3.3.2 Alkanes

Question 1

What are alkanes?

Answer 1

• Homologous series of saturated hydrocarbons with general formula of C2H2n+1

Question 2

Are alkanes reactive?

Answer 2

Alkanes are very unreactive but react with halogens

Question 3

What does saturated mean?

Answer 3

Saturated is a molecule containing no double bonds

Question 4

What does hydrocarbon mean?

Answer 4

Hydrocarbon is a molecule containing hydrogen and carbon only

Question 5

Why are alkanes saturated hydrocarbons?

Answer 5

Alkanes contain no double bonds, only single bonds and contains only hydrogen and carbon atoms

Question 6

How does crude oil and natural gas usually form?

Answer 6

Formed by the slow decay of marine animals and plants under heat and pressure in the absence of air over millions of years

Question 7

What does crude oil/ petroleum usually consist of?

Answer 7

Petroleum is a mixture consisting mainly of alkane

hydrocarbons

Question 8

Why is crude oil separated using fractional distillation?

Answer 8

• Crude oil no use in raw form

- Useful products need separating

Question 9

How does fractional distillation work?

Answer 9

Compounds have different boiling points which is used to separate each component

Question 10

What are C-H bonds?

Answer 10

They are virtually non-polar therefore there are only Van der Waals forces between the molecules

Question 11

Explain the processes of fractional distillation?

Answer 11

• Crude oil is vaporised
• Vapour is passed into a tower which is hot at the bottom and cold at the top
• As the vapour rises, it cools
• Molecules condense at different heights as they have different boiling points
• The larger the molecule, the lower down the tower it condenses
• Residue from primary distillation contains useful substances that boil above 350’C at atmospheric pressure

Question 12

Why does residue need to be separated at a lower pressure (vacuum distillation)?

Answer 12

• Residue would decompose at temperatures higher than 350’C

Question 13

What are compounds produced at 25’C during fractional distillation called and what are examples of it?

Answer 13

Fuel gases

• Calor gas
• LPG

Question 14

What are compounds produced at 40’C during fractional distillation called and what are examples of it?

Answer 14

Gasoline

- Petrol for cars

Question 15

What are compounds produced at 110’C during fractional distillation called and what are examples of it?

Answer 15

Naphtha

- Petrochemicals

Question 16

What are compounds produced at 200’C during fractional distillation called and what are examples of it?

Answer 16

Kerosene

- Jet fuel

Question 17

What are compounds produced at 300’C during fractional distillation called and what are examples of it?

Answer 17

Diesel

- Diesel engine fuels

Question 18

What does residue contain?

Answer 18

Useful substances such as:

• Fuel oil
• Bitumen
• Lubricating oil

Question 19

As the C chain gets bigger of alkanes, what happens to the consistency?

Answer 19

• The bigger the C chain, the hydrocarbons become more viscous

Question 20

As the C chain gets bigger of alkanes, what happens to the ease of ignition?

Answer 20

• The bigger the C chain, the hydrocarbons become harder to ignite

Question 21

As the C chain gets bigger of alkanes, what happens to the evaporation?

Answer 21

• The bigger the C chain, the hydrocarbons become more volatile

Question 22

As the C chain gets bigger of alkanes, what happens to the boiling point?

Answer 22

• The bigger the C chain, the hydrocarbons have bigger boiling points

Question 23

What are boiling points?

Answer 23

Boiling points measure the strength of intermolecular forces between molecules

Question 24

What does a high boiling point mean?

Answer 24

Higher boiling points mean stronger intermolecular forces

Question 25

What are Van der Waals?

Answer 25

Electron movement in a molecule causes a temporary dipole resulting in an attractive force between molecules

Question 26

Why do straight chain molecules have higher boiling/melting points?

Answer 26

• More neatly arranged/ packed closer together therefore more surface contact
• Stronger Van der Waals forces

Question 27

Why do single branch alkanes have a lower boiling/melting points?

Answer 27

• Less neatly packed therefore weaker Van der Waals