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AQA A Level Chemistry S4L > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

How does fractional distillation separate alkanes?

A

Crude oil heated until it has evaporated
Gas then rises through fractioning tower
Fractioning tower cools as you go up
The longer the fraction (C chain), the higher the boiling/condensation point - therefore condense fairly near the bottom
The shorter chains have lower condemnation points, and so condense nearer the top as the temp. cools

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2
Q

What are the conditions of thermal cracking?

A

High temps (500-1000*C) and high pressure

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3
Q

What kind of length chain does thermal cracking produce?

A

Shorter chain alkanes and alkenes

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4
Q

What are the conditions of catalytic cracking?

A

Takes place at lower temp (450*C) and pressure than thermal

Uses a zeolite catalyst

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5
Q

What kind of products does catalytic cracking produce?

A

Products mainly branched and cyclic alkanes and aromatic compounds (benzene based)

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6
Q

What are the products of catalytic cracking mostly used for?

A

Motor fuels

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7
Q

What are the products of complete combustion?

A

CO2 and H2O ONLY

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8
Q

What is the difference between complete and incomplete combustion?

A

Complete only produces CO2 and H2O

Incomplete may also produce CO or particulates

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9
Q

Why is CO harmful?

A

Colourless and poisonous - reduces carrying O2 capacity of blood

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10
Q

What is an effect of particulates?

A

Smog and global dimming

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11
Q

CO2, H2O and possibly CO or particulates are formed in the combustion of petrol. What other pollutants are formed?

A

SO2 gas from sulphur impurities, various oxides of nitrogen

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12
Q

What is an effect of sulphur of the environment?

A

Acid rain

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13
Q

What is an effect of nitrogen oxides of the environment?

A

Acid rain

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14
Q

What does a catalytic converter do?

A

Remove pollutant gases formed from the combustion in the engine

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15
Q

What kind of structure does a catalytic converter have and why?

A

Honeycomb

Increases SA so increases efficiency and reduces cost

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16
Q

A catalytic converter consists of a mixture of metals. What are they?

A

Platinum, palladium and rhodium

17
Q

The chlorination of alkanes involves what kind of mechanism reaction?

A

Free radical substitution

18
Q

What are the stages of free radical substitution?

A

Initiation, propagation 1 and 2, termination

19
Q

What happens in the initiation stage of free radical substitution (chlorination of an alkane)?

A

UV light breaks the Cl-Cl bond (homolytic fission)

2 free radicals of Cl produced

20
Q

What is a free radical?

A

An atom or species with an unpaired electron

21
Q

What happens in the propagation stages of free radical substitution (chlorination of an alkane)?

A

1 - Cl + CH4 –> HCl + CH3 (C = free radical)

2 - CH3 + Cl2 –> Ch3Cl + Cl

22
Q

Why is propagation hard to control?

A

It’s a chain reaction - a free radical Cl is produced at the end each time, and so starts a new reaction

23
Q

What happens in the termination stage of free radical substitution (chlorination of an alkane)?

A

2 free radicals combine
CH3 + Cl –> CH3Cl
CH3 + CH3 –> C2H6

24
Q

How are further substitutions able to take place in free radical substitution?

A

It is possible for more than one H to be replaced if the product meets more Cl*

25
Q

In free radical substitution, how can you increase the likelihood of CCl4 being produced?

A

Excess Cl2

26
Q

Name the raw material from which hexane is obtained

A

Crude oil

27
Q

Explain the economic reasoning for the cracking of alkanes

A

There is a higher demand for shorter chain alkanes

28
Q

Give the general formula for alkanes

A

CnH2n+2

29
Q

Sulphur dioxide is sometimes produced from reactions in the engine. How has it come to be present there?

A

Present as impurities in petroleum fractions

30
Q

What is an alkane?

A

A saturated hydrocarbon

31
Q

What does cracking involve?

A

Breaking C-C bonds in alkanes

32
Q

What is an essential condition for the initiation stage of free radical substitution to occur?

A

UV light must be present

AQA A Level Chemistry S4L (34 decks)

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