Section 7 - Alkanes

Question 1

What are alkanes?

Answer 1

Alkanes are hydrocarbons - they only contain hydrogen and carbon atoms

Question 2

Describe the two types of bonds that alkanes contain

Answer 2

• All of the carbon-carbon bonds are single covalent bonds.
• All the other bonds are carbon-hydrogen covalent bonds (which are always single)

Question 3

In alkanes all the available bonds have been formed, therefore they are ____________ molecules.

Answer 3

saturated

Question 4

What are the first four straight-chain alkanes?

Answer 4

Methane, Ethane, Propane, Butane

Question 5

3D alkane molecules are not rigid. What does this mean?

Answer 5

There is free rotation around a carbon-carbon single bond. This means that the carbon chains are quite flexible and gives the molecules the ability to change shape, particularly as the chain length increases.

Question 6

The bonds in alkanes are very strong and it requires a lot of energy to break them. How can this be used to explain some of their properties? (3)

Answer 6

1) They are very unreactive.
2) They are not able to form polymers.
3) They burn cleanly, tending to undergo complete combustion to form carbon dioxide and water. The flame is usually a faint blue colour.

Question 7

The attractive forces between molecules get stronger as the chain length increases. How can this be used to explain some of alkanes’ properties?

Answer 7

1) Boiling point increases as the length of the carbon chain increases
2) Viscosity (resistance to flow) increases as chain length increases.
3) Volatility (ease of evaporation) decreases as chain length increases.