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A-Level Chemistry > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

What is petroleum mainly made of?

A

Alkanes

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2
Q

What are alkanes?

A

Alkanes are saturated hydrocarbons

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3
Q

What is the general formula of alkanes?

A

CnH2n+2

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4
Q

What are hydrocarbons?

A

Compounds that only contain carbon and hydrogen atoms

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5
Q

What is the definition of saturated?

A

There are only single bonds

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6
Q

What is the general formula of cycloalkanes?

A

CnH2n

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7
Q

What are the bonds like between the C and H bonds?

A

They are non-polar

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8
Q

What is petroleum?

A

It is a mixture of hydrocarbons ranging from small to long alkanes

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9
Q

What process allows the hydrocarbons in petroleum to be separated?

A

Fractional distillation

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10
Q

What happens to the van der Waals forces as the length of the chain increases?

A

As the chain length increases, the melting/boiling points increases which result in stronger van der Waals forces

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11
Q

Describe the fractional distillation process of petroleum

A
  • crude oil is vapourised at about 350 degrees C
  • the vapourised crude oil is passed into a fractionating column
  • it is hot at the bottom and cold at the top
  • the vapourised crude oil cools as it rises up the column
  • the molecules condense at their boiling points
  • larger molecules are collected lower down the column
  • molecules with the lowest boiling points are collected as gases at the top of the column
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12
Q

What does primary distillation produce?

A

Primary distillation produces useful substances that boil above 350 degrees C at atmospheric pressure

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13
Q

What happens to the products of primary distillation at high temperatures?

A

The substances decompose at high temperatures and have to be distilled at lower pressure

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14
Q

What is cracking?

A

Cracking is the breaking of long alkane chains into more useful shorter alkane and alkene chains

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15
Q

What is the general formula for alkenes?

A

CnH2n

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16
Q

What are the two types of cracking?

A

Thermal cracking

Catalytic cracking

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17
Q

Describe the conditions for thermal cracking?

A

High temperatures up to 1000 degrees C

High pressure up to 70atm

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18
Q

What are the products made from thermal cracking?

A

Alkenes

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19
Q

Give an example of what alkenes are used to make

A

Polymers

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20
Q

Describe the conditions for catalytic cracking

A

High temperature of about 450 degrees C
Low-pressure 1-2atm
Zeolite catalyst

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21
Q

What are the products made from catalytic cracking?

A

Motor fuels

Aromatic hydrocarbons

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22
Q

Why is zeolite used?

A

Zeolite has a honeycomb structure and an enormous surface area

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23
Q

Why is a catalyst used?

A

Catalysts cut costs because lower conditions can be used(temperature/pressure) saving energy
The reaction is speeded up which saves time

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24
Q

What are the economic reasons for cracking?

A
  • products of cracking are more valuable

- shorter alkane chains are in more demand than longer alkane chains

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25
Q

Describe how the zeolite catalyst works

A
  • A zeolite catalyst has sites which remove a hydrogen from the alkane along with the 2 electrons in the bond
  • This leaves a positive charge on the carbon atom(carbocation)
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26
Q

What makes alkanes great fuels?

A

Burning a small amount releases a large amount of energy

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27
Q

What is complete combustion?

A

Complete combustion occurs when alkanes are burnt with plenty of oxygen

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28
Q

What are the products of complete combustion?

A

Carbon dioxide

Water

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29
Q

What is the equation for the combustion of propane?

A

C3H8(g) + 5O2 -> 3CO2(g) + 4H2O(g)

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30
Q

What is incomplete combustion?

A

Incomplete combustion occurs when alkanes burn with a limited supply of oxygen

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31
Q

What are the products of incomplete combustion?

A

Carbon(soot)

carbon monoxide

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32
Q

What are the problems caused by burning crude oil?

A

Carbon monoxide(poisonous gas) is produced
Nitrogen oxides are formed
Acid rain(sulfuric acid,nitrogen oxides)
Photochemical smog(nitrogen oxides, greenhouse gases)
Particulates
Greenhouse gases

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33
Q

Give examples of greenhouse gases

A

Water vapour
Carbon dioxide
Methane

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34
Q

What is the equation for nitrogen oxide?

A

N2(g) + O2(g) -> 2NO(g)

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35
Q

What is the equation for sulfuric acid?

A

SO2 + H2O -> H2SO3 + 0.5O2 -> H2SO4

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36
Q

What problems can particulates cause?

A

Cancer

Asthma

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37
Q

How does acid rain affect the environment?

