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AQA A Level Organic Chemistry > 2. Alkanes > Flashcards

2. Alkanes Flashcards

1
Q

What is petroleum another name for?

A

Crude oil

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2
Q

What is crude oil a mixture of?

A

Hydrocarbons, the majority of which are alkanes

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3
Q

What needs to be done to crude oil in order for it to be useful?

A

It needs to be separated into mixtures of hydrocarbons with similar chain lengths

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4
Q

What are fractions?

A

Mixtures with similar chain lengths

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5
Q

What is a hydrocarbon?

A

A molecule that contains only hydrogen and carbon atoms

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6
Q

What is an alkane?

A

A hydrocarbon with the general formula C₂H₂ₙ₊₂

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7
Q

Why are alkanes relatively unreactive?

A

They only contain strong C-C and C-H bonds

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8
Q

Why are alkanes useful as fuels?

A

They burn

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9
Q

What are alkanes held together by?

A

Weak van der waal’s forces

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10
Q

What happens to van der waal’s forces as size of the alkane increases?

A

The strength increases

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11
Q

What happens to boiling point of alkanes as size increases?

A

It increases

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12
Q

Why does boiling point of alkanes increase with size?

A

Because the strength of the van der waal’s forces in between increases

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13
Q

Why do branched alkanes tend to have lower boiling points than straight chain alkanes?

A

Because there is reduced surface area contacts with branched isomers, and so fewer van der waal’s forces - and so less energy required to break them

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14
Q

How is petroleum separated into useful fractions?

A

In an oil refinery by fractional distillation

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15
Q

What equipment is used for fractional distillation?

A

A fractionating column

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16
Q

Which fractions condense at the top of the fractionating column?

A
  • those with short chains

* those with low melting points

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17
Q

Which fractions condense at the bottom of the fractionating column?

A
  • those with long chains

* those with high melting points

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18
Q

What happens during fractional distillation?

A
  • vaporised crude oil enters a fractionating column - which is hot at bottom and cool at top
  • vapour from the oil rise
  • vapour condense when they become cool enough
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19
Q

What is cracking?

A

The chemical splitting of long-chain alkanes into hydrocarbons of shorter chain length by breaking a C-C bond

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20
Q

What is produced when long chain alkanes are cracked?

A

Short chain alkanes and alkenes

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21
Q

What are the two main types of cracking?

A
  • thermal cracking

* catalytic cracking

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22
Q

What are the conditions required for thermal cracking?

A
  • 700-1200K (900*C)

* 7000 kPa

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23
Q

Why are intense conditions required to break alkanes down?

A

They are unreactive

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24
Q

What provides the energy to break C-C bonds in thermal cracking?

A

High temperatures

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25
Q

What is there an absence of in the conditions for thermal cracking?

A

Air

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26
Q

What is a high percentage of the products made from thermal cracking?

A

Alkenes

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27
Q

How many free radicals does cracking form?

A

Two

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28
Q

In thermal cracking, where do long-chain hydrocarbons tend to break at the lower end of the temperature scale?

A

In the middle of the molecule

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29
Q

In thermal cracking, where do long-chain hydrocarbons break as temperature is increased?

A

The cracking shifts from the middle of the molecule towards one end of the molecule

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30
Q

What does using even higher temperatures in thermal cracking lead to?

A

A higher percentage of short chain alkenes as molecules are cracked closer to the end

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31
Q

How would you get shorter-chained molecules in thermal cracking?

A

Use higher temperatures

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32
Q

What is thermal cracking also known as?

A

Free radical reaction

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33
Q

When happens when the C-C bond breaks in thermal cracking?

A

Each of the carbon atoms gets one of the shared electrons - producing two free radicals

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34
Q

What happens after two free radicals are formed during thermal cracking?

A

One free radical can take a hydrogen atom to form an alkane and the other will lose a hydrogen atom to form an alkene

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35
Q

Why are free radicals highly reactive?

A

Due to an unpaired electron

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36
Q

How long are conditions in thermal cracking maintained for? Why is this?

A

Approximately 1 second, as they are so intense

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37
Q

What type catalyst of is used in catalytic cracking?

