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Organic Chemistry > Organic 2: Alkanes > Flashcards

Organic 2: Alkanes Flashcards

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1
Q

What is an alkane?

A

A saturated hydrocarbon
Contains only carbon-carbon and carbon-hydrogen single bonds

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2
Q

What are alkanes used for?

A

Fuels and lubricants

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3
Q

What is the main source of alkanes?

A

Crude oil

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4
Q

What is the general formula of alkanes?

A

CnH2n+2

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5
Q

What is the general formula of ring alkanes?

A

CnH2n

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6
Q

What is the bond angle for alkanes?

A

109.5 (tetrahedral)

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7
Q

Describe the polarity of alkanes.

A

1) Alkanes are almost non-polar
2) Because there is only a small difference in electronegativities for carbon and hydrogen
3) The only intermolecular forces are weak van Der Waal forces

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8
Q

Describe boiling points of alkanes.

A

Longer alkanes have higher melting and boiling points due to stronger van Der Waal forces between molecules
Shorter alkanes have less, weaker van Der Waal forces so have lower melting and boiling points

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9
Q

What states are short chain alkanes and long chain alkanes at room temperature?

A

Short chain alkanes are gases at room temperature
Long chain alkanes are solids or liquids at room temperature

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10
Q

How does the melting and boiling point of branched alkanes differ from straight alkanes with the same number of carbon atoms?

A

Branched alkanes have lower melting points than straight chains
Because they are less compact, so the van Der Waal forces are not as effective

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11
Q

Are alkanes soluble in water, and why?

A

Alkanes are insoluble in water
Because water molecules are held together by strong hydrogen bonds
Which are much stronger than the weak van Der Waal forces that hold alkanes together

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12
Q

What substances can alkanes mix with?

A

Other non-polar liquids

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13
Q

What substances can or can’t alkanes react with?

A

Can react with:
1) Halogens (under appropriate conditions)
2) Oxygen (during combustion)

Can’t react with:
1) Acids or bases
2) Reducing agents or oxidising agents

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14
Q

What is crude oil, or petroleum?

A

A mixture of branched and unbranched hydrocarbons, mainly alkanes

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15
Q

Why does crude oil contain impurities?

A

It often contains small amounts of other compounds from the original plant or animal that it was formed from
E.g. some might contain sulfur, so would release sulfur dioxide when burnt

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16
Q

How is crude oil separated into useful products for industry?

A

By fractional distillation

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17
Q

Describe the process of fractional distillation.

A

1) The crude oil is heated in a furnace
2) A mixture of liquid and vapour passes into a fractionating tower with a temperature gradient (cooler at the top, hotter at the bottom)
3) Vapours pass up the tower and condense at a lower temperature than their boiling point, and are then piped off at that fraction
4) Shorter chain hydrocarbons condense nearer the top, because they have lower boiling points
5) Thick residue at the bottom of the tower (tar, or bitumen) is made of long hydrocarbon chains so they have very high boiling points

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18
Q

Why are shorter chain hydrocarbons more valuable?

A

They are in higher demand as they are more useful in industry
For example, as petrol

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19
Q

How are longer chain hydrocarbons broken into smaller chain hydrocarbons?

A

Industrial cracking

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20
Q

Why are alkenes useful in industry?

A

1) As chemical feedstock (they supply industries with starting materials to make different products)
2) Can be converted into other products like polymers

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21
Q

What temperature is used for thermal cracking?

A

700K-1200K (High temperature)
7000kPa (High pressure)

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22
Q

Is a catalyst used in thermal cracking?

A

No

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23
Q

What are the products of thermal cracking?

A

Alkenes
Hydrogen gas

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24
Q

How does thermal cracking produce alkenes?

A

1) C-C bonds break so that one electron from the covalent pair goes to each carbon atom
2) The fragments produced are free radicals
3) Free radicals are highly reactive so can react in a variety of ways to form shorter chain molecules
4) There are not enough hydrogen atoms to produce two alkanes
5) So one of the new chains forms a C=C bond

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25
Q

How are the products of cracking separated?

A

By fractional distillation

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26
Q

What temperature is used for catalytic cracking?

A

720K (Lower temperature)

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27
Q

What pressure is used for catalytic cracking?

A

Lower pressure than thermal cracking, but still higher than atmospheric pressure

28
Q

What catalyst is used for catalytic cracking?

A

Zeolite catalyst:
- Silicon dioxide and aluminium oxide
- In a honeycomb structure to vastly increase the surface area
- Zeolites are acidic

29
Q

What are the products of catalytic cracking?

A

Motor fuels
Branched alkanes
Cycloalkanes
Aromatic compounds

30
Q

How is catalytic cracking carried out in a lab?

A

Using lumps of aluminium oxide as a catalyst

31
Q

What type of reaction is a combustion reaction?

A

Exothermic

32
Q

Why are combustion reactions useful for fuels?

A

Because they are highly exothermic
So lots of heat energy is released

33
Q

Describe the complete combustion of alkanes.

A

Alkanes burn completely in an excess of oxygen
To produce carbon dioxide and water

34
Q

Describe the incomplete combustion of alkanes.