A

Vegetation and trees are destroyed
Buildings and statues are corroded
Marine life is killed because of the acidic conditions

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38
Q

How can sulfur be removed from flues?

A

Calcium oxide or limestone are now used in flues to absorb sulfur dioxide

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39
Q

What salt is formed when the sulfur reacts with the calcium compounds?

A

Calcium sulfate

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40
Q

What is the process of removing sulfur from flues called?

A

Flue gas desulfurisation

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41
Q

What is the greenhouse effect?

A

The greenhouse effect is the absorption of infrared energy by greenhouse gases and trapping them in the Earth’s atmosphere keeping it warm

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42
Q

What is global warming?

A

Global warming is the increase in the Earth’s temperature ,due to what most scientists believe to be caused by the increase in the level of carbon dioxide in the atmosphere

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43
Q

why is infra-red radiation important?

A

It is important as it heats up the Earth or else it would be too cold and life couldn’t be sustained

44
Q

Why does the concentration of water vapour in the atmosphere stay roughly the same?

A

This is because of the equilibrium that exists between water and water vapour

45
Q

Why are catalytic converters important?

A

They remove unburnt hydrocarbons and oxides of nitrogen from the exhaust

46
Q

Describe catalytic converters

A
  • ceramic material coated with platinum and rhodium metals which act as catalysts
  • honeycomb structure which provides an enormous surface area
47
Q

What is the equation involving carbon monoxide and nitrogen oxide passing through a catalytic converter?

A

2CO(g) + 2NO(g) -> N2(g) + 2CO2(g)

48
Q

What is the equation involving a hydrocarbon and nitrogen oxide passing through a catalytic converter?

A

hydrocarbon + nitrogen oxide -> nitrogen + carbon dioxide + water
e.g. C8H18 + 25NO ->12.5N2 + 8CO2 + 9H2O

49
Q

What would happen to the amount of water vapour in the atmosphere if the temperature increased?

A

The amount of water vapour would increase resulting in an increase of the greenhouse effect

50
Q

What is a haloalkane?

A

An alkane with halogen atoms

51
Q

What type of bond is there between a carbon and a halogen?

A

A polar bond

52
Q

Why is the bond between carbon and a halogen polar?

A

Halogens are more electronegative than carbon

53
Q

Where is the delta positive charge in a carbon-halogen bond?

A

On the carbon

54
Q

What is the delta positive carbon atom susceptible to?

A

Nucleophiles

55
Q

What is a nucleophile?

A

A nucleophile is an electron pair donor

56
Q

Which are the only nucleophiles to react with haloalkanes?

A

OH-
CN-
NH3

57
Q

What happens to the bond between a carbon-halogen as you go down group 7?

A

The bond becomes less polar as the electronegativity decreases

58
Q

What are the main intermolecular forces in haloalkanes?

A

Dipole-dipole attractions

van der Waals forces

59
Q

Why are haloalkanes not soluble in water?

A

They are not polar enough

60
Q

What happens to the boiling point as you go down the halogen group?

A

The boiling point increases

61
Q

Why do haloalkanes have higher boiling point than similar chained alkanes?

A

They have higher relative molecular mass

They are more polar

62
Q

What is a free radical?

A

A particle with an unpaired electron

63
Q

When are free radicals formed?

A

When a covalent bond splits

64
Q

In what two ways can a covalent bond break?

A

Homolytic fission

Heterolytic fission

65
Q

What is homolytic fission?

A

The covalent bonds breaks equally and each atom/molecule gets one electron

66
Q

What is heterolytic fission?

A

The covalent bond breaks unequally and one atom /molecule gets both electrons and the other none

67
Q

How can a free radical in a mechanism be shown?

A

Using a dot

68
Q

What property does the unpaired electron give to the atom/molecule?

A

It makes them very reactive

69
Q

In what types of reactions do halogens react with alkanes?

A

Photochemical reactions

70
Q

What are photochemical reactions?

A

Reactions started by UV light

71
Q

What is a free radical substitution reaction?

A

A reaction where a hydrogen is replaced by a halogen

72
Q

What is a reaction mechanism?

A

A series of steps that show what happens in a chemical reaction

73
Q

What is photodissocation?

A

When the sun provides enough energy to break a bond

74
Q

What are the three steps in free radical substitution?

A

Initiation
Propagation
Termination

75
Q

What happens during the initiation reaction?

