3.3.2 Alkanes

Question 1

what is the general formula of an alkane?

Answer 1

CnH2n+2

Question 2

what is the general formula of cycloalkanes?

Answer 2

CnH2n

Question 3

What is an alkane?

Answer 3

saturated hydrocarbon containing C-H bonds only

Question 3

Are alkanes’ bonds polar? why/why not?

Answer 3

non-polar - carbon and hydrogen have similar electronegativities

Question 4

Which intermolecular forces do alkanes have? Why?

Answer 4

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

Question 5

Are alkanes soluble in water? Why?

Answer 5

No, hydrogen bonds in water are stronger than alkanes’ Van der Waals forces

Question 6

How reactive are alkanes?

Answer 6

Very unreactive

Question 7

Which reactions will alkanes undergo?

Answer 7

Combustion
Reaction with halogens

Question 8

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

Answer 8

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
Non renewable

Question 9

What is fractional distillation? How does it work?

Answer 9

Crude oil heated until it is vaporised
Passed into a fractioning tower that is cooler at the top than the bottom
Vapours rise up the column and - via trays and bubble caps - condense when temperature is lower than their boiling point
Shorter chain alkanes condense at the top - they have the lowest boiling points

Question 10

Why are alkanes cracked?

Answer 10

to turn a long chain alkane into shorter ones and an alkene.
Shorter chains are more useful

Question 11

What are the conditions for Thermal cracking?

Answer 11

700-1200K temp
Up to 7000kPa pressure

Question 12

What is the intermediate for the thermal cracking reaction?

Answer 12

Free radicals

Question 13

What are the main products of thermal cracking?

Answer 13

Alkenes

Question 14

What are the conditions for catalytic cracking?

Answer 14

Lower temperature
Lower pressure
Zeolite catalyst with a honeycomb structured give a large SA

Question 15

What are the main products of catalytic cracking?

Answer 15

Cycloalkanes, aromatic hydrocarbons, branched alkanes

Question 16

Write an equation for the combustion of propane.

Answer 16

C3H8 + 5O2 —> 3CO2 + 4H2O

Question 17

What is a fuel?

Answer 17

Something which releases heat energy when combusted

Question 18

What are the five main fuels comprising of alkanes?

Answer 18

Methane, butane, propane, petrol, paraffin

Question 19

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

Answer 19

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

Question 20

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

Answer 20

Longer chains

Question 21

What is the environmental impact of carbon monoxide?

Answer 21

Toxic / poisonous

Question 22

What is the environmental impact of nitrogen oxides?

Answer 22

Form nitric acid —> acid rain, smog

Question 23

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

Answer 23

Form sulphuric acid —> acid rain

Question 24

What is the environmental impact of soot (carbon)?

Answer 24

Asthma, cancer, global dimming

Question 25

What is the environmental impact of unburnt hydrocarbons?

Answer 25

Smog

Question 26

What is the environmental impact of carbon dioxide?

Answer 26

Greenhouse gas —> global warming

Question 27

What is the environmental impact of water vapour?

Answer 27

Greenhouse gas —> global warming

Question 28

What are flue gases?

Answer 28

Gases given out by power stations

Question 29

What can we use to desulphurise flue gases?

Answer 29

CaO CaCO3

Question 30

What are catalytic converters made up of?

Answer 30

Ceramic honeycomb coated with platinum, palladium and rhodium metals

Question 31

What are greenhouse gases?

Answer 31

Gases which trap infrared radiation, making earth like a greenhouse

Question 32

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

Answer 32

Greenhouse gases trap infrared radiation in the atmosphere, atmosphere heats up

Question 33

Define carbon neutral activities?

Answer 33

Produce no net / overall carbon dioxide emissions

Question 34

How are halogenoalkanes formed from alkanes?

Answer 34

Free radical substitution reactions

Question 35

What are the three stages of free radical substitution?

Answer 35

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 36

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

Answer 36

Presence of UV light

Question 37

What is the ozone layer’s function?

Answer 37

Protects the earth from harmful exposure to too many UV rays

Question 38

How do CFCs break the ozone layer down?

Answer 38

Free radical substitution