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A-Level Organic Chemistry AQA (Paper 2) > Alkanes And Haloalkanes > Flashcards

Alkanes And Haloalkanes Flashcards

1
Q

What are alkanes?

A

•Saturated hydrocarbons- they only contain carbon and hydrogen atoms.

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2
Q

What is the general formula for alkanes?

A

•CnH2n+2.

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3
Q

What bonds does each carbon atom form in an alkane?

A

•Four single bonds- alkanes are saturated (they only contain single bonds).

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4
Q

What are cycloalkanes? (2)

A
  • They have a ring of carbon atoms with two hydrogens attached to each carbon.
  • They are still saturated.
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5
Q

What is the general formula of cycloalkanes?

A

•CnH2n (assuming they only have one ring and are saturated).

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6
Q

What is petroleum?

A

•Known as crude oil.

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7
Q

What is petroleum made up of? (2)

A
  • A mixture of hydrocarbons (mainly alkanes).

* Ranging from small alkanes like pentane and large alkanes of more than 50 carbons.

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8
Q

Why does petroleum need to be separated?

A

•To get the useful products/fractions.

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9
Q

What is the process by which petroleum is separated?

A

•Fractional distillation.

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10
Q

Describe the process of fractional distillation. (6)

A
  • 1). Crude oil is vaporised at 350°C.
  • 2). Vaporised crude oil goes into a fractionating column and rises up through the trays. The largest hydrocarbons don’t vaporise at all because their boiling points are too high (they run to the bottom forming a gooey residue).
  • 3). As the crude oil goes further up the fractionating column, it gets cooler.
  • 4). Because the alkane molecules have different chain lengths, they have different boiling points, so each fraction condenses at a different temperature.
  • 5). Fractions are drawn off at different levels in the column.
  • 6). Hydrocarbons with the lowest boiling points don’t condense, they’re drawn off as gases at the top of the column.
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11
Q

As the carbon chain gets longer in fractional distillation, the hydrocarbons become… (4)

A
  • More viscous.
  • Harder to ignite.
  • Less volatile.
  • Have a higher boiling point.
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12
Q

What is the use of hydrocarbons of chains with 1-4 carbons?

A

•Gases- liquified petroleum gas (LGP), camping gas.

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13
Q

What is the use of hydrocarbons of chains with 5-12 carbons?

A

•Petrol (gasoline).

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14
Q

What is the use of hydrocarbons of chains with 7-14 carbons?

A

•Naphtha- processed to make petroleum chemicals.

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15
Q

What is the use of hydrocarbons of chains with 11-15 carbons?

A

•Kerosene (paraffin)- jet fuel, petrochemicals, central heating fuel.

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16
Q

What is the use of hydrocarbons of chains with 15-19 carbons?

A

•Gas oil (diesel)- diesel fuel, central heating fuel.

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17
Q

What is the use of hydrocarbons of chains with 20-30 carbons?

A

•Mineral oil- lubricating oil.

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18
Q

What is the use of hydrocarbons of chains with 30-40 carbons? (Residue)

A

•Fuel oil- ships, power stations.

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19
Q

What is the use of hydrocarbons of chains with 40-50 carbons? (Residue)

A

•Wax, grease- candles, lubrication.

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20
Q

What is the use of hydrocarbons of chains with 50+ carbons? (Residue)

A

•Bitumen- roofing, road surfacing.

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21
Q

How are heavy fractions made into smaller molecules?

A

•By cracking.

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22
Q

Why do heavy fractions need to be cracked? (2)

A
  • There is a higher demand for lighter fractions of crude oil, like petrol and naphtha.
  • Heavier fractions like bitumen is not as useful.
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23
Q

What is cracking? (2)

A
  • The breaking of long-chain alkanes into smaller hydrocarbons (which can include alkenes).
  • It involves breaking the C-C bonds.
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24
Q

Give an example of the cracking of decane. (2)

A
  • C10H22 —> C2H4 + C8H18

* Decane —> Ethene + Octane

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25
Q

What are the two types of cracking? (2)

A
  • Thermal cracking.

* Catalytic cracking.

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26
Q

What is thermal cracking? (2)

A
  • Takes place at a high temperature (up to 1000°C) and a high pressure (up to 70 atm).
  • Produces a lot of alkenes.
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27
Q

What are the alkenes produced from thermal cracking used for?

A

•To make products, like polymers (plastics) e.g. poly(ethene) made from ethene.

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28
Q

What is catalytic cracking? (2)

A
  • Uses a zeolite catalyst (hydrated aluminosilicate), at a slight pressure and high temperature (about 450°C).
  • Produces aromatic hydrocarbons and other fuels.
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29
Q

What do aromatic compounds contain? (2)

A
  • Benzene rings- they have six carbon atoms with three double bonds.
  • They are pretty stable because the electrons are delocalised around the carbon ring.
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30
Q

Why does using a catalyst lower the cost? (2)

A
  • The reaction can be done at a low pressure and lower temperature.
  • It speeds up the reaction, saving time and money.
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31
Q

What is a fuel?

