How the physical properties of hydrocarbons change with molecule size

4 marks · Normal

Hydrocarbon fractions are collected at different heights in a fractionating column, depending on their boiling points.

(a)

State what is meant by the volatility of a liquid.

[1 mark]

(b)

Explain why the fraction collected at the top of the column is more volatile than the one at the bottom.

[2 marks]

(c)

Give one use each for the top fraction and the bottom fraction.

[1 mark]

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(a) Volatility is how easily a liquid evaporates (turns into a gas).

(b) The top fraction contains smaller molecules with weaker intermolecular forces, so they evaporate easily and are more volatile; the bottom fraction contains large molecules with strong forces which are less volatile.

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Marking points

B1 (a) · 1 mark · Defines volatility as how easily a liquid evaporates (or turns to gas).

M1 (b) · 1 mark · Links volatility to smaller/larger molecules and intermolecular forces.

A1 (b) · 1 mark · Correctly explains that the top fraction has smaller molecules and is more volatile because it evaporates more easily.

B2 (c) · 1 mark · States a correct use for both the top and bottom fractions.

Worked solution

a) Volatility is how easily a liquid evaporates (becomes a gas).

b) The fraction collected at the top of the column is more volatile because it contains smaller molecules with weaker forces, so it evaporates more easily. The bottom fraction has large molecules with strong intermolecular forces, so it is less volatile and harder to evaporate.

c) Top fraction: used as bottled gas. Bottom fraction: used for road surfacing (bitumen).