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Organic Chemistry 1 > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

Define alkanes

A

Saturated hydrocarbons — contain only C—C and C—H single bonds

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2
Q

How do alkanes react?

A

Relatively unreactive. Have strong C—C and C—H bonds.
Do not react with acids, bases, oxidising agents, and reducing agents.
They do burn and will react with halogens under suitable conditions.

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3
Q

Describe solubility of alkanes

A

Insoluble in water.
This is because water molecules are held together by hydrogen bonds which are much stronger than the VDWs forces that act between alkane molecules.
However, alkanes do mix with other relatively non-polar liquids.

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4
Q

Describe polarity of alkanes

A

Almost non-polar because the electronegativities of carbon and hydrogen are so similar. As a result, the only intermolecular forces between their molecules are weak VDWs forces, and the larger the molecule, the stronger the VDWs forces.

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5
Q

Why does boiling point of alkanes increase as chain length increases?

A

Because of increasing intermolecular forces (which increases the VDWs forces)

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6
Q

Why do alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms?

A

Because they cannot pack together as closely as unbranched chains and so the VDWs forces are not so effective

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7
Q

Why is crude oil called a fossil fuel?

A

Because it was formed millions of years ago by the breakdown of plant and animal remains at the high pressures and temperatures deep below the Earth’s surface. Because it forms very slowly, it is effectively non-renewable

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8
Q

What is crude oil a mixture mostly of?

A

Alkanes (both unbranched and branched)

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9
Q

Where the small amount of other compounds (not alkanes) that is dissolved in crude oil come from? Give an example of one and its effect.

A

Other elements in the original plants and animals the oil was formed from.
E.g. some contain sulphur. These produce sulphur dioxide when they’re burnt. This can react with oxygen high in the atmosphere to form sulphur trioxide. This reacts with water in the atmosphere to form sulphuric acid.

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10
Q

Process of fractional distillation

A

The crude oil is first heated in a furnace.
A mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom.
The vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool. Then they condense to liquid.
The mixture of liquids that condenses on each tray is piped off.

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11
Q

During fractional distillation, why do the shorter chain hydrocarbons condense in the trays nearer to the top of the tower (where it is cooler)?

A

Because they have lower boiling points (due to smaller chain length + weaker VDWs forces)

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12
Q

Order of fuels that condense from top to bottom during fractional distillation

A 
Gases
Petrol
Naphtha
Kerosene 
Gas oil
Fuel oil and wax
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13
Q

Are covalent bonds within molecules broken during fractional distillation?

A

No because it is a physical process.

It is the VDWs forces between the molecules that are broken during vaporisation and reform on condensing.

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14
Q

What is fractional distillation and how is it different from distillation?

A

Fractional distillation separates a mixture into several components with different ranges of boiling temperatures whereas distillation simply separates all the volatile components of a mixture from the non-volatile ones

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15
Q

Gave the names of two gases produced in fractional distillation.

A

Methane, ethane, propane, butane etc.

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16
Q

Why are many long chain hydrocarbons cracked?

A

Because shorter chain products are more economically more valuable than the longer chain material. To meet the demand for the shorter chain hydrocarbons, many of the longer chain fractions are broken into shorter lengths

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17
Q

Why are harsh conditions required to break down alkanes?

A

Because they’re very unreactive

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18
Q

Name 2 types of cracking

A

Thermal cracking, catalytic cracking

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19
Q

Describe the conditions required for thermal cracking

A

High temperatures (700-1200K), high pressure (up to 7000kPa)

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20
Q

How do C—C bonds break during thermal cracking (initially)?

A

In such a way that one electron from the pair in the covalent bond goes to each carbon atom. So initially two shorter chains are produced, each ending in a carbon atom with an unpaired electron (free radical).

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21
Q

What does thermal cracking tend to produce a high proportion of?

A

A high proportion of alkenes

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22
Q

What does thermal cracking produce?

A

An alkane and an alkene

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23
Q

What is done to avoid too much decomposition of alkanes during thermal cracking?

A

The alkanes are kept in the essential conditions for a very short time, typically one second

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24
Q

What conditions are required for catalytic cracking?

A

Lower temperatures (~720K), lower pressure (but more than atmospheric), a zeolite catalyst (consisting of silicon dioxide and aluminium oxide)

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25
Q

Give two features of zeolite catalysts

A

Have a honeycomb structure with an enormous surface area, acidic

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26
Q

What does catalytic cracking mainly produce?

A

Motor fuels

27
Q

What are the products of catalytic cracking mostly?

A

Branched alkanes, cycloalkanes, aromatic compounds

28
Q

How are the products obtained from cracking separated?

A

Fractional distillation

29
Q

In the laboratory cracking of alkanes, how can you tell that the products have shorter chains than the starting material?

A

Many of the products are gases rather than liquids

30
Q

Why would we not crack octane industrially?

A

Octane itself has a short enough chain length to be in demand.

31
Q

Give two economic reasons for cracking long chain alkanes.

A

Short chain products are in greater demand than long chain ones.
Alkenes are more useful than alkanes as starting materials for further chemical reactions.

32
Q

What is complete combustion?

A

When alkanes burn completely in a plentiful supply of oxygen to give carbon dioxide and water

33
Q

What is a fuel?