A

•A substance that burns exothermically.

32
Q

Why are alkanes useful as fuels? (2)

A
  • If you burn (oxidise) alkanes (and other hydrocarbons) with plenty of oxygen- a combustion reaction.
  • If you burn a small amount, it releases a humongous amount of energy.
33
Q

What are the other products of a complete combustion reaction? (3)

A
  • Alkane + oxygen —> carbon dioxide + water
  • Carbon dioxide.
  • Water.
34
Q

How do you test for carbon dioxide?

A

•It turns a lime water solution from colourless to a cloudy precipitate.

35
Q

How do you test for water? (2)

A
  • Boiling at 100°C, it evaporates.

* E.g. Adding water to anhydrous copper sulfate = turns blue.

36
Q

Where are alkanes burnt?

A

•In power stations, central heating systems, to power car engines.

37
Q

What are the disadvantages of burning alkanes?

A

•Pollutants are produced.

38
Q

What is incomplete combustion?

A

•When there is not enough oxygen to fully oxidise all the carbon, so the products of the reaction are changed.

39
Q

What are the additional products of incomplete combustion? (3)

A
  • Carbon or soot (C).
  • Carbon monoxide (CO).
  • Carbon dioxide (CO2).
40
Q

What does carbon particles result in? (4)

A
  • Global dimming- the soot particles clock out the sun’s rays, preventing plants from photosynthesising.
  • Breathing problems.
  • Making things dirty.
  • Building up in engines- preventing them from working properly.
41
Q

What is the negative effect of carbon monoxide? (2)

A
  • It is a toxic gas:
  • Carbon monoxide molecules bind to the same site on haemoglobin molecules in red blood cells as oxygen molecules, so oxygen molecules cannot be transported around the body.
42
Q

How is carbon monoxide removed from exhaust gases?

A

•By catalytic converter on cars.

43
Q

What does carbon dioxide and water vapour contribute to?

A

•Global warming (they are greenhouse gases).

44
Q

What is the greenhouse effect? (2)

A
  • Greenhouse gases in the atmosphere are really good at absorbing infrared radiation (heat energy).
  • They emit some of the energy they absorb back towards the Earth, keeping it warm.
45
Q

What is global warming?

A

•The rise in the temperature of the Earth’s atmosphere thought to be caused by the greenhouse effect.

46
Q

What causes smog? (2)

A
  • Unburnt hydrocarbons and oxides of nitrogen- engines don’t burn all the fuel molecules.
  • Hydrocarbons and nitrogen oxides react in the presence of sunlight to form ground-level ozone (O3), a major component of smog- it causes lung damage and respiratory problems.
47
Q

How are oxides of nitrogen (NOx) produced?

A

•When the high temperature and pressure of car engines cause the nitrogen and oxygen atoms from the air to react together.

48
Q

How is acid rain caused? (3)

A
  • When fossil fuels containing sulphur burn, the sulfur reacts with the oxygen in the air to form sulfur dioxide gas.
  • Sulfur dioxide dissolves in the moisture and is converted into sulphuric acid (acid rain).
  • Destroys trees, vegetation, buildings, statues and fish.
49
Q

How is sulfur dioxide removed from power station flue gases before it gets into the atmosphere? (5)

A
  • By flue gas desulfurisation:
  • E.g. CaO + SO2 —> CaSO3
  • Base + Sulfur Dioxide –> Calcium sulfate
  • Powdered limestone or calcium oxide is mixed with water to make an alkaline slurry.
  • When the flue gases mix with the alkaline slurry, the acidic sulfur dioxide gas reacts with the calcium compounds to form a harmless salt (calcium sulfate).
50
Q

What are catalytic converters? (2)

A

•Placed in the car exhaust system to reduce pollution from CO, NOx and unburnt hydrocarbons.
2CO + 2NO –(catalyst)–> 2CO2 + N2

51
Q

What are catalytic converters made from?

A

•Platinum or rhodium catalysts mounted on a ceramic honey comb (it is cheaper as it reduces the amount of metal used and increases surface area).

52
Q

What are the disadvantages of catalytic converters? (2)

A
  • Expensive.

* Needs to be warm- not good on cold days, not good on short journeys (short start up time).

53
Q

What are free radicals? (4)

A
  • A particle with an unpaired electron.
  • Formed when a covalent bond splits equally, giving one electron to each atom (homolytic fission).
  • The unpaired electron makes them very reactive.
  • Shown by ‘•’.
54
Q

How are haloalkanes formed? (2)

A
  • When halogens react with alkanes in photochemical reactions- reactions started by ultraviolet light.
  • A hydrogen atom is substituted by chlorine or bromine (a free radical substitution reaction).
55
Q

What is the overall reaction of chlorine and methane to produce chloromethane?