A

A zeolite catalyst

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38
Q

What are zeolite catalysts?

A

Crystalline aluminosilicates

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39
Q

What are the conditions required for catalytic cracking?

A
  • high temperature - 720K (450*C)
  • slight pressure - 1-2 atm
  • zeolite catalyst
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40
Q

What structure does the catalyst used in catalytic cracking have?

A

Honeycomb

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41
Q

Why does the zeolite catalyst in catalytic cracking have a honeycomb structure?

A

To ensure a large surface area

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42
Q

How does one carbon atom end up with a positive charge in catalytic cracking?

A

The C-C bond breaks in a way that both electrons from the shared pair remain with one carbon (leaving the other with a positive charge)

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43
Q

What is mainly produced during catalytic cracking?

A

Branched alkanes, cycloalkanes and aromatic compounds

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44
Q

Why are mainly branched alkanes, cycloalkanes and aromatic compounds produced in catalytic cracking?

A

The C-C bond breaks in a way that leaves one carbon with a positive charge

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45
Q

Why are short-chain branched chain hydrocarbons used for motor fuels?

A

They burn more easily

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46
Q

What are short-chain branched chain hydrocarbons used for?

A

Motor fuels

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47
Q

Which burns easiest; straight or branched chain hydrocarbons?

A

Branched chain

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48
Q

What are the aromatic compounds produced in catalytic cracking used for?

A

Starting materials for many substances such as dyes and polymers

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49
Q

What do the products of cracking need to be separated?

A

Fractional distillation

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50
Q

What type of hydrocarbons are more in demand?

A

Short-chained

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51
Q

What happens to long-chained hydrocarbons that are produced from fractional distillation? Why?

A

They are cracked again, as short-chained hydrocarbons are more in demand

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52
Q

What are the most useful fractions produced from fractional distillation of crude oil? Why is this?

A

Those with shorter chains as they are more in demand

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53
Q

Does the supply of useful fractions from fractional distillation only meet demand?

A

No

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54
Q

How does demand and supply compare for long and short chained hydrocarbons?

A
  • short chained - supply exceeds demand

* long chained - demand exceeds supply

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55
Q

How is the demand for short-chained hydrocarbons met?

A

Long chain hydrocarbons are cracked into more useful fractions to help meet demand

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56
Q

Why are alkenes industrially valuable?

A

They have a reactive nature due to the presence of a double bond

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57
Q

What are alkenes used as a starting material for?

A

Different products such as polymers and pharmaceuticals

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58
Q

What are alkanes used for?

A

Fuels

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59
Q

What are the types of combustion of organic compounds?

A
  • complete

* incomplete

60
Q

When does complete combustion occur?

A

When there is a plentiful supply of oxygen

61
Q

What are the products of complete combustion?

A

Carbon dioxide and water

62
Q

Why are carbon dioxide and water produced in the complete combustion of alkanes?

A
  • carbon dioxide due to oxidation of carbon in the hydrocarbon
  • water due to oxidation of hydrogen in the hydrocarbon
63
Q

When does incomplete combustion occur?

A

When there is an insufficient supply of oxygen

64
Q

What can be produced instead of carbon dioxide in incomplete combustion?

A
  • carbon monoxide

* soot

65
Q

What is soot?

A

A black solid

66
Q

Where might incomplete combustion take place?

A

An internal combustion engine (car engine)

67
Q

What is formed in incomplete combustion?

A
  • carbon monoxide and water

* soot and water

68
Q

What additional products are formed during combustion in internal combustion (car) engines?

A

Oxides of nitrogen (NOx) and unburned hydrocarbons

69
Q

What happens in combustion engines at high temperatures and pressures?

A

Oxygen and nitrogen in the air can react together to form the nitrogen oxides:

  • NO
  • NO2
70
Q

Under what conditions is NO produced in a combustion engine?

A

High pressure/temperature

71
Q

When is NO2 produced in a combustion engine?

A

Under high temperatures/pressures, when NO reacts further

72
Q

What is produced in combustion engines when NO reacts further?

73
Q

Which compound is responsible for the haze on hot days (ie. smog)?

