(U2) Alkanes

Question 1

What is a saturated hydrocarbon?

Answer 1

Contains no C=C or C≡C bonds

Question 2

What is a hydrocarbon?

Answer 2

Compounds only containing hydrogen and carbon atoms

Question 3

What is substitution?

Answer 3

Process of replacing 1 atom or group with a different atom or group

Question 4

What is homolytic fission?

Answer 4

Bond breaking in which 1 of the shared electrons goes to each atom

Question 5

What is heterolytic fission?

Answer 5

Bond breaking in which both electrons in the shared pair go to a single atom

Question 6

What is a free radical?

Answer 6

A species with an unpaired electron

Question 7

How do you name alkanes? (4)

Answer 7

The IUPAC naming system is used.

1. Find longest continuous chain of carbon atoms. This gives the name of the parent hydrocarbon
2. Identify the substituent groups attached to the parent hydrocarbon
3. Name each substituent and place these names in alphabetical order before the name of the parent hydrocarbon
4. Number the parent hydrocarbon chain in such a way as to use the smallest numbers for the carbons attached to the substituent groups. To each substituent prefix a number that denotes its position of attachment.

Question 8

What is the percentage of carbon dioxide in the atmosphere?

What was it previously and why did it change?

Answer 8

• 0.03%, increased to 0.04%
• largely due to combustion reactions
• the effect of this increase is believed to be climate change

Question 9

Why do longer chain alkanes have higher boiling points?

What about branched structural isomers?

Answer 9

• Greater Van der Waals forces between adjacent chains
• following the same trend, they have weaker Van der Waals forces than their unbranched isomers (less carbons on the main chain), therefore having a lower b.p.

Question 10

What is the skeletal formula of propane?

Answer 10

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Question 11

How many structural isomers do:

• methane
• ethane
• propane
• butane
• pentane
• hexane

Have?

Answer 11

• 1
• 1
• 1
• 2
• 3
• 5

Question 12

How does combustion of alkanes appear?

How can you tell how much carbon is present?

Answer 12

• a yellow flame
• the ‘smokier’ the flame (i.e. more soot) indicates more carbon

Question 13

What are the products of complete combustion?

Answer 13

• carbon dioxide
• water

Question 14

What are the products of incomplete combustion?

Answer 14

• carbon monoxide / soot
• water

Question 15

How does a catalytic converter reduce the environmental impact of combusting alkane fuels? (3)

Answer 15

• they are a honeycomb structure made of a thin layer of palladium, platinum or rhodium - cuts costs and creates large SA
• molecules bind to the surface in adsorption
• this allows reactions to occur, creating less harmful products e.g. CO converted to CO₂

Question 16

What pollutants can be produced during the combustion of impure alkane fuels? (4)

Answer 16

• sulphur oxides
• nitrogen oxides
• carbon or carbon monoxide
• unburned hydrocarbons

Question 17

Outline the:

1. chain initiation
2. chain propegation (x2)
3. chain termination (x3)

Stages of the free radical substitution reaction of methane by chlorine

Answer 17

• Cl₂ —> 2Cl^. (under UV light)

• CH₄ + Cl^. —> CH₃^. + HCl
• CH₃^. + Cl₂ —> CH₃Cl + Cl^.

3.

• CH₃^. + Cl^. —> CH₃Cl
• Cl^.+ Cl^. —> Cl₂
• CH₃^. + CH₃^. —> C₂H₆

Question 18

What is the general formula of alkanes?

Answer 18

C_nH_2n+2

Question 19

By which process are alkanes converted to alkenes?

Answer 19

Cracking