3.2 Alkanes

Question 1

What is the general formula of an alkane?

Answer 1

CnH2n+2

Question 2

Are their bonds polar? Why/why not?

Answer 2

Nonpolar- carbon and hydrogen have similar electronegativities

Question 3

Intermolecular forces? Why?

Answer 3

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

Question 4

Solubility in water? why?

Answer 4

Insoluble because hydrogen bonds in water are stronger than alkanes’ van der Waals forces of attraction

Question 5

How reactive are alkanes?

Answer 5

Very unreactive

Question 6

Which reactions will alkanes undergo?

Answer 6

Combustion and reaction with halogens

Question 7

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

Why?

Answer 7

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

Question 8

Name the fractions from high to low boiling point.

Answer 8

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

Question 9

What is fractional distillation/how does it work?

Answer 9

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

Question 10

What is fracking and how is it done?

Answer 10

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

Question 11

Pros/cons of fracking?

Answer 11

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

Question 12

Why are alkanes cracked?

Answer 12

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)

Question 13

What are the conditions for thermal cracking?

Answer 13

700-1200 K temperature

Up to 7000 kPa pressure

Question 14

What is the intermediate for the reaction?

Answer 14

Free radicals

Question 15

What are the main products of thermal cracking?

Answer 15

Alkenes

Question 16

What are the conditions for catalytic cracking?

Answer 16

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

Question 17

What are the main products of catalytic cracking?

Answer 17

Cycloalkanes, aromatic hydrocarbons, branched alkanes

Question 18

What is a fuel?

Answer 18

Something which releases heat energy when combusted

Question 19

What are the five main fuels comprising of alkanes?

Answer 19

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

Question 20

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

Answer 20

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

Question 21

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

Answer 21

Longer chains

Question 22

What is the environmental impact of carbon monoxide?

Answer 22

It is toxic/poisonous

Question 23

What is the environmental impact of nitrogen oxides?

Answer 23

form nitric acid → acid rain, photochemical smog

Question 24

What is the environmental impact of sulfur impurities/ sulfur dioxide?

Answer 24

form sulphuric acid → acid rain

Question 25

What is the environmental impact of soot (carbon)?

Answer 25

Asthma, cancer, soot

Question 26

What is the environmental impact of unburnt hydrocarbons?

Answer 26

Photochemical smog

Question 27

What is the environmental impact of carbon dioxide?

Answer 27

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

Question 28

What is the environmental impact of water vapour?

Answer 28

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

Question 29

What are flue gases?

Answer 29

Gases given out by power stations

Question 30

What are catalytic converters made up of?

Answer 30

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

Question 31

What do they catalyse (equations)?

Answer 31

They catalyse these reactions of products from

car exhausts.

Question 32

What are greenhouse gases?

Answer 32

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

Question 33

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

Answer 33

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

Question 34

Define carbon neutral activities

Answer 34

Activities that produce no net / overall carbon dioxide emissions

Question 35

How are halogenoalkanes formed from alkanes?

Answer 35

Free radical substitution reaction

Question 36

What are the three stages of free radical substitution?

Answer 36

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

Question 37

What are the conditions needed for the formation of

a free radical chlorine atom?

Answer 37

Presence of UV light

Question 38

What is the ozone layer’s function?

Answer 38

protects the earth from harmful exposure to too many UV rays

Question 39

How do CFCs break the ozone layer down?

Answer 39

Free radical substitution