthalpy so is the bond that breaks
photodissociation:
CF2Cl2 —-> CF2Cl∙ + Cl∙
Chlorine radical formed, is very reactive so reacts with ozone molecules.
Propogation step 1:
Cl∙ + 03 —–> ClO∙ + O2
propogation step 2:
ClO∙ + O —–> Cl∙ + 02
overall equation:
O3 + O —–> 2O2
propogation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in propogation step 1. The two propogation steps repeat in a cycle over and over again in a chain reaction

Question 6

other radicals that catalyse ozone breakdown

Answer 6

• nitrogen oxide radicals are formed naturally during lightning strikes and as a result of aircraft travel in the stratosphere
• NO∙ + O3 —> NO2 ∙ + O2
• NO2∙ + 0 —–> NO∙ + 02
• Overall equation:
• O3 + O —–> 2O2