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AQA A Level Chemistry > 3.2 Alkanes > Flashcards

3.2 Alkanes Flashcards

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1
Q

What is an alkane ?

A

A saturated hydrocarbons containing C-H bonds only

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2
Q

What is the general formula of an alkane

A

CnH2n+2

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3
Q

Are their bonds polar? Why / why not?

A

Nonpolar- carbon and hydrogen have similar electronegativities

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4
Q

Which intermolecular forces do they have?

A

Only Van de Waals forces of attraction - bonds are non polar

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5
Q

Are they soluble in water? Why?

A

Insoluble because hydrogen bonds in water are stronger than Van de Waals forces of attraction

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6
Q

How reactive are alkanes?

A

Very unreactive

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7
Q

Which reactions will alkanes undergo?

A

Combustion and reactions with halogens

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8
Q

What is crude oil? How is it formed? Is it renewable? Why?

A

Mixture of fractions (hydrocarbons with similar boiling points and properties)
Formed at high temperatures and pressures deep below the earths surface over millions of years —> therefore non-renewable

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9
Q

Name the fractions from high to low boiling points

A

Gases - fuels on sight
Gasoline/ petroleum/ naphtha- cars
Kerosene/ paraffin - jet fuel / lighting
Diesel fuel - lorries / taxis
Lubricating oils/ waxes - candles/ engine oil
Fuel oil- ships/ power stations
Tar / bitumen- roads / roofing

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10
Q

What is fractional distillation and how does it work?

A

Crude oil heated til mostly vaporised
Passed into a fractioning tower that is cooler at the top than at the bottom
Liquid fractions are piped off at the bottom
Vapours rise up the column and - via trays and bubble caps - condense when temperature < their boiling point
Shortest chain hydrocarbons condense at the top as they have the lowest boiling points

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11
Q

What is fracking and how is it done?

A

Natural gas held within a shale rock
Drill into shale, force pressurised water and sand in the rock to fracture it, collect gas
HCl and methanol added to break up shale and prevent corrosion

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12
Q

Pros/cons of fracking

A

Advantages - gas supply for many years, reduces imported gas and electricity
Disadvantages - lots of traffic to local area, concern about amount of water used, chemical additives can pollute water supplies, can cause small earthquakes, combustion of CH3 —> CO2 —> global warming

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13
Q

Why are alkanes cracked?

A

To turn a long chain alkane, which is not very economically valuable, into a shorter chain alkane chain (more economically valuable as used as fuels) and an alkene (more reactive, starting point for many products)

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14
Q

What are the conditions for thermal cracking?

A

700-1200K temperature
Up to 7000kPa pressure

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15
Q

What is the intermediate for the reaction?

A

Free radicals

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16
Q

What are the main products of thermal cracking?

A

Alkenes

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17
Q

What are the conditions for catalytic cracking

A

Lower temperature (720K)
Lower pressure (but above atmospheric)
Zeolite catalyst (SiO2 and Al2O3) with a honey comb structure to give a large surface area

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18
Q

What are the main products of catalytic cracking ?

A

Cycloalkanes, aromatic hydrocarbons, branched alkanes

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19
Q

Write an equation for the combustion of propane

A

C3H8 + 5O2 —> 3CO2 + 4H2O

20
Q

What is a fuel ?

A

Something which releases heat energy when combusted

21
Q

What are the 5 main fuels comprising of alkanes ?

A

Methane, Butane, propane, petrol (about C8) and paraffin (C10-C18)

22
Q

What is incomplete combustion and what products are formed in the case of alkanes ?

A

Combustion in a limited supply of oxygen
CO - carbon monoxide - poisonous
C - carbon - particulates - soot - global dimming

23
Q

Which type of hydrocarbons are most likely to undergo incomplete combustion

A

Longer chains

24
Q

What is the environmental impact of CO

A

Toxic/poisonous

25
Q

What is the environmental impact of NO3

A

Form nitric acid —> acid rain
Photochemical smog

26
Q

What is the environmental impact of Sulphur impurities / sulphur dioxide

A

Form sulphuric acid —> acid rain

27
Q

What is the environmental impact of Soot (Carbon)

A

Asthma, cancer, global dimming

28
Q

What is the environmental impact of unburnt hydrocarbons

A

Photochemical smog

29
Q

What is the environmental impact of carbon dioxide

A

Greenhouse gas —> global warming
Increases global temperatures
Speeds up climate change

30
Q

What is the environmental impact of water vapour

A

Greenhouse gas —> global warming
Increases global temperatures
Speeds up climate change

31
Q

What are flue gases

A

Gases given out by power stations

32
Q

Write two different equations for two different ways of desulphurising flue gases

A

Ca (s) + 2H2O (l) + SO2 (g) + 1/2O2 (g) —> CaSO4.2H2O
Or
CaCO3 (s) + SO2 (g) + 1/2 O2 (g) —> CaSO4 + CO2 (g)

33
Q

What are catalytic converters made of

A

Ceramic honeycomb coated with platinum palladium and rhodium

34
Q

What do catalytic converters catalyse

A

They exhaust these products from car exhausts:
2CO (g) + 2NO (g) —> N2 (g) + 2CO2 (g)
Hydrocarbons + NO —> N2 + CO2 + H2O

35
Q

What are greenhouse gases

A

Gases which trap infrared radiation, which make the earth act like a greenhouse

36
Q

What is the greenhouse effect and how does contribute to global warming

A

Trap infrared radiation in the atmosphere, atmosphere heats up —> global warming

37
Q

Define carbon neutral activities

A

Activities which have no net / overall carbon dioxide emissions

38
Q

How are halogenoalkanes formed from alkanes

A

Free radical substitution reaction

39
Q

What are the three stages of free radical substitution

A

Initiation - breaking halogen bond to form free radicals
Propagation - chain part of the reaction where products are formed but free radical remains
Termination - free radicals removed, stable products formed

40
Q

What are the conditions needed for the formation of a free radical of chlorine

A

UV light

41
Q

Write equations for the reaction of CH4 with Cl2 to form CH3Cl

A

Initiation
Cl2 —> 2Cl• (in presence of UV light)
Propagation
Cl• + CH4 —> HCl + •CH3
•CH3 + Cl2 —> CH2Cl + Cl•
Termination
•CH3 + Cl• —> CH3Cl
2Cl• —> Cl2
•CH3 + •CH3 —> CH3CH3

42
Q

What is the ozone layers function

A

Protect the earth from too much harmful UV radiation

43
Q

How do CFCs break down the ozone layer

A

Free radical substitution

44
Q

Write an equation for the overall decomposition of the ozone layer

A

2O3 —> 3O2

45
Q

Write free radical substitution reactions to show how Cl• catalyse the breakdown of O3

A

Cl2 —> 2Cl• (in the presence of UV light)
Cl• + O3 —> ClO• + O2
ClO• + O3 —> 2O2 + Cl•

Overall: 2O3 —> 3O2

AQA A Level Chemistry (34 decks)

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