Alkanes

Question 1

What are the key properties of Alkanes?

Answer 1

• Alkanes are saturated Hydrocarbons (all have single C-C bond).
• Have Cn H2n+2 as general formula.
• Each carbon atom has 4 other bonds.

Question 2

What shape are Alkane molecules around each Carbon?

Answer 2

Tetrahedral. Each Carbon atom has four pairs of bonding electrons around it, so has a bond angle of 109.5°.

Question 3

What bond exists between the molecules in a Alkane?

Answer 3

Covalent Bonds.

Question 4

What forces occur between Alkane chains, holding them together?

Answer 4

Induced Dipole-Dipole interactions (London Dispersion Forces).

Question 5

What happens to the forces between Alkane chains as they get longer?

Answer 5

The induced Dipole-Dipole interactions get stronger due to a greater surface area and therefore more electrons available to interact.

Question 6

What happens to the boiling point as the molecule gets longer?

Answer 6

It increases as more energy is needed to overcome the increase in Dipole-Dipole interactions.

Question 7

Does a branched Alkane have a lower or higher melting point than a straight chain alkane?

Answer 7

The boiling point is lower as the molecules cant pack as closely together and so have smaller surface areas, so the induced Dipole-Dipole interactions are reduced.

Question 8

What state does combustion happen in?

Answer 8

Combustion happens between gasses. So liquid Alkanes must be vaporised first. Also, smaller Alkanes are more volatile (turn into gases) so are easier to burn.

Question 9

What are the products and characteristics of flame for complete combustion of an Alkane.

Answer 9

Alkanes form CO₂ and H₂O when combusted fully in Oxygen with a clean blue flame and no soot. This gives maximum energy transfer.

Example Reaction: C₃H₈ + 5O₂ = 3CO₂ + 4H₂O.

Question 10

Do longer Alkanes release much energy?

Answer 10

Yes. They releases more energy per mole as more bonds react. Alkanes make excellent fuels as they release so much energy.

Question 11

What are the products and characteristics of flame for incomplete combustion of an Alkane.

Answer 11

Alkanes form CO (Carbon Monoxide) and H₂O when incompletely combusted. Has a yellow flame and emits soot.

Example Reaction: 2CH₄ + 3O₂ = 2CO + 4H₂O.

Question 12

Why is Carbon Monoxide so dangerous?

Answer 12

• Haemoglobin carries blood around the body.
• CO bonds to Haemoglobin in the blood better than Oxygen can so binds before Oxygen can.
• Therefore less oxygen is carried around the body, leading to oxygen deprivation.