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A-Level Chemistry > Alkanes > Flashcards

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

Cn H2n+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

Intermolecular forces? Why?

A

Only van der Waals forces of attraction - bonds are non-polar.

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5
Q

Solubility in water? why?

A

Insoluble because hydrogen bonds in water are stronger than alkanes’ van der 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 reaction 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 earth’s surface over millions of years

→ therefore non-renewable

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9
Q

Name the fractions from high to low boiling point.

A

Gases - fuel on site.

Gasoline/petrol/naphtha - cars.

Kerosene/paraffin - jet fuel, lighting.

Diesel oil - lorries/taxis.

Lubricating oil/waxes - candles, engine oil.

Fuel oil - ships, power stations.

Tar/bitumen - roads/roofing

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10
Q

What is fractional distillation/how does it work?

A

Crude oil heated until mostly vapourised.

Passed into a fractionating tower that is cooler at the top than 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 shale rock.

Drill into shale, force pressurised water and sand into 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, combust CH4 → CO2

→ global warming

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13
Q

Why are alkanes cracked?

A

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

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14
Q

What are the conditions for thermal cracking?

A

700-1200 K temperature.

Up to 7000 kPa 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 temp (720K).

Lower pressure (but above atmospheric).

Zeolite catalyst (SiO2 and Al2O3) with a honeycomb 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 five main fuels comprising of alkanes?

A

Methane, butane, propane, petrol (about C8), 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 carbon monoxide?

A

It is toxic/poisonous

25
Q

What is the environmental impact of nitrogen oxides?

A

form nitric acid → acid rain.

photochemical smog

26
Q

What is the environmental impact of sulfur impurities/ sulfur 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 equations for two different ways of desulfurising flue gases.

A

CaO (s) + 2H2O (l) + SO2 (g) + ½O2 (g) → CaSO4.2H2O (s)

Or

CaCO3 (s) + SO2 (g) + ½O2 (g) → CaSO4+ CO2 (g)

33
Q

What are catalytic converters made up of?

A

Ceramic honeycomb coated with platinum, palladium and rhodium (Pt, Pd and Rh) metals

34
Q

What do they catalyse (equations)?

A

They catalyse these reactions of 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, making the earth act like a greenhouse

36
Q

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

A

Greenhouse gases trap infrared radiation in the atmosphere, atmosphere heats up → global warming.

37
Q

Define carbon neutral activities

A

Activities that produce 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 chlorine atom?

A

Presence of 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 → CH3Cl + Cl

Termination:
CH3 + Cl → CH3Cl 2Cl* → Cl2
*CH3+ *CH3→ CH3CH3

42
Q

What is the ozone layer’s function?

A

protects the earth from harmful exposure to too many UV rays

43
Q

How do CFCs break the ozone layer down?

A

Free radical substitution

44
Q

Write an equation for the overall decomposition of ozone into oxygen (O2)

A

2O3→ 3O2

45
Q

Write free radical substitution equations to show how Cl free radicals catalyse the decomposition of O3

A

Cl2 → 2Cl* (in presence of UV light)
Cl* + O3→ ClO* + O2
ClO* + O3 → 2O2 + Cl*

Overall: 2O3→ 3O2

A-Level Chemistry (16 decks)

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