15.2 - Organohalogen Compounds in the Environment Flashcards
What are organohalogen compounds?
Molecules with at least 1 halogen atom joined to a carbon chain.
Name some uses of organohalogen compounds.
- Pesticides
- Solvents
- Dry cleaning solvents
- Polymer making
- Flame retardant
- Refrigerant
What is the ozone layer?
Formed at the outer edge of the stratosphere (10-40km above sea level)
- Small proportion of gases making up ozone is ozone (O3).
- Enough ozone gas to absorb harmful UV-B radiation from the sun.
What is the effect of UV-B radiation and what will happen if the ozone layer continues to deplete?
- Linked to sunburn
- If ozone continues to deplete - more UV-B will enter, leading to increased genetic damage and greater risk of skin cancer.
How is ozone formed?
O2 > 2O (broken by UV radiation) (O is a radical, no need dot)
Then
O2 + O (equilibrium) O3 - this process is constantly occuring, the rate of ozone formation is equivalent to its breakdown.
- Use of CFCs and human activity upsets this equilibrium
What happens when CFCs reach the stratosphere?
- CFCs are stable compounds due to the C-X bond strenggth.
- They are stable until the stratosphere where they form Cl. (radicals) - which catalyses ozone breakdown.
How do CFC make chlorine radicals/
- Stability keeps them in the troposphere (10km above sea level) long enough - takes years to reach stratosphere
- At stratosphere UV radiation provides energy to break C-Cl bond.
Describe the photodissociation and propagation steps of CFC depleting the ozone layer.
Example CF2Cl2
Photodissociation - CF2Cl2 > CF2Cl. + Cl. (very reactive Cl. - breaks down
Propagation 1 - Cl. + O3 > ClO. + O2
Propagation 2 - ClO. + O > Cl. + O2
Overall - O3 + O > 2O2
What other radicals are responsible for ozone-depletig reactions?
Nitrogen oxide radicals can break down O3 (comes from lightning due to aircraft in stratosphere.
P1: NO. + O3 > NO2. + O2
P2: NO2. + O > NO. + 2O2
Same overall equation
