12. Alkanes

Question 1

What is the functional group of alkanes?

Answer 1

CnH2n+2

Question 2

Is alkane saturated or unsaturated?

Answer 2

Saturated

Question 3

Why is alkane saturated?

Answer 3

Because it only contains single bonds/sigma bonds.

Question 4

Which alkanes don’t have any isomers?

Answer 4

1. methane
2. ethane
3. propane

Question 5

Are alkanes polar or non-polar? Why?

Answer 5

Non-polar, because:
1. the electronegativity of hydrogen and carbon are very similar
2. the intermolecular forces between molecules are weak London forces

Question 6

How do the lengths of chain affect the boiling point of alkanes? Why?

Answer 6

As the chain is longer, the boiling point increases, because:
1. greater surface area of contact
2. stronger London forces

Question 7

How do branches affect the melting point of alkanes? Why

Answer 7

Alkanes with branches have lower melting points, because:
1. less surface area of contact
2. weaker London forces

Question 8

Are alkanes soluble or insoluble?

Answer 8

Insoluble

Question 9

Are alkanes reactive? Why?

Answer 9

No, because they have strong carbon-carbon bonds.

Question 10

What is the equation of incomplete combustion of alkanes?

Answer 10

Alkane + oxygen > carbon dioxide + water

Question 11

What is the equation of complete combustion of alkanes?

Answer 11

Alkane + oxygen > carbon monoxide + water

Question 12

What does the reaction of alkanes with halogens called?

Answer 12

Free radical substitution reaction

Question 13

What is required in order to start the reaction?

Answer 13

UV light

Question 14

What are the three stages in free radical substitution reaction?

Answer 14

1. initiation
2. propagation
3. termination

Question 15

What happens in initiation?

Answer 15

The covalent bond in a halogen molecule is broken by homolytic fission. This results in each halogen atom takes one electron from the pair.

Question 16

What is homolytic fission?

Answer 16

The evenly breaking of a covalent bond with one of the bonded electron going to each atom, forming two free radicals.

Question 17

What happens in propagation?

Answer 17

1. a halogen free radical takes a hydrogen atom from the alkane to form hydrogen halogen and a methyl free radical.
2. the methyl free radical is also reactive and reacts with a halogen molecule to form halogenoalkane and a new halogen free radical

Question 18

What happens in termination?

Answer 18

To remove free radicals:
1. two halogen free radicals react together to give a halogen molecule
2. two methyl free radicals react together to give an alkane
3. one halogen free radical reacts with one methyl free radical to give a halogenoalkane

Question 19

Why is free radical substitution reaction not a suitable way for making halogenoalkane?

Answer 19

Because a mixture of isomers will be formed which are difficult to separate.