OZ opqr: Bond fission; radical reaction mechanisms; formation, effects & depletion of ozone

Question 1

Define heterolytic and homolytic bond fission.

Answer 1

Heterolytic: breakage of covalent bond where both electrons go to one atom (forms anion + cation if molecule is neutral)

Homolytic: breakage of covalent bond where one electron goes to each atom (forms 2 uncharged radicals if molecule is neutral)

Question 2

1. Suggest which type of fission is more likely to occur in polar and non-polar bonds.
2. Suggest conditions which would make the less likely type of fission of polar bonds more probable.

Answer 2

1. Polar: heterolytic fission. Non-polar: homolytic fission
2. Homolytic fission of polar bonds is more likely in the gas phase + in the presence of light

Question 3

What name(s) is/are given to the following reaction?

CH₃Cl + hv → CH3* + Cl*

Answer 3

Homolytic fission / homolysis / photodissociation / photolysis

Question 4

Name and describe the stages of a radical chain reaction.

Answer 4

• Initiation: non-radical(s) → 2 radicals
• Propogation: radicals → new radicals
• Termination: 2 radicals → non-radical(s)

Question 5

Use equations to show how ozone is formed:

1. In the stratosphere
2. In the troposphere

Answer 5

1) Stratosphere: UV dissociates O₂ molecules, forming radicals: O₂ + hν → 2O^•. These react with O₂ molecules, forming ozone: O^• + O₂ → O₃^•.
2) Troposphere: Light dissociates NO₂ molecules in photochemical smogs, forming oxygen radicals: NO₂ + hν → 0.5N₂ + 2O^•. These react with O₂ molecules, forming ozone: O^• + O₂ → O₃^•​.

Question 6

1. Why does stratospheric ozone act as a sunscreen? Include an equation in your answer.
2. What are the main effects on humans of the radiation screened by ozone?

Answer 6

1. Ozone absorbs the high-energy UV responsible for sunburn, and photodissociates: O₃^• + hν → O₂ + O^•.
2. Sunburn, skin cancer, eye damage, agricultural damage.

Question 7

Ozone acts as a sunscreen in the stratosphere. What effects does it have in the troposphere?

Answer 7

Acts as a pollutant which contributes to photochemical smogs:

• Low visibility
• Irritation + respiratory problems

Question 8

Draw the mechanism which occurs when a chlorine radical reacts with a hydrogen molecule.

Answer 8

Half-headed arrow should be used to represent movement of one electron

Question 9

This equation shows the overall effect of a radical chain reaction:

H₂ + Cl₂ → 2HCl

Describe the stages of this reaction. Use equations to support your answer.

Answer 9

Initiation: UV light photodissociates chlorine molecule, producing chlorine radicals

Cl₂ + hν → 2Cl^•

Propogation: radicals react with molecules, producing new radicals

Cl^• + H₂ → HCl + H^•

H^• + Cl₂ → HCl + Cl^•

So on…

Termination: 2 radicals react, producing molecules

2H^• → H₂

H^• + Cl^• → HCl

2Cl^• → Cl₂

Question 10

1. The overall equation of the the radical chain reaction between methane and chlorine is shown below. Describe the stages of the reaction, using equations to support your answer.

CH₄ + Cl₂ → CH₃Cl + HCl

2. Explain why the products also contain some dichloromethane, trichloromethane and tetrachloromethane.

Answer 10

1. Initiation: UV light photodissociates chlorine molecule, forming chlorine radicals

Cl₂ + hν → 2Cl^•

Propogation: radicals react with molecules, producing new radicals

Cl^• + CH₄ → HCl + CH₃^•

CH₃^• + Cl₂ → CH₃Cl + Cl^•

Termination: 2 radicals react, producing molecules

CH₃^• + Cl^• → CH₃Cl

• Also 2Cl^• → Cl₂, 2CH₃^• → C₂H₆, etc*
  2. CH₃Cl reacts with Cl^• radicals:

CH₃Cl + Cl^• → CH₂Cl^• + HCl

CH₂Cl^• + Cl₂ → CH₂Cl₂ + Cl^•

CH₂Cl₂ + Cl^• → CHCl₃ + H^•

CHCl₃ + Cl^• → CCl₄ + H^•

Question 11

Suggest why radical polymerisation tends to produce highly branched, irregular chains.

Answer 11

Radicals are highly reactive with any species, so it’s hard to control where monomers join the polymer.

Question 12

1. Representing a radical with X, write the propogation and termination stages of a catalytic cycle, and the overall equation, responsible for the depletion of stratospheric ozone.
2. State the role X plays.

Answer 12

1. X + O₃ → XO + O₂

XO + O → X + O₂

Overall: O + O₃ → 2O₂ (also happens directly)

2. Homogeneous catalyst

Question 13

What are CFCs?

Answer 13

Chlorofluorocarbons

Human-derived haloalkanes containing chlorine and/or fluorine

Question 14

Small amounts of chloromethane and bromomethane reach the stratosphere due to natural processes.

Using relevant equations, explain how chloromethane can lead to the depletion of stratospheric ozone.

Answer 14

Initiation: photolysis of chloromethane

CH₃Cl + hv → CH₃* + Cl*

Propogation:

Cl* regenerated in catalytic cycle; acts as homogeneous catalyst in ozone depletion:

Cl* + O₃* → ClO* + O_{2<strong> </strong>}

ClO* + O* → Cl* + O₂

Overall: O* + O₃* → 2O₂

Question 15

Chlorine radicals contribute to the depletion of stratospheric ozone.

Explain why the effect of chlorine radicals in the stratosphere is significant, despite the fact that their concentration is very small.

Answer 15

• They act as homogeneous catalysts since they are regenerated by ClO* + O* → O₂ + Cl*
• So they go through catalytic cycle many times before being converted into something else

Question 16

Why are bromoalkanes much more effective in depleting stratospheric ozone than chloroalkanes?

Answer 16

• Bromoalkanes are more reactive than chloroalkanes, so photodissociate more easily (since carbon-halogen bond enthalpy decreases down group)
• More bromine radicals are formed, depleting more ozone

Doesn’t add up bc there is a lower conc of bromine radicals, yet it’s still more effective??

Question 17

Using relevant equations, explain:

• how hydroxyl radicals are generated.
• how hydroxyl radicals contribute to the depletion of stratospheric ozone.

Answer 17

Initiation: photolysis of oxygen

O₂ + hv → 2O*

Propogation:

O* + H₂O → 2OH*​

OH* regenerated in catalytic cycle; acts as homogeneous catalyst in ozone depletion:

OH* + O₃* → HO₂* + O₂

HO₂* + O* → HO* + O₂

Overall effect: O₃* + O* → 2O₂

Question 18

Using relevant equations, explain:

• how nitrogen monoxide radicals are generated.
• how nitrogen monoxide radicals contribute to the depletion of stratospheric ozone.

Answer 18

Initiation: photolysis of nitrogen dioxide

NO₂ + hv → NO* + O*

Propogation:

NO* regenerated in catalytic cycle; acts as homogeneous catalyst in ozone depletion:

NO* + O₃* → NO₂* + O₂

NO₂* + O* → NO* + O₂

Overall effect: O₃* + O* → 2O₂

Question 19

Answer 19

• Reactions show regeneration of NO₂, which does occur
• But reactions are incorrect
• Correct reactions are NO + O₃ → NO₂ + O₂ and NO₂ + O → NO + O₂