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AS Chemistry > OZ - Haloalkanes and ozone > Flashcards

OZ - Haloalkanes and ozone Flashcards

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1
Q

How many types of bond fission are there?

A

2

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2
Q

What are the 2 types of bond fission?

A

Homolytic and heterolytic.

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3
Q

What is bond fission?

A

Breaking a covalent bond.

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4
Q

What is breaking a covalent bond called?

A

Bond fission.

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5
Q

What is a single covalent bond?

A

A shared pair of electrons between two atoms.

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6
Q

What is a shared pair of electrons between two atoms?

A

A single covalent bond.

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7
Q

In what two ways can a single covalent bond break?

A

Heterolytic fission and homolytic fission.

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8
Q

What is heterolytic fission?

A

When two different substances are formed - a positively charged cation (X+), and a negatively charged anion (X-).

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9
Q

What does ‘hetero’ mean?

A

‘Different’.

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10
Q

What is homolytic fission?

A

Two electrically uncharged ‘radicals’ are formed. Radicals are particles that have an unpaired electron. Because of the unpaired electron, these radicals are very reactive.

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11
Q

What are radicals?

A

Particles that have an unpaired electron.

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12
Q

Are radicals very reactive or not?

A

They are very reactive.

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13
Q

Why are radicals very reactive?

A

Because of the unpaired electron they have.

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14
Q

What does a double-headed arrow show?

A

That a pair of electrons move.

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15
Q

What does a single-headed arrow show?

A

The movement of a single electron.

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16
Q

What do radicals take part in?

A

Chain reactions.

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17
Q

When do chain reactions take place?

A

If a product causes more reactions to take place.

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18
Q

How many main stages do radical chain reactions have?

A

3

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19
Q

What are the 3 main stages of radical reactions?

A

Initiation reactions.
Propagation reactions.
Termination reactions.

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20
Q

What happens in initiation reactions?

A

Free radicals are produced.

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21
Q

What happens in propagation reactions?

A

Free radicals react with molecules and produce new radicals. These go on to react with more molecules, producing even more radicals. This is the chain part of the reaction.

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22
Q

What happens in termination reactions?

A

Two radicals react together to form a stable molecule.

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23
Q

In what stage of a radical chain reaction are free radicals are produced?

A

Initiation reactions.

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24
Q

In what stage of a radical chain reaction do free radicals react with molecules and produce new radicals. These go on to react with more molecules, producing even more radicals. This is the chain part of the reaction?

A

Propagation reactions.

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25
Q

In what stage of a radical chain reaction do two radicals react together to form a stable molecule?

A

Termination reactions.

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26
Q

What can halogen radicals react with and what does this form?

A

Alkanes to form haloalkanes.

27
Q

In what reactions do halogens react with alkanes?

A

Photochemical reactions.

28
Q

What are photochemical reactions started by?

A

Light.

29
Q

What happens in the photochemical reaction between a halogen and an alkane?

A

A hydrogen atom is substituted (replaced) by chlorine or bromine. This is a radical chain reaction.

30
Q

Apart from a photochemical reaction, what type of reaction is the reaction between a halogen and an alkane?

A

A radical chain reaction.

31
Q

Describe the three stages of the radical chain reaction between chlorine and methane to form chloromethane

A

Initiation reaction:

  • Sunlight provides enough energy to break the Cl-Cl bond - this is photodissociation.
  • The bond splits equally and each atom gets to keep one electron - homolytic fission.
  • The molecule forms two highly reactive free chlorine radicals.

Propagation:

  • The chlorine radical attacks a methane molecule, forming a hydrogen halide and a methyl radical.
  • The new methyl free radical can attack another chlorine (Cl2) molecule.
  • The new chlorine radical can attack another methane molecule, and so on, until all the chlorine or methane molecules are wiped out.

Termination:

  • If two free radicals join together, they make a stable molecule.
  • There are heaps of possible termination reactions.
32
Q

What is photodissociation?

A

When sunlight provides enough energy to break bonds between/in molecules.

33
Q

What will some of the products be in a termination reaction of a radical chain reaction?

A

Trace impurities in the final sample.

34
Q

What is the ozone layer?

A

A ‘layer’ in the stratosphere containing most of the atmosphere’s ozone molecules (O3).

35
Q

Where is the ozone layer?

A

In the layer of the atmosphere called the stratosphere.

36
Q

When is ozone formed?

A

When UV radiation from the sun hits oxygen molecules.

37
Q

Explain how ozone molecules are formed by oxygen and UV radiation from the sun:

A

If the right amount of UV radiation is absorbed by an oxygen molecule, the oxygen molecule splits into separate atoms or free radicals.

The free radicals then combine with other oxygen molecules to form ozone molecules, O3.

38
Q

What does ‘hv’ stand for in the formation of ozone?

A

A quantum of UV radiation.

39
Q

Why is the ozone layer important to us?

A

Because it protects us from most of the harmful effects of the Sun’s ultraviolet (UV) radiation.

