Alkanes 3.3.2

Question 1

Physical properties of alkanes

Answer 1

Non-polar so intermolecular forces are weak van Der Waals forces
The greater the chain length or greater the Mr, the greater the VDW forces because there are more electrons

Boiling point increases with chain length (even if they have the same Mr)

Are insoluble in water because water molecules are held together by strong H-bonds which are stronger than VDWs. So they cannot overcome the forces that hold water molecules together
But soluble in non-polar solvents

Question 2

Fractional distillation process

Answer 2

1. Crude oil heated in a furnace
2. A mixture of liquid and vapour passes into a tower that decreases in temperature with height
3. Vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is at their boiling point. They condense into a liquid
4. The sorter chain hydrocarbons condense in the trays nearer to the top of the tower, where it is cooler because they have lower boiling points
5. The mixture of liquids that condenses on each tray is piped off
6. The thick residue that collects at the base of the tower is called tar or bitumen

Question 3

What forces are involved in fractional distillation

Answer 3

VDW forces between molecules break during vaporisation and reform on condensing