3.2 Alkanes

Question 1

What is an alkane ?

Answer 1

A saturated hydrocarbon containing only carbon-hydrogen bonds.

Question 2

What is the general formula of an alkane?

Answer 2

CnH2n+2.

Question 3

Do alkenes have polar bonds ? Why / why not ?

Answer 3

They don’t, this is because they have similar electronegativities.

Question 4

What intermolecular forces do alkanes have ? Why ?

Answer 4

They only have van der waals, they are non polar.

Question 5

Are alkanes soluble in water ?

Answer 5

They are insoluble because the 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 and reactions with halogens.

Question 8

What is crude oil? Is it renewable?

Answer 8

•It is a mixture of fractions (hydrocarbons with different boiling points)
•They are formed from biological matter at high temperatures of pressure over millions of years - unrenewable.

Question 9

What is fractional distillation and how does it work with crude oil ?

Answer 9

•Crude oil is heated until mostly vaporised
•It is then passed into a fractionating column (cooler at top)
•Vapours rise up the column and condense when temperature < their bp
•Shorter chains will condense at the top as their bp is lowest.

Question 10

Why are alkanes cracked ?

Answer 10

Long chain alkenes can be cracked into a shorter chain alkane and shorter chain alkene, which are both more valuable.

Question 11

What are the conditions for thermal cracking ?

Answer 11

700-1200K
Up to 7000kPa.

Question 12

What is the intermediate for the thermal cracking ?

Answer 12

Free radicals.

Question 13

What are the main products of thermal cracking ?

Answer 13

Shorter chain alkenes.

Question 14

What are the conditions for catalytic cracking?

Answer 14

Lower temp (720K)
Lower pressure but higher than atmospheric
Zeolite catalyst with a honeycomb structure for high SA.

Question 15

What are the main products of catalytic cracking ?

Answer 15

Cycloalkanes, aromatic hydrocarbons and branched alkanes.

Question 16

Write an equation for the complete combustion of propane

Answer 16

C3H8 + 5O2 -> 3CO2 + 4H2O

Question 17

What is a fuel ?

Answer 17

Something that releases heat energy when combusted.

Question 18

What are the 5 main alkane fuels ?

Answer 18

Methane, butane, propane, diesel (~C8), paraffin (C10-18).

Question 19

What are the products for the incomplete combustion of alkanes ? What are their harmful effects ?

Answer 19

CO - poisonous
C - soot, carbon dimming.

Question 20

What type of hydrocarbons are more likely to undergo incomplete combustion ?

Answer 20

Longer chain alkanes.

Question 21

What is the environmental impacts of CO being released ?

Answer 21

It is toxic / poisonous.

Question 22

What is the environmental impact of nitrous oxides being released ?

Answer 22

They form nitric acid -> acid rain.

Question 23

What is the environmental impact of sulfur impurities being released ?

Answer 23

Form sulphuric acid -> acid rain.

Question 24

What is the environmental impact of C being released into the atmosphere ?

Answer 24

Asthma, cancer, global dimming.

Question 25

What is the environmental impact of CO2 being released into the atmosphere?

Answer 25

Greenhouse gas -> global warming.

Question 26

What are flue gasses ?

Answer 26

The waste gasses released by power stations - often contain sulfur impurities.

Question 27

What are catalytic converters made up of ?

Answer 27

Ceramic honeycomb coated with platinum, palladium or rhodium.

Question 28

What reactions do catalytic converters catalyse ?

Answer 28

2CO (g) +2NO (g) -> N2 (g) + 2CO2 (g) Hydrocarbons + NO -> N2 + CO2 + H2O

Question 29

What are greenhouse gasses ?

Answer 29

Gasses which trap IR, increasing the temp of the earths atmosphere.

Question 30

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

Answer 30

Greenhouse gasses trap infrared radiation in the atmosphere, atmosphere heats up -> global warming.

Question 31

Define carbon neutral activities

Answer 31

Activities that produce no net / overall CO2 emissions.

Question 32

What reaction produces halogenoalkanes from alkanes ?

Answer 32

Free radical substitution.

Question 33

What are the stages of free radical substitution ?

Answer 33

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

Question 34

What are the conditions needed to form free radicals ?

Answer 34

Presence of UV light.

Question 35

Write all stages for the free radical reaction of CH4 with Cl2

Answer 35

I •Cl2 -> 2Cl• P1 •CH4 + Cl• -> C•H3 +HCl P2 •C•H3 + Cl2 -> CClH3 + Cl• T • C•H3 + C•H3 -> C2H6.

Question 36

How is the ozone layer important for the environment ?

Answer 36

It protects the earth from exposure to too many harmful UV rays.

Question 37

How do CFCs break the ozone layer down ?

Answer 37

Free radical substitution.

Question 38

Write an overall equation for the decomposition of ozone into oxygen

Answer 38

2O3 -> 3O2.

Question 39

Write the free radical substitution equation to show how CFCs convert ozone into oxygen

Answer 39

Cl2 -> 2Cl• Cl• + O3 -> ClO• + O2 ClO• + O3 -> 2O2 + Cl•.