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A Level Chemistry > 15.2- Organohalogen Compounds In The Environment > Flashcards

15.2- Organohalogen Compounds In The Environment Flashcards

1
Q

What are organohalogen compounds ?

A

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

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2
Q

What are some uses of organohalogen compounds ?

A
  • general solvents
  • dry cleaning solvents
  • making polymers
  • flame retardants
  • refrigerants
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3
Q

What is CHCl3 used for ?

A

General solvents

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4
Q

Where is the ozone layer ?

A

Found at the outer edge of the stratosphere at a height that varies from about 10-40 km above the earths surface

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5
Q

What is the ozone layer ?

A
  • absorbs harmful UV-B radiation
  • and prevents UV-B from reaching the earth’s surface
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6
Q

What can occur if UV-B reaches the earths surface ?

A

Leads to genetic damage

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7
Q

What is UV-B commonly associated with ?

A

Linked to sunburn

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8
Q

What is continually happening in the stratosphere ?

A
  • ozone is continually being formed and broken down by the action of UV radiation
  • initially very high energy UV breaks oxygen molecules into oxygen radicals

O2 —-> 2O

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9
Q

What are CFCs ?

A

Chlorofluorocarbons

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10
Q

What do CFCs do ?

A
  • break down the ozone layer in the atmosphere
  • they are molecules that have had all of there hydrogens replace by chlorine and fluorine
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11
Q

What is the ozone layer made or ?

A

O3 molecules

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12
Q

What are some properties of CFCs ?

A

They are very stable due to the strength of the carbon- halogen bond

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13
Q

Are CFCs always stable

A

They’re stable till they reach the stratosphere

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14
Q

What do CFCs do in the stratosphere ?

A
  • they begin to break down forming chlorine radicals
  • these chlorine radicals catalyse the breakdown of the ozone layer
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15
Q

What occurs in the initiation stage of this process ?

A
  • bonds break via homolyctic fission
  • the C-Cl bond has the lowest bond enthalpy and so if the bond that breaks
  • UV breaks the C-Cl bond and produces 2 radicals which will react with the ozone molecule O3

CFCl3 ——> CF3• + Cl•

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16
Q

State the equation to show the break down of CFCl2

A

CF2Cl2 ——> CF2Cl• + Cl•

  • the chlorine radicals formed is a very reactive intermediate
17
Q

What happens in propagation step 1 ?

A

Cl• + O3 —-> ClO• + O2

18
Q

What happens in the propagation step 2 ?

A

ClO• + O• —-> Cl• + O2

19
Q

What is the overall equation?

A

O3 + O• —-> 2O2

20
Q

How many molecules of ozone can a single CFC molecule break ?

A

100,000

21
Q

What other type of radical can also catalyse the breakdown of the ozone layer ?

A

Nitrogen oxide radicals

22
Q

When are nitrogen oxide radicals formed naturally ?

A
  • During lighting strikes
  • as a result of aircraft travel in the stratosphere
23
Q

State the equations of nitrogen oxide radicals that cause the breakdown of ozone

A

NO• + O3 ——> NO2 + O2

NO2• + O• ——> NO• + O2

Overall equation is the same as with the chlorine radicals :

O3 + O• ——> 2O2

24
Q

What is depletion ?

A

Gradual thinning of the earths ozone layer in the upper atmosphere caused due to the release of chemical compounds containing gaseous bromine or chlorine from industries or other human activities

25
Q

Why is ozone depletion a major concern for humans ?

A
  • depletion of the ozone layer will slow more harmful UV-B to penetrate to the troposphere
  • leading to an increased incidence of skin cancer and genetic damage
26
Q

Why is the chlorine radicals described as a catalyst ?

A

It is used in the first prorogation step then regenerated in the second prorogation step

27
Q

Name some uses of CFCs

A
  • refrigerants
  • air conditioning units
  • aerosol propellants
28
Q

What are some actions to prevent depletion of the ozone layer ?

A
  • removal of CFCs using alternatives
  • signing the Montreal protocol
  • pump sprays
  • new coolants
29
Q

Why are HCFs preferred to CFCs ?

A
  • CFCs destroy the ozone layer = exposed to UV radiation + greater risk of skin cancer
  • HFCs are more stable and do not contain a chlorine atom so thus cannot form a Cl• and start a chain reaction
  • CFCs produce Cl• which create a chain reaction and contribute to ozone depletion
  • HFCS contain C-H / C-F bonds and these are too strong to be broken by UV light
30
Q

What is a homogeneous catalyst ?

A

Radicals that are at the same phase as ozone molecules

31
Q

What is a heterogeneous catalyst ?

A

Catalysts that are at a different physical state to their reactants

A Level Chemistry (44 decks)

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