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3.3.2 Alkanes Flashcards

1
Q

What are alkanes?

A

Alkanes are saturated hydrocarbons.

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2
Q

What is a hydrocarbon?

A

A hydrocarbon is a molecule containing only hydrogen and carbon atoms.

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3
Q

What is a saturated hydrocarbon?

A

A saturated hydrocarbon contains only single bonds, and no carbon-carbon double bonds.

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4
Q

What is the general formula of alkanes?

A

CnH2n+2

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5
Q

What is the general formula of cyclic alkanes?

A

CnH2n

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6
Q

What are the three types of alkanes?

A

Unbranched (straight)
Branched (chain)
Cyclic

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7
Q

Explain the polarity of alkanes?

A

Alkanes re non-polar, so the only intermolecular forces that exist between them are van-der-waals forces of attraction.
The greater the chain length, the greater the Mr, so the greater the van-der-waals forces because larger molecules have larger electron clouds.

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8
Q

Explain the trends in the boiling point and melting point of alkanes?

A

Alkanes have relatively low boiling points due to there only being weak van-der-waals forces between molecules.
The greater the chain length, the higher the mp/bp as the van-der-waals forces are greater.
Branching decreases the boiling point as it changes the surface area of molecules so they are further apart from each other and cannot be packed closely together, meaning van-der-waals forces decrease.

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9
Q

Explain why alkanes are insoluble in water

A

Alkanes are insoluble in water as water molecules are held together by strong hydrogen bonds which are much stronger than van-der-waals forces, so alkanes cannot overcome the forces of attraction between water molecules.

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10
Q

Explain why alkanes are soluble in non-polar substances

A

Alkanes are soluble in non-polar substances because they both have van-der-waals forces only.

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11
Q

What is crude oil?

A

Crude oil is a substance that contains a mixture of different hydrocarbons, making it the main source of organic chemicals.

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12
Q

What is fractional distillation?

A

Fractional distillation is the process by which the hydrocarbons in crude oil are separated into different fractions.

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13
Q

Describe the process of fractional distillation

A

First, crude oil is heated in a furnace, so that it vaporises.
The vapour enters the fractionating column, which is hotter at the bottom and cooler at the top.
Hydrocarbons with a high boiling point will condense into liquid lower down in the fractionating column.
Hydrocarbons with low boiling points will rise up and condense into a liquid high up in the fractionating chamber.
The condensed hydrocarbons are tapped off as liquids and collected.

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14
Q

What is the environmental concern with fractional distillation?

A

Crude oil contains sulfur which can react with oxygen to form sulfur dioxide.
This causes acid rain (damage to buildings and harmful to aquatic life).

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15
Q

What are the fractions in order of the tower, and their uses?

A

Refinery gases- bottled gas
Petroleum/gasoline- fuel for cars
Kerosene- aircraft fuel
Diesel- fuel for cars, vans, lorries and buses
Fuel oil- fuel for ships and power stations
Bitumen- road and roof surfacing

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16
Q

Where do long and short hydrocarbons condense in the fractionating column?

A

Shorter chain hydrocarbons have a lower boiling point so condense high up in the column, whereas longer chain hydrocarbons have a higher boiling point so condense low down in the column.

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17
Q

What is cracking?

A

Cracking is the separation of long chain hydrocarbons into shorter chain hydrocarbons

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18
Q

What is the purpose of cracking economically?

A

There is a high demand for shorter chain hydrocarbons in industry, as they are easier to burn.
This means they can be used to make more fuel and useful products.

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19
Q

What does cracking produce?

A

Alkanes and alkanes (and sometimes hydrogen)

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20
Q

How does cracking occur?

A

Through the breaking of the C-C bonds in the hydrocarbon chain

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21
Q

Is cracking exothermic or endothermic?

A

Endothermic- as carbon-carbon bonds require energy to break

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22
Q

What are the two types of cracking called?

A

Catalytic cracking and Thermal cracking

23
Q

What is the reaction mechanism of thermal cracking?

A

Free radical

24
Q

What is the reaction mechanism of catalytic cracking?