74
Q

What happens when NO2 dissolves in rainwater?

A

Produces acid rain (weak solution of nitric acid)

75
Q

What are catalytic converters made from?

A

Platinum or palladium

76
Q

What are cars fitted with to remove harmful gases?

A

Catalytic converters

77
Q

What are catalytic converters supposed to do?

A

Remove most of the harmful gases produced in engines

78
Q

Example of the equations used to show how catalytic converters work?

A
  • C3H8 + 5O2 -> 3CO2 + 4H20

* 2NO + 2CO -> N2 + 2CO2

79
Q

What do most crude oil and coal deposits contain as an impurity?

80
Q

Why is sulphur present in many fuels?

A

Most crude oil contains sulphur as an impurity

81
Q

What happens to sulphur when hydrocarbon fuel undergoes combustion?

A

The sulphur reacts with the oxygen to produce sulphur dioxide

82
Q

What do sulphur-containing hydrocarbons burn to produce?

83
Q

What is sulphur dioxide?

A

An acidic gas that causes air pollution

84
Q

What happens when sulphur dioxide dissolves in rainwater?

A

Produces weak solution of sulphuric acid - acid rain

85
Q

What are flue gases?

A

Gases coming out of chimneys/flues

86
Q

What do flues (chimneys) in factories contain to remove flue gases?

A

Calcium carbonate or calcium oxide

87
Q

What does the calcium carbonate/oxide in flues react with?

A

SO2 to form calcium sulphate

88
Q

How is SO2 removed from factory flue gases?

A

Reacts with calcium carbonate/oxide in flues to produce calcium sulphate

89
Q

What is the process known as to remove SO2 from flue gases?

A

Flue gas desulphurisation

90
Q

What is flue gas desulphurisation?

A

The removal of SO2 from flue gases

91
Q

Equation for reaction of SO2 with calcium oxide in flue gas desulphurisation?

A

CaO + SO2 -> CaSO3

92
Q

Equation for reaction of SO2 with calcium carbonate in flue gas desulphurisation?

A

CaCO3 + SO2 -> CaSO3 + CO2

93
Q

Which is the better calcium compound to use in flue gas desulphurisation?

A

Calcium oxide as calcium carbonate produces carbon dioxide

94
Q

How can sulphur dioxide emissions be reduced?

A
  • flue gas desulphurisation

* sulphur impurities removed from petrol before it is burnt

95
Q

Why is water vapour harmful to the environment?

A

It is a greenhouse gas

96
Q

What are halogenoalkanes?

A

Alkanes in which one or more hydrogen atoms are replaced by a halogen atom

97
Q

What does the halogen replace in a halogenoalkane?

A

A hydrogen atom

98
Q

What happens in the substitution reaction for the formation of halogenoalkanes?

A

When a hydrocarbon and a halogen react, at least one H atom will be replaced by a halogen

99
Q

What must happen when methane reacts with chlorine to form a halogenoalkane?

A

At least one H atom will be replaced with chlorine

100
Q

Under what conditions will methane undergo a substitution reaction with chlorine?

A

In UV light

101
Q

What will the substitution reaction of methane with chlorine produce?

A

Chloromethane and hydrogen chloride

102
Q

Symbol equation for the substitution reaction of methane with chlorine?

A

CH4 + Cl2 → CH3Cl + HCl

103
Q

What can happen after the substitution reaction of methane with chlorine?

A

The reaction can continue until all hydrogen atoms are replaced (one at a time)

104
Q

What is formed after all hydrogen atoms in methane are replaced with chlorine?

A

Tetrachloromethane

105
Q

In which type of reaction can all of the hydrogen atoms in an alkane be replaced with halogens?

A

Substitution reaction

106
Q

What does the equation for a substitution reaction not show?

A

Which bonds are broken and formed, when and how

107
Q

How is the substitution of halogens in alkanes shown in more detail?

A

Through the reaction mechanism - in this case free radical substitution

108
Q

What is the reaction mechanism for when halogenoalkanes are formed?

A

Free radical substitution

109
Q

How many stages does free radical substitution occur in?