A

Free radicals are formed
e.g. Cl2 –> 2Cl(free radicals)
The bond splits equally

76
Q

What happens during the propagation reaction?

A

free radicals are used up and created in a chain reaction
CH4 + Cl(radical) –> HCl + CH3(radical)
CH3(radical) +Cl2–> CH3Cl + Cl(radical)
All propagation steps have a free radical in the reactants and products

77
Q

What happens in the termination reaction?

A

When 2 free radicals collide, the chain reaction is terminated as the radicals make a stable molecule
CH3Cl(radical) + Cl(radical) –> CH3Cl

78
Q

What are CFCs?

A

Chlorofluorocarbons that contain no hydrogen

Hydrogen atoms are replaced by chlorine and flourine atoms

79
Q

Why were CFCs used for refrigerators, air conditions and aerosols?

A

They have low reactivity, low volatility and non-toxic

80
Q

What is the ozone layer?

A

A layer in the upper atmosphere that filters out much of the Sun’s harmful UV radiation

81
Q

What is the equation that shows the formation and depletion of the ozone layer?

A

O + O2 O3

Reversible reaction

82
Q

Why is there a constant amount of ozone in the atmosphere?

A

The rate of formation = the rate of depletion

83
Q

What is the formula for the ozone formation?

A

O2 + UV –> O(radical) + O(radical)

84
Q

What is the formula for the ozone depletion?

A

O3 +UV –> O2 + O

85
Q

Radicals from what can affect the breakdown of the ozone layer?

A

Thunderstorms
Aircraft
CFCs
NOx

86
Q

How can one regenerated chlorine atom destroy thousands of ozone molecules?

A

Chlorine free radicals catalyse the decomposition of the ozone as they provide an alternative route with a lower activation energy

87
Q

Why were CFCs banned?

A

They were destroying the ozone layer

88
Q

What did chemists develop as a safer alternative to CFCs?

A

HFCs- hydrofluorocarbon

They contain no chlorine

89
Q

What problems can UV radiation cause?

A

Skin cancer

Sunburn

90
Q

What is a nucleophilic substitution reaction?

A

A reaction where a nucleophile reacts with a polar molecule by kicking out the functional group and taking its place

91
Q

What do curly arrows in a nucleophilic substitution reaction show?

A

The movement of electron pairs

92
Q

How does a nucleophilic substitution reaction work?

A
  • The lone pair of electrons on the nucleophile attack the delta positive carbon and the carbon-halogen bond breaks
  • The halogen leaves taking both electrons with it
  • A new bond forms between the carbon and nucleophile
93
Q

What is formed when haloalkanes react with hydroxides?

A

Alcohols

for example, bromoethane can react to form ethanol

94
Q

What is nucleophilic substitution sometimes called?

A

Hydrolysis

95
Q

What is hydrolysis?

A

Splitting a molecule apart by reacting it with water

96
Q

What is formed when cyanide reacts with haloalkanes?

A

Nitrile

97
Q

What is formed when haloalkanes react with ammonia?

A

Amines

98
Q

What factor decides the reactivity in haloalkanes?

A

The bond enthalpy

99
Q

Which haloalkane bnd has the highest bond enthalphy?

A

C-F bond

they undergo nucleophilic substitution more slowly

100
Q

What happens to the speed of the substitution as the bond enthalphy decreases?

A

The substitution is faster as it is easier to break the bonds

101
Q

what would happen if you warm a haloalkane with excess ethanoic ammonia?

A

The ammonia swaps places with the halogen

102
Q

What can the amine group in the product of the reaction do?

A

The amine group has a lone pair of electrons and can act as a nucleophile

103
Q

What other types of reaction can haloalkanes also undergo?

A

Elimination reactions

104
Q

What happens if you warm a haloalkane with hydroxide ions in ethanol?

A

An alkene is formed

105
Q

How does the elimination reaction with ethanol and haloalkane work?

A

e.g. CH3CHBrCH3 + OH- –> CH2CHCH3 + H2O + Br-
OH- acts as a base and takes a proton, H+, from a carbon and forms water
The carbon has a spare electron and forms a double bond with the next carbon
To form the bond, the carbon has to let go of the Br which becomes a Br- ion

106
Q

In aqueous conditions, what type of reaction happens between haloalkanes and hydroxides?

A

Nucleophilic substitution

OH- acts as a nucleophile

107
Q

In anhydrous conditions, what type of reaction happens between haloalkanes and hydroxides?

A

Elimination

OH- acts as a base

A-Level Chemistry (30 decks)

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