A

Alkanes burn incompletely in a limited supply of oxygen to form carbon monoxide, or carbon particulates (soot)
Often happens with longer chain hydrocarbons which need more oxygen to combust than shorter chains

35
Q

What is photochemical smog?

A

The chemical reaction of sunlight, nitrogen oxides, and volatile organic compounds in the atmosphere
Results in airborne particles and ground-level ozone

36
Q

How does carbon monoxide act as a pollutant?

A

Formed by incomplete combustion
Is a poisonous gas

37
Q

How do nitrogen oxides act as pollutants?

A

Formed when nitrogen and oxygen combine in the air under high temperatures (happens in petrol engines)
Contributes to acid rain and photochemical smog

38
Q

How does sulfur dioxide act as a pollutant?

A

Formed by sulfur containing impurities in crude oil
Combines with water vapour and oxygen in the air to form sulfuric acid
Contributes to acid rain

39
Q

How do carbon particulates act as a pollutant?

A

Produced by incomplete combustion
Exacerbates asthma
Can cause cancer

40
Q

How do unburnt hydrocarbons act as a pollutant?

A

They can enter the atmosphere and act as greenhouse gases
Can also contribute to photochemical smog

41
Q

How does carbon dioxide act as a pollutant?

A

Produced when hydrocarbons burn completely
Acts as a greenhouse gas

42
Q

How does water vapour act as a pollutant?

A

Produced when hydrocarbons burn completely
Acts as a greenhouse gas

43
Q

Give an equation to show how sulfuric acid is formed in the atmosphere.

A
44
Q

What are flue gases?

A

Gases given out by power stations

45
Q

Describe how flue gas desulfurisation is done with calcium oxide.

A

1) A slurry of calcium oxide and water is sprayed into flue gas
2) The flue gas reacts with calcium oxide and water to form calcium sulfite
3) Which can be oxidised to form calcium sulfate (gypsum)

46
Q

Give an equation for flue gas desulfurisation using calcium oxide and water.

A
47
Q

Why is calcium sulphate a useful product?

A

Gypsum
Can be used to make builder’s plaster or plasterboard

48
Q

Give an equation for flue gas desulfurisation using calcium carbonate.

A
49
Q

Why are catalytic converters useful?

A

They reduce the output of carbon monoxide, nitrogen oxides, and unburnt hydrocarbons in exhaust gas

50
Q

What are catalytic converters made of?

A

Ceramic honeycomb (provides a vast surface area)
Coated with platinum and rhodium (catalysts)

51
Q

Give an equation to show how carbon monoxide reacts with nitrogen oxides in a catalytic converter.

A
52
Q

Give a word equation for how unburnt hydrocarbons react with nitrogen oxides in a catalytic converter.

A
53
Q

How does carbon dioxide act as a greenhouse gas?

A

It traps infrared radiation in the Earth’s atmosphere, so it heats up

54
Q

What are the three main greenhouse gases?

A

Carbon dioxide
Methane
Water vapour (most abundant)

55
Q

What is a carbon neutral activity?

A

An activity that has no net production of carbon dioxide

56
Q

Give an overall equation for the formation of a halogenoalkane from an alkane and halogen.

A

Alkane + Halogen –> Halogenoalkane + Hydrogen Halide

57
Q

What are the three steps for a free radical substitution reaction?

A

Initiation
Propagation
Termination

58
Q

What happens during initiation?

A

UV light provides the energy to break the covalent bond between halogen atoms
One electron from the covalent pair goes to each atom
Forming two free radicals

59
Q

What happens during propagation?

A

Two steps
1) Formation of an alkyl free radical
2) Formation of halogenoalkane

Propagation is a chain reaction, could keep occurring until free radicals are terminated

60
Q

What happens during termination?

A

Removal of free radicals in one of three ways:
1) Two halogen free radical react to produce the halogen molecule
2) Two alkyl free radicals can react to produce an alkane
3) A halogen free radical could react with an alkyl free radical to produce a halogenoalkane

61
Q

Why aren’t chain reactions useful?

A

They produce a varied mixture of products which then have to be separated

62
Q

What is ozone?

A

A molecule made from three oxygen atoms (O3)
Which decomposes to oxygen

63
Q

What is the problem with ozone at ground-level?

A

It can cause lung irritation
It degrades paints and plastics

64
Q

Why is the ozone layer in the atmosphere important?

A

Protects Earth from harmful exposure to too many UV rays:
1) Causes skin cancer by damaging DNA
2) Can damage sensitive sea life (e.g. plankton) which has a knock on impact on the rest of the food chain

65
Q

How are chlorine free radicals formed in the atmosphere by chlorofluorocarbons (CFCs)?

A

1) C-Cl bond breaks homolytically in the presence of UV radiation to form chlorine free radicals
2) Chlorine radicals attack ozone molecules
3) The resulting radicals attack ozone to regenerate the chlorine radicals

66
Q

What do chlorine free radicals do in the atmosphere?

A

Act as a catalyst in the breakdown of ozone to oxygen
2O3 –> 3O2

Organic Chemistry (16 decks)

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