15.2 Organohalogen Compounds In The Environment
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What are organohalogen molecules?
Molecules that contain at least one halogen atom joined to a carbon chain
List some of the uses of organohalogen molecules.
Pesticides, flame retardants, refrigerants, solvents
Where is the ozone found?
At the outer edge of the stratosphere
What does ozone do?
Absorbs ultraviolet radiation from the Sun’s rays
What is the molecular formula for ozone?
O3
What is the symbol for ultraviolet radiation from the Sun’s rays?
UV-B
What will the depletion of the ozone layer cause?
More UV-B radiation to reach the Earth’s surface
Describe the process of the formation of ozone.
• Very high energy UV breaks down oxygen molecules into radicals • Then, ozone is formed and broken down at an equal rate (equilibrium)
Write down the equations demonstrating the formation of ozone.
First O2 -> 2O: Then O2 + O: ⇌ O3
What upsets the equilibrium of the formation of ozone being broken down and formed?
Human activity, especially in the production and use of chlorofluorocarbons
What are CFCs?
Chlorofluorocarbons
Why are chlorofluorocarbons stable?
Because of the high strength of the carbon-halogen bonds within the molecules
When do CFCs stop being stable?
When they reach the stratosphere
What happens when CFCs reach the stratosphere?
• They become unstable and break down, forming chlorine radicals. • UV radiation provides sufficient energy to break the Carbon-Halogen bonds in CFCs by homolytic fission to form radicals
Which bond is the first to break during homolytic fission of CFCs? Why?
The C-Cl bond, because it has the lowest bond enthalpy
What is the name of the process of chlorofluorocarbons being broken down to form radicals under UV radiation?
Photodissociation
State the equation for the photodissociation of CF2Cl2
CF2Cl2 -> CF2Cl* + Cl*
State the equations for the reactions that take place when a chlorine radical reacts with ozone.
Step 1: Cl* + O3 -> ClO* + O2 Step 2: ClO* + O -> Cl* + O2
State the overall equation for the reaction that takes place when a chlorine radical reacts with ozone.
O3 + O -> 2O2
What are the sources of nitrogen oxides in the atmosphere?
Thunderstorms/lightning and aircraft
Write equations to show how nitrogen monoxide catalyses the breakdown of ozone.
NO + O3 -> NO2 + O2 NO2 + O -> NO + O2