110
Q

What are the three stages in free radical substitution?

A
  1. Initiation
  2. Propagation
  3. Termination
111
Q

What does the reaction stage in free radical substitution involve?

A

The homolytic fission of a covalent bond

112
Q

What is the first stage of free radical substitution?

A

Initiation

113
Q

What is the second stage of free radical substitution?

A

Propagation

114
Q

What is the third stage of free radical substitution?

A

Termination

115
Q

What is homolytic fission?

A

When a covalent bond breaks between two identical atoms, so that one electron from the shared pair goes to each atom

116
Q

What conditions are required for homolytic fission?

117
Q

Why is UV light required for homolytic fission?

A

Provides energy to split Cl-Cl bond

118
Q

Why are the C-H bonds not broken in homolytic fission, but the Cl-Cl are?

A

It takes more energy to break C-H bonds than Cl-Cl, so there is not enough energy

119
Q

What is the equation for the initiation step in free radical substitution (chlorine and methane)?

A

Cl - Cl → Cl• + Cl•

120
Q

Where do the electrons go in homolytic fission?

A

One electron from the shared pair foes to each atom

121
Q

What type of bond is broken in homolytic fission?

122
Q

What is a radical?

A

A species with an unpaired electron

123
Q

What is propagation?

A

The reaction of a free radical with a molecule to produce a different free radical as well as a stable molecule

124
Q

What is produced in propagation?

A

A stable molecule and a free radical

125
Q

How many steps are involved in propagation?

126
Q

What is the first step of propagation (chlorine and methane)?

A

The chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride, leaving a methyl free radical, •CH3

127
Q

What is the equation for the first step of propagation (chlorine and methane)?

A

Cl• + CH4 → HCl + •CH3

128
Q

What is the second step of propagation (chlorine and methane)?

A

The methyl free radical reacts with a chlorine molecule - this produces another chlorine free radical and a stable compound (e.g. chloromethane)

129
Q

What is the equation for the second step of propagation (chlorine and methane)?

A

•CH3 + Cl2 → CH3Cl + Cl•

130
Q

What are the propagation equations for chlorine and methane?

A
  1. Cl• + CH4 → HCl + •CH3

2. •CH3 + Cl2 → CH3Cl + Cl•

131
Q

What is the effect of the propagation steps in free radical substitution? (e.g. chlorine and methane)

A

Produces HCl, chloromethane, and a new Cl• free radical

132
Q

What is the chain part of the chain reaction that is free radical substitution?

A
  • in propagation
  • HCl, chloromethane and Cl• produced
  • free radical ready to react with more methane
133
Q

What is termination?

A

The combination of any two free radicals to form a single stable molecule with no unpaired electrons

134
Q

Equation examples of how termination can occur?

A
  • Cl + •Cl → Cl2
  • Cl + •CH3 → CH3Cl
  • CH3 + •CH3 → C2H6
135
Q

How many free radicals are involved in termination?

136
Q

What does termination show?

A

That other products are formed as well the main ones (e.g. chloromethane and hydrogen chloride)

137
Q

How is it ensured that chloromethane is the major product in a reaction of methane and chlorine?

A

Using excess methane

138
Q

What is excess methane used for in creating chloromethane?

A

In order to ensure that chloromethane is the major product

139
Q

Why are a range of products formed in the termination step?

A

There are a range of free radicals which can combine

140
Q

How can the three steps of free radical substitution be easily identified?

A
  • initiation produces free radicals from stable molecules
  • propagation has free radicals as both reactants and products
  • termination has radicals as reactants but only stable molecules as products
141
Q

What are chloroalkanes used for?

142
Q

What are chlorofluroalkanes used for?

143
Q

What are chlorofluroalkanes?

A

An alkane where one or more hydrogen atoms have been replaced by chlorine and fluorine atoms

144
Q

What happens if long-chained alkanes are used in free radical substitution?

A

Many products and isomers will be formed

145
Q

Why are chain reactions of long-chained alkanes not very useful?

A

Due to the mixture of products formed

146
Q

Is UV required in the termination step?

AQA A Level Organic Chemistry (16 decks)

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