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Chemistry revision > Alkanes > Flashcards

Alkanes Flashcards

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

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 and 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 of fracking?

A
  • Gas supply for many years

- reduces imported gas and electricity

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13
Q

Cons of fracking?

A
  • 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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14
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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15
Q

What are the conditions for thermal cracking?

A

700-1200 K temperature

Up to 7000 kPa pressure

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16
Q

What is the intermediate for the thermal cracking reaction?

A

Free radicals

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17
Q

What are the main products of thermal cracking?

A

alkenes

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18
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 SA
19
Q

What are the main products of catalytic cracking?

A

Cycloalkanes, aromatic hydrocarbons, branched alkanes

20
Q

Write an equation for the combustion of propane

A

C3H8 + 5O2 → 3CO2 + 4H2O

21
Q

What is a fuel?

A

Something which releases heat energy when combusted

22
Q

What are the five main fuels comprising of alkanes?

A

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

23
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

24
Q

Which type of hydrocarbons are most likely to undergo incomplete combustion?

A

Longer chains

25
Q

What is the environmental impact of carbon monoxide?

A

It is toxic/poisonous

26
Q

What is the environmental impact of nitrogen oxides?

A

form nitric acid → acid rain,

photochemical smog

27
Q

What is the environmental impact of sulfur impurities/

sulfur dioxide?

A

form sulphuric acid → acid rain

28
Q

What is the environmental impact of soot (carbon)?

A

asthma, cancer, global dimming

29
Q

What is the environmental impact of unburnt

hydrocarbons?

A

Photochemical smog

30
Q

What is the environmental impact of carbon dioxide?

A

greenhouse gas → global warming, increases global temperatures, speeds up climate change

31
Q

What is the environmental impact of water vapour?

A

greenhouse gas → global warming, increases global temperatures, speeds up climate change

32
Q

What are catalytic converters made up of?

A

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

33
Q

What do catalytic converters 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

34
Q

What are greenhouse gases?

A

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

35
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

36
Q

Define carbon neutral activities

A

Activities that produce no net / overall CO2 emissions

37
Q

How are halogenoalkanes formed from alkanes?

A

Free radical substitution reaction

38
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

39
Q

What are the conditions needed for the formation of

a free radical chlorine atom?

A

Presence of UV light

40
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
41
Q

What is the ozone layer’s function?

A

protects the earth from harmful exposure to too many UV rays

42
Q

How do CFCs break the ozone layer down?

A

Free radical substitution

43
Q

Write an equation for the overall decomposition of

ozone into oxygen (O2)

A

2O3→ 3O2

44
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

Chemistry revision (46 decks)

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