A

Carbocation

25
What are the conditions for thermal cracking?
High temperatures of 750*c High pressure of 70 atmospheres
26
What are the conditions for catalytic cracking?
High temperatures of 500*c Slight pressure Zeolite catalyst
27
What does thermal cracking produce, and what are the products used for?
Alkenes mostly, but also alkanes The alkenes are used in the production of polymers
28
What does catalytic cracking produce, and what are the products used for?
Produces mostly alkanes, but also alkenes. The alkanes are used to produce motor fuels and aromatic hydrocarbons
29
What type of alkanes does cracking produce?
Branched and cyclic alkanes (they burn more efficiently)
30
Why are alkanes combusted?
They are combusted on a large scale for their use of fuels.
31
What is complete combustion of alkanes and what are the products?
When alkanes are burnt in an excess of oxygen, complete combustion will take place. All carbon and hydrogen will be oxidised to carbon dioxide and water.
32
What is incomplete combustion of alkanes and what are the products?
When alkanes are burnt in a limited supply of oxygen, incomplete combustion will take place. Not all of the carbon is oxidised, so some carbon is partially oxidised to form carbon monoxide, or in even less oxygen, carbon particulates (soot).
33
What are the steps to balance combustion equations?
1. Balance carbons first 2. Balance hydrogens second 3. Balance oxygens third 4. Half numbers can be used if needed
34
Describe how carbon monoxide is produced and problems associated with it?
Toxic and odourless gas formed by incomplete combustion of a fossil fuel. It prevents oxygen from binding to haemoglobin so oxygen cannot be transported to organs. It causes dizziness, loss of consciousness and death.
35
Describe how sulfur dioxide is produced and problems associated with it?
Crude oil contains sulfur so sulfur dioxide is produced from the combustion of sulfur-containing impurities in crude oil. It combines with water vapour and oxygen in the atmosphere to form sulfuric acid. The resultant acid rain damages lakes, fish, plant life and buildings.
36
Describe how particulates is produced and problems associated with it?
Formed from the incomplete combustion of fossil fuels. Causes global dimming, athsma, and is carcinogenic
37
Describe how nitrogen oxides is produced and problems associated with it?
Produced by the oxidation of nitrogen in car engines at high temperatures. Combine with water vapour and oxygen in the atmosphere to form nitric acid and so acid rain. Nitrous oxides (NOx) form photochemical smog
38
Describe how unburnt hydrocarbons are produced and problems associated with it?
Formed from fuels in engines It is a greenhouse gas and forms photochemical smog
39
Describe how carbon dioxide is produced and problems associated with it?
Formed from the complete combustion of fossil fuels It is a greenhouse gas so contributes to global warming.
40
Describe how water vapour is produced and problems associated with it?
Formed in both complete and incomplete combustion. It is a greenhouse gas so contributes to global warming.
41
Describe how sulfur is removed from flue gas?
Sulfur is removed from coal or natural gas (flue gas) by carrying out flue gas desulfurisation. 1. At the power station, calcium carbonate thermally decomposes at high temperatures. 2. At the power station sulfur dioxide reacts with calcium oxide at high temperatures. This produces calcium sulphate/gypsum used in plaster.
42
Describe how a catalytic converter works?
Many cars are fitted with catalytic converters to reduce the amount of pollutants released in car exhaust fumes. They use a catalyst such as platinum, which is coated onto a honeycomb which increases its surface area.
43
Describe how the enhanced greenhouse effect causes global warming?
Short wavelength (UV) radiation penetrates the atmosphere and warms the earth. Long wavelength (IR) radiation is re-emitted from earth. It is absorbed by greenhouse gases and trapped in the atmosphere, heating the atmosphere.
44
Describe how the enhanced greenhouse effect causes global warming?
Short wavelength (UV) radiation penetrates the atmosphere and warms the earth. Long wavelength (IR) radiation is re-emitted from earth. It is absorbed by greenhouse gases and trapped in the atmosphere, heating the atmosphere.
45
Define global warming
Global warming is an increase in the average temperature of the earths, and is caused by the enhanced greenhouse effect.
46
What is a free radical?
An atom or molecule with one or more unpaired electrons in its outer shell, making it highly reactive and unstable.
47
What happens if an alkane undergoes free radical substitution?
A hydrogen atom is substituted by a halogen to form a halogenoalkane.
48
What are the conditions for free radical substitution of alkanes?
Ultraviolet light (because alkanes are very unreactive)
49
What are the three stages of free radial substitution?
Initiation Propagation Termination
50
What is initiation?
The first stage in a free radical substitution reaction. The halogen is broken by UV light, so each atom takes one electron from the covalent bond (homolytic fission). This forms two halogen free radicals
51
What is homolytic fission?
Where each atom in a covalent bond gets one electron.
52
What is propagation?
The second stage in free radical substitution. Very reactive free radicals attack the unreactive alkanes, causing a C-H bond to break homolytically. This produces an alkyl free radical. This can then attack another halogen molecule to form a halogenoalkane. It also regenerates the original halogen free radical (repeat cycle).
53
What is termination?
The final step in free radical substitution. The chain reaction stops when two free radicals react together to form a single unreacitve molecule. Multiple termination processes are possible.
54
What is the shape and bond angles in alkanes?
Tetrahedral as all bonds are sigma bonds. 109.5*

Chemistry (22 decks)

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