A

CH4 + Cl2 -(UV)-> CH3Cl + HCl

56
Q

What are the steps to the reaction mechanism of free-radical substitution? (3)

A
  • Initiation.
  • Propagation.
  • Termination.
57
Q

What happens during initiation? (5)

A

•Free radicals are produced.
•Sunlight provides enough energy to break the Cl-Cl bond (photodissociation).
Cl2 -(UV)-> 2Cl•
•The bond splits equally and each atom gets to keep one electron.
•The atom becomes a highly reactive free radical, Cl•, because of its unpaired electron.

58
Q

What happens during propagation? (6)

A

•Free radicals are used up in a chain reaction.
•Cl• attacks a methane molecule:
Cl• + CH4 —> •CH3 + HCl
•The new methyl free radical, CH3•, can attack another Cl2 molecule:
•CH3 + Cl2 —> CH3Cl + Cl•
•The new Cl• can attack another CH4 molecule, and son on, until all the CH4 or Cl2 molecules are used up.

59
Q

What happens during the termination reaction? (3)

A
  • If two free radicals join together, they make a stable molecule.
  • The two unpaired electrons form a covalent bond.
  • E.g. Cl• + •CH3 —> CH3Cl
60
Q

What is the molecular formula of dichloromethane?

A

•CH2Cl2.

61
Q

What is the molecular formula of trichloromethane?

A

•CHCl3.

62
Q

What is the molecular formula of tetrachloromethane?

A

•CCl4.

63
Q

What is the overall equation for the conversion of 2-iodopropane to 1,2-diiodoproane?

A

CH3-CHI-CH3 + I2 —> CH3-CHI-CHI + HI

64
Q

Give the pair of propagation steps to form the conversion of 2-iodopropane to 1,2-diiodoproane? (2)

A

CH3-CHI-CH3 + I• —> •CH2-CHI-CH3 +HI

•CH2-CHI-CH3 + I2 —> CH2I-CHI-CH3 + I•

65
Q

Give the termination step that forms a substance with the empirical formula of C3H6I

A

2•CH2-CHI-CH3 —> CH3-CHI-CH2-CH2-CHI-CH3

66
Q

What is the polarity of alkanes? (2)

A
  • They are almost non-polar because the electronegativities of carbon (2.5) and hydrogen (2.1) are so similar.
  • The only IMFs are weak vdW forces, the larger the molecule, the stronger the vdW forces.
67
Q

What’s the solubility of alkanes? (2)

A
  • Insoluble in water- water molecules are held together by hydrogen bonds, which are much stronger than the vdW forces that act between the alkane molecules.
  • They mix with other relatively non-polar liquids.
68
Q

How do alkanes react? (4)

A
  • They are relatively unreactive.
  • They have strong carbon-carbon and carbon-hydrogen bonds.
  • They do not react with acids, bases, oxidising agents and reducing agents.
  • They burn and will react with halogens under suitable conditions.
69
Q

With longer-chain alkanes in chain reactions…

A

•Many isomers will be formed because the Cl• can replace any of the hydrogen atoms.

70
Q

Why are chain reactions not very useful? (2)

A
  • They produce such a mixture of products.

* They can occur without light at high temperatures.

71
Q

What is ozone? (2)

A
  • A molecule made from three oxygen atoms, O3.

* It decomposes to oxygen.

72
Q

What does too much ozone at ground levels cause?

A

•Lung irritation and degradation of paints and plastics.

73
Q

What is the importance of the role of ozone in the atmosphere? (2)

A
  • It protects the Earth from the harmful exposure to too many ultraviolet (UV) rays.
  • Without this, more people would get skin cans as DNA is damaged by the UV rays.
74
Q

What are believed to be destroying the ozone layer?

A

•Chlorofluorocarbons (CFCs) in the stratosphere.

75
Q

How do CFCs destroy the ozone layer? (4)

A
  • Chlorine free radicals are formed from CFCs when the C-Cl bond breaks homolytically in the presence of UV radiation to produce chlorine free radicals, Cl•.
  • The chlorine free radicals then attack ozone molecules.
  • The resulting free radicals also attack ozone and regenerate Cl•.
  • The chlorine free radical is not destroyed in the process, it acts as a catalyst in the breakdown of ozone to oxygen.
76
Q

Write out the equations to show how CFCs deplete the ozone layer. (3)

A

Cl• + O3 —> ClO• + O2
ClO• + O3 —> 2O2 + Cl•
Overall: 2O3 —> 3O2

77
Q

Outline the essential features of fractional distillation of crude oil that enable the crude oil to be separated into fractions. (4)

A
  • Fractions have diff bp - high bp = larger fractions which are piped off at the bottom.
  • Bp depends on the chain length of molecules.
  • Top of tower cooler than bottom (vice versa) = temperature gradient.
  • Short-chain molecules found at top (vice versa).

A-Level Organic Chemistry AQA (Paper 2) (17 decks)

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