A

Substances that release heat energy when they undergo combustion. They also store a large amount of energy for a small amount of weight.

34
Q

What are combustion reactions?

A

Give out heat and have large negative enthalpies of combustion.
The more carbons present, the greater the heat output.

35
Q

Draw the skeletal formula for 3-methylpentane

A

Book (P191)

36
Q

Give 3 reasons for why many people are against fracking

A

They do like the infrastructure of wells and the associated traffic in their ‘backyard’

There is concern about the amount of water used

Burning natural gas produces carbon dioxide - a cause of global warming

37
Q

Write a balanced equation equation for complete combustion of ethane

A

C2H6(g) + 3.5O2(g) -> 2CO2(g) + 3H2O

38
Q

What is incomplete combustion?

A

In a limited supply of oxygen, alkanes burn to produce the poisonous gas carbon monoxide. With even less oxygen, carbon (soot) is produced.

39
Q

Why does incomplete combustion usually happen more with long chain hydrocarbons than short chain hydrocarbons?

A

Because longer chain hydrocarbons need more oxygen to burn compared with shorter chains

40
Q

Write a balanced equation for incomplete combustion of propane.

A

C3H8(g) + 3.5O2(g) -> 3CO(g) + 4H2O(l)

41
Q

Name pollutants that burning hydrocarbon-based fuels could produce (7)

A 
Carbon monoxide
Nitrogen oxides
Sulphur dioxide 
Carbon particles
Unburnt hydrocarbons
Carbon dioxide
Water vapour
42
Q

What effect does nitrogen oxides have on the atmosphere?

A

These oxides may react with water vapour and oxygen in the air to form nitric acid. They are therefore contributors to acid rain and photochemical smog.

43
Q

What effect does sulphur dioxide have on the atmosphere?

A

These oxides combine with water vapour and oxygen in the air to form sulphuric acid.
This then falls as acid rain.

44
Q

What effect does carbon particles have on the atmosphere?

A

Can exacerbate asthma and cause cancer

45
Q

What effect does unburnt hydrocarbons have on the atmosphere?

A

They’re significant greenhouse gases. They contribute to photochemical smog which can cause a variety of health problems.

46
Q

What effect does carbon dioxide have on the atmosphere?

A

It’s a greenhouse gas. It’s rising levels has caused an increase in the Earth’s temperature and consequent climate change

47
Q

What are the gases given out by power stations called? What is the process of removing sulphur dioxide called?

A

Flue gases

Flue gas desulphurisation

48
Q

Write an equation to show how sulphur dioxide contributes to acid rain

A

Book (P199)

49
Q

Give one method of flue gas desulphurisation (+ an alternative process)

A

A slurry of of calcium oxide and water is sprayed into the flue gas which reacts with the calcium oxide to form calcium sulfite, which can be further oxidised to calcium sulfate (gypsum).

An alternative process uses calcium carbonate rather than calcium oxide.

50
Q

Write equation for flue gas desulphurisation (using calcium oxide)

A

Book (P199)

51
Q

Write equation for flue gas desulphurisation (using calcium carbonate)

A

Book (P199)

52
Q

Where are catalytic converters fitted in cars?

A

The exhaust systems of petrol engines

53
Q

What do catalytic converters reduce the output of?

A

Carbon monoxide, nitrogen oxides, and unburnt hydrocarbons in the exhaust gas mixture

54
Q

Describe what catalytic converters are (catalysts used, shape + why, how it works - excluding equations)

A

Honeycomb made of a ceramic material coated with platinum and rhodium metals (these are the catalysts).

The honeycomb shape provides an enormous surface area, so a little of these expensive metal goes a long way.

As the polluting gases pass over the catalyst, they react with each other to form less harmful products.

55
Q

Write 2 equations to illustrate how catalytic converters work

A

Book (P200)

56
Q

How do greenhouses work?

A

They become very warm inside. This is because the visible rays from the sun pass through the glass. Rather than escaping, their energy is absorbed by everything inside the greenhouse and re-radiated as IR energy (heat). IR energy has a longer wavelength and cannot pass back out through the glass.

57
Q

Describe the greenhouse effect

A

Book (P200)

58
Q

What conditions are needed for the formation of halogenoalkanes?

A

Light (UV rays)

59
Q

What type of reaction occurs during the formation of halogenoalkanes?

A

Free-radical substitution

It starts off a chain reaction which takes place in three stages — initiation, propagation, and termination

60
Q

What is the danger of too much ozone at ground level?

A

Causes lung irritation and degradation of paints and plastics

61
Q

Why is ozone important high in the atmosphere?

A

The ozone layer protects Earth from the harmful exposure to too many UV rays.

Too much UV radiation causes skin cancer in people by damaging DNA.

62
Q

Write the 3 equations that are involved in destruction of the ozone layer.

A

Book (P203)

63
Q

Why are chlorine free radicals formed from CFCs?

A

Because the C—Cl bond breaks homolytically in the presence of UV radiation to produce chlorine free radicals

64
Q

What test can you use to determine if something is a hydrogen halide?

A

Waft the fumes from a bottle of ammonia over the mouth of the test tube

White fumes of ammonium bromide are formed

Organic Chemistry 1 (7 decks)

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