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A Level Chemistry > Hydrocarbons: Alkanes and Alkenes > Flashcards

Hydrocarbons: Alkanes and Alkenes Flashcards

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1
Q

Are alkanes saturated or unsaturated hydrocarbons?

A

They are saturated hydrocarbons.

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2
Q

What does it mean if fuels are refined before use?

A

They have their sulphur compounds removed.

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3
Q

Describe the process of fractional distillation of crude oil.

A 
Crude oil is vaporised.
Evaporates.
Rises.
Cools.
Condensates.
Collected as fractions.
Smaller molecules in fractions closer to the top of column where it is cooler.
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4
Q

Why do branched molecules have lower boiling points than straight chain molecules?

A

They have a reduced surface area leading to reduced magnitude of London forces.

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5
Q

The octane number of a fuel is a measure of its what?

A

Performance.

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6
Q

Higher octane numbers mean what?

A

Less straight chains and a smoother start when starting an engine (less knocking).

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7
Q

How can we make branched or cyclic chains?

A

By reforming or cracking.

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8
Q

What are the conditions for reforming?

A

A catalyst with a high temperature.

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9
Q

Conditions for thermal cracking?

A

Temperature and pressure (alkenes are a product).

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10
Q

Conditions for catalytic cracking?

A

Catalyst, temperature and pressure (forms aromatic compounds).

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11
Q

Alkanes react with halogens under what conditions?

A

UV light or 300C.

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12
Q

The process of halogenation of alkanes takes place in what three stages?

A

Initiation.
Propagation.
Termination.

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13
Q

A chlorine molecule can be split into two chlorine radicals by what process?

A

Homolytic fission.

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14
Q

Under what conditions can homolytic fission take place?

A

UV light or 300C.

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15
Q

Why does a large amount of homolytic fission take place in the upper atmosphere?

A

There is a large exposure to UV light in the upper atmosphere.

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16
Q

What is the popagation stage of the halogenation of alkanes?

A

When a halogen radical goes on to react with an alkane.

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17
Q

The propagation stage can be described as a _________ reaction.

A

A chain reaction because it is a rapid process that continues until no more reactant remains.

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18
Q

What is the step at the end of free radical substitution?

A

Termination.

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19
Q

What is the termination stage?

A

Where two radicals combine to form a molecule. There are number of different possibilities due to the variety of radicals that are formed.

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20
Q

If there is too much of a halogen what could happen in free radical substitution?

A

You could produce di, tri or tetra of that haloalkane.

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21
Q

How can we reduce the chance of forming di, tri or tetra haloalkanes?

A

By ensuring there is excess alkane.

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22
Q

If we have longer molecule alkanes for free radical substitution what could be an issue?

A

You may get a mixture of structural isomers.

23
Q

Fractional distillation gives a large supply of heavier fractions but what are in high demand?

A

Smaller fractions for fuel.

24
Q

Cracking converts long chain alkanes into…

A

…smaller more useful molecules in a mixture of branched alkanes, cycloalkanes, alkenes and branched alkenes.

25
Q

How can a higher proportion of alkenes be produced from cracking?

A

When cracking is conducted at a high temperature with steam.

26
Q

When conducted in the presence of a catalyst a higher proportion of what are produced from cracking?

A

Branched and cyclic alkanes.

27
Q

What is the catalyst for cracking?

A

Synthetic sodium aluminium silicate which is a zeolite.

28
Q

Zeolites have a three dimensional structure similar to what?

A

Clay.

29
Q

Inside the zeolite the silicon, aluminium and oxygen atoms form what?

A

Tunnels into which small molecules can fit into.

30
Q

Cracking takes place in the surface of the zeolite catalyst at what temperature?

A

500C.

31
Q

Synthetic zeolites are excellent catalysts, why?

A

They can be developed to have an active site of specific shape to favour the desired products.

32
Q

Reforming converts straight chain alkanes into…

A

…branched chain or cyclic alkanes or arenes such as benzene and cyclic alkanes into arenes.

33
Q

What is a valuable by-product of reforming that can be used elsewhere in the refinery?

A

Hydrogen.

34
Q

What is the catalyst for reforming?

A

Precious metals, such as platinum or rhodium supported on an inert material, such as aluminium oxide.

35
Q

What temperature does reforming take place?

A

500C.

36
Q

What causes incomplete combustion?

A

Insufficient air supply.

37
Q

General formula of alkenes:

A

CxH2x

38
Q

Are alkenes saturated or unsaturated?

A

Alkenes are unsaturated.

39
Q

Test for saturation?

A

Bromine water decolourises in unsaturated substances.

40
Q

Why don’t alkenes dissolve in water?

A

Hydrogen bonding in water is far stronger than the

London forces in between the alkenes.

41
Q

How do you make an alkane from an alkene?

A

You hydrogenate the alkene at 150C with a nickel catalyst to saturate the alkene.

42
Q

Making dihalogenalkanes from alkenes is an example of what type of reaction?

A

Electrophilic addition.

43
Q

Hetrolytic fission forms what?

A

Ions.

44
Q

Tertiary carbocations are attached to how many carbons?

A

3 carbon groups.

45
Q

Secondary carbocations are attached to how many carbons?

A

2 carbon groups.

46
Q

Primary carbocations are attached to how many carbons?

A

1 carbon group.

47
Q

List the carbocations in order of stability.

A

Tertiary (most stable), secondary, primary (least stable).

48
Q

In order to form halogenalkanes from alkenes we must react it with what?

A

A hydrogen halide.

49
Q

How do we make alcohols from alkenes?

A

React alkenes with steam over an acid catalyst (not HCl).

50
Q

Reacting alkenes with steam is a what type of reaction?

A

Hydration.

51
Q

How can we make diols from alkenes?

A

React alkene with water and an oxidising agent to make a diol.

52
Q

What is the oxidising agent for making a diol?

A

Dilute acidified potassium permanganate (VII).

53
Q

Many monomers join together to form…

A

…a long chain polymer through polymerisation.

A Level Chemistry (13 decks)

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