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AQA A Level Chemistry > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

Saturated

A

a compound which contains only carbon-carbon single bonds

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2
Q

Hydrocarbon

A

A compound consisting of carbon and hydrogen ONLY

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3
Q

General molecular formula of alkanes

A

CnH2n+2

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4
Q

what are the different states of alkanes

A

alkanes 1-4 = gases
5-17 = liquids
18+ = solids

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5
Q

why can different alkanes in crude oil be separated by fractional distillation

A

Different alkanes have different boiling points due to their different chain lengths

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6
Q

structural isomers

A

Compounds with the same molecular formula but different structural formulae

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7
Q

where does isomerism start from in alkanes

A

butane

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8
Q

Physical properties of alkanes

A

alkanes are only slightly soluble in water
There is a gradual change in the physical properties as Mr increases e.g BP and MP increase

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9
Q

State and explain the trend in boiling points from methane to octane

A

The boiling points increases from methane to octane. There are more electrons per molecule from methane to octane, so the strength of the van der walls forces increases and more energy is needed to break the stronger intermolecular forces.

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10
Q

what is the trend in melting points and boiling points of alkanes with increasing chain length

A

The melting points and boiling points of the alkanes increases as chain length increases. Larger molecules have larger electron clouds meaning stronger Van der Waals forces.

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11
Q

What is the effect of branching in the carbon chain on the boiling points of alkanes

A

As the molecule becomes more branched, the boiling point decreases.

There is less contact between molecules with a smaller surface area, therefore there are fewer van der waals forces between these molecules compared to molecules with a larger surface area.

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12
Q

why does 2,2-dimethylpropane have a lower boiling point than pentane

A

2,2-dimethylpropane is a spherical molecule so has a smaller surface area than pentane.

There is less contact between molecules of 2,2-dimtheylpropane therefore there are fewer wan der waals forces between molecules of 2,2-dimethylpropane.

Less energy is needed to break the weaker intermolecular forces in 2,2-dimethylpropane.

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13
Q

why does butane have a higher boiling point than 2 - methylpropane

A

Butane is a linear molecule so has a greater surface area than 2-methylpropane.

There is more contact between molecules of butane therefore there are more van der waals forces between molecules of butane.

More energy is needed to break the stronger intermolecular forces in butane

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14
Q

What happens when alkanes burn in a plentiful supply of air

A

Carbon dioxide and water and formed. The reaction is exothermic

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15
Q

When does incomplete combustion take place

A

When the fuel burns in an insufficient amount of oxygen/air. Carbon monoxide and carbon soot are produced

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16
Q

What are alkanes used as

A

fuels as they produce large amounts of heat energy when burned

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17
Q

What are the main environmental problems arising from the complete combustion of alkane fuels

A

CO2 is produced when fuels are burned.

CO2 is a greenhouse gas linked to global warming

Coastal flooding, rising sea levels, change in weather patterns

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18
Q

Problems caused by incomplete combustion of alkane fuels

A

CO is formed.

CO is a toxic gas which combines with haemoglobin in red blood cells, preventing oxygen from binding with haemoglobin. This reduces the amount of oxygen in the bloodstream

Carbon soot is formed which causes breathing problems and damages the lungs

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19
Q

What are the 3 different incomplete combustion reactions

A

alkane + oxygen = CO + water
alkane + oxygen = C + water
alkane + oxygen = C + CO + water

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20
Q

why are liquid fuels preferred to gaseous fuels

A

they are easier to store and transport
Gaseous fuels need to be cooled to a very low temp to be liquefied
Liquids occupy a smaller volume than gases so more fuel can be carried

21
Q

Disadvantages of using hydrogen fuel

A

It is a gas and needs to be stored under pressure in large heavy containers
It needs to be cooled to a very low temp to be liquefied

22
Q

what is a hazard associated with alkanes

A

they are flammable

23
Q

what is the principal source of alkanes

A

crude oil

24
Q

In a fractionating column where is it the hottest

A

At the bottom

25
Q

what are the conditions for free radical substitution

A

Ultraviolet radiation

26
Q

Why is ultraviolet radiation necessary for free radical substitution

A

The ultraviolet radiation provides the energy to break the Cl-Cl bonds producing chlorine free radicals. The reaction is initiated by chlorine free radicals

27
Q

Why are free radicals needed to attack the alkane

A

The molecules have no significant polarity in their strong C-H bonds

28
Q

Why does propagation lead to a chain reaction

A

Because a chlorine free radical is regenerated by the propagation reactions each time a molecule of product is formed

29
Q

What happens in termination step

A

2 free radicals react with each other in the termination step. The reaction ends because the product is a stable species and no free radicals are formed

30
Q

State 2 limitations of using free radical substitution in organic synthesis

A

Further substitution takes place so a mixture of products is formed
There could be substitution at different positions along the chain

31
Q

why do C-H bonds have no significant polarity

A

the electronegativity of C is similar to that of H

32
Q

What is cracking

A

Breaking large alkanes/hydrocarbons into smaller hydrocarbon molecules by breaking C-C bonds

33
Q

What are the economic reasons for cracking

A

The petroleum fractions with shorter carbon chain such as petrol are more in demand

To make use of excess larger hydrocarbons and to supply demand for shorter

The products of cracking are more valuable than the starting materials

34
Q

what are the 2 different types of cracking

A

Thermal cracking and catalytic cracking

35
Q

What are the conditions for thermal cracking

A

High temperature and High pressure

36
Q

What are the conditions for catalytic cracking

A

moderate pressure
High temperature
Zeolite catalyst

37
Q

What does thermal cracking produce

A

Mostly alkenes and sometimes hydrogen

38
Q

What does catalytic cracking produce

A

branched and cyclic alkanes and aromatic hydrocarbons and motor fuels

39
Q

What effect on the environment does carbon soot have

A

Causes global dimming - reflection of sun’s light

40
Q

What pollutants does internal combustion engine produce

A

nitrogen oxides, carbon monoxide, carbon and unburned hydrocarbons

41
Q

How is sulfur dioxide produced

A

Sulphur containing impurities are found in petroleum fractions which produce sulphur dioxide when they are burned

42
Q

What is produced when sulphur dioxide dissolves in atmospheric water

A

acid rain

43
Q

How can sulphur dioxide be removed from flue gases

A

Using calcium oxide or calcium carbonate because they are bases and react with the acidic sulphur dioxide and neutralises it

44
Q

How are the oxides of nitrogen produced

A

When nitrogen and oxygen inside a car engine react under high temperature.

45
Q

what do catalytic convertors remove

A

CO, NOx and unburned hydrocarbons

46
Q

What do catalytic convertors do to the harmful gases when they remove them

A

they convert them into the ‘harmless’ CO2, N2, and H20

47
Q

What does the combustion of hydrocarbons containing sulphur impurities lead to

A

Air pollution and acid rain

48
Q

What are catalytic convertors made of

A

Platinum, palladium etc….

49
Q

Describe the separation process of petroleum in order to separate the fractions

A

Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top

Crude oil enters the fractionating column and is heated strongly so vapours rise

Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column

Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top

The different fractions condense at different heights according to their boiling points

The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases

The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column

AQA A Level Chemistry (27 decks)

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