40
Q

What happens to ozone when it breaks down that is so dangerous to most life on Earth?

A

When ozone breaks down, it absorbs high energy UV radiation, so the ozone layer removes all the high energy UVC radiation and about 90% of the UVB. These types of UV radiation are harmful to humans and most other life on Earth.

41
Q

What is UV radiation from the sun made up of?

A

Different frequencies, grouped into three bands: UVA, UVB and UVC (increasing in frequency and energy).

42
Q

Why is UVB bad?

A

Because it can damage the DNA in cells and cause skin cancer and is the main cause of sunburn. It also causes the skin to age faster.

43
Q

Why is UVA bad?

A

Can lead to skin cancer and causes the skin to age faster.

44
Q

Are UVA and UVB bad or good?

A

Mainly bad.

45
Q

It UV radiation good or bad?

A

Both.

46
Q

Why is UV radiation good?

A

It is essential for humans as wee need it to produce vitamin D. Also, it causes the skin to tan when the skin is exposed to UV which helps protect deeper tissues from the effects of the radiation.

47
Q

Why does ozone occur at ground level?

A

Occurs in the troposphere (the lowest part of the atmosphere) due to the effect of sunlight on mixture of nitrogen dioxide and hydrocarbons (which occur naturally from a variety of sources but vehicle engines and power stations contribute large amounts).

48
Q

What is ‘ground level’/the lowest part of the atmosphere called scientifically?

A

The troposphere.

49
Q

Why are there large amount of nitrogen dioxide and hydrocarbons in the troposphere?

A

They occur naturally from a variety of sources but vehicle engines and power stations contribute large amounts.

50
Q

What can happen to ozone in the troposphere in industrialised areas and cities with lots of cars?

A

The ozone mixes with solid particles of carbon and many other substances to create an air pollutant called photochemical smog.

51
Q

What problems are there around photochemical smog?

A

It can cause respiratory problems.
It can be dangerous for animals.
It can cause damage to plants and materials.
Ozone itself is toxic to humans.

52
Q

How does photochemical smog form?

A

In the troposphere in industrialised areas and cities with lots of cars, the ozone mixes with solid particles of carbon and many other substances to create an air pollutant called photochemical smog.

53
Q

What do CFCs do?

A

Break down ozone in the stratosphere.

54
Q

What breaks down ozone in the stratosphere?

A

CFCs.

55
Q

What are CFCs?

A

Chlorofluorocarbons are haloalkanes that have all their hydrogen atoms replaced by chlorine and fluorine atoms.

56
Q

Why/how can CFCs break down ozone in the stratosphere?

A

The C-Cl bond can be broken down by high energy UV radiation in the stratosphere to form chlorine radicals - these act as catalysts on the breakdown of ozone.

57
Q

Why is ozone less likely to be broken down by CFCs in the troposphere?

A

Because most of the high frequency UV has been absorbed by the ozone layer. Though ozone can still be broken down in the troposphere, it’s much less likely to happen because there are less suitable catalysts.

58
Q

What factor of UV radiation determines whether it can break bonds?

A

The frequency no the intensity of UV radiation.

59
Q

Why/how can UV radiation break haloalkanes?

A

All haloalkanes contain bonds between carbon atoms and halogen atoms. UV radiation can break these bonds - the carbon-halogen bond splits homolytically to create two free radicals.

The ease with which the carbon-halogen bond it broken by UV depends on the halogen. It turns out that the carbon-iodine bond is the most likely to break, and the carbon-fluorine bond the least likely. This is because the C-I bond has the lowest bond enthalpy, and the C-F bond the highest.

60
Q

How does the ease with which UV can break carbon-halogen bonds change with different halogens?

A

The ease with which the carbon-halogen bond it broken by UV depends on the halogen. It turns out that the carbon-iodine bond is the most likely to break, and the carbon-fluorine bond the least likely. This is because the C-I bond has the lowest bond enthalpy, and the C-F bond the highest.

61
Q

By what process is the ozone layer being destroyed?

A

Homogenous catalysis.

62
Q

Explain what is happening when the ozone layer is being destroyed by CFCs/homogenous catalysis

A
  • Chlorine free radicals, Cl. , are formed when CFCs are broken down by high energy UV radiation in the stratosphere.
  • These free radicals are catalysts. They react with ozone to form an intermediate (ClO.), and an oxygen molecule.
  • The reaction can only terminate if two radicals react together (e.g. Cl. (g) + Cl. (g) –> Cl2 (g))
  • So the overall reaction is 2O3(g) –> 3O2(g) and Cl. is the catalyst.
63
Q

Other than free chlorine radicals, what other free radicals can destroy ozone?

A

NO from nitrogen oxides,
e.g. NO2 (g) + hv –> NO. (g) + O (g)

Other halogen free radicals from haloalkanes.

64
Q

How can the overall reactions between free radicals and ozone be represented?

A

R + O3 –> RO + O2
O + RO –> O2 +R

Where R represented the free radical.

AS Chemistry (44 decks)

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