A student collects four samples, labelled Fraction P, Q, R and S, from different heights of a laboratory fractionating column used to separate a sample of petroleum. The table shows data recorded for each fraction, listed in a random order. Fraction Average number of carbon atom…

Answer: ((a)) Fraction Q. It has the lowest boiling point range (20–100 °C) and shortest average chain length, so it is the most volatile fraction and rises furthest up the column before condensing. ((b)) Q, then S, then R, then P (5–8, then 9–14, then 15–20, then 25–40 carbon atoms). ((c)) Fraction R's molecules are much larger than fraction Q's. Larger molecules have a greater surface area of contact with neighbouring molecules, so the intermolecular (van der Waals) forces between them are stronger. These stronger forces make it harder for the molecules to slide… ((d)) Although petroleum is a mixture, the different hydrocarbon molecules it contains still have boiling points that depend on their chain length. As the vapour rises and cools in the column, hydrocarbons with similar chain lengths (and therefore similar boiling points) condense toge…. Part (a): Which fraction is collected nearer the top? The top of a fractionating column is cooler than the bottom, so only the most volatile hydrocarbons (those with the lowest boiling points) reach the top before condensing. Comparing P a…

IGCSE Chemistry 0620: Fractional Distillation of Petroleum Data Table

Fraction	Average number of carbon atoms per molecule	Boiling point range / °C	Ease of flow
P	25–40	300–400	does not flow; solid at room temperature
Q	5–8	20–100	flows very easily
R	15–20	250–300	flows slowly
S	9–14	150–250	flows fairly easily

Fraction

Average number of carbon atoms per molecule

Boiling point range / °C

Ease of flow

25–40

300–400

does not flow; solid at room temperature

5–8

20–100

flows very easily

15–20

250–300

flows slowly

9–14

150–250

flows fairly easily

Part (a): Which fraction is collected nearer the top?

The top of a fractionating column is cooler than the bottom, so only the most volatile hydrocarbons (those with the lowest boiling points) reach the top before condensing.

Comparing P and Q in the table:

Fraction P: boiling range 300–400 °C, 25–40 carbon atoms, does not flow at room temperature.
Fraction Q: boiling range 20–100 °C, 5–8 carbon atoms, flows very easily.

Fraction Q has the much lower boiling point range and the shorter chain length, so it is the more volatile fraction. It travels further up the column before condensing, so fraction Q is collected closer to the top.

Part (b): Ordering by chain length

Reading the “average number of carbon atoms” column for each fraction:

Q: 5–8
S: 9–14
R: 15–20
P: 25–40

In order of increasing average chain length: Q, then S, then R, then P.

Part (c): Explaining the viscosity difference

Fraction R’s molecules (15–20 carbon atoms) are considerably longer than fraction Q’s molecules (5–8 carbon atoms).

Longer hydrocarbon molecules have a larger surface area over which they can interact with neighbouring molecules, so the intermolecular (van der Waals) forces of attraction between fraction R’s molecules are stronger than those between fraction Q’s molecules.

Stronger intermolecular forces mean the molecules in fraction R resist sliding past one another, so fraction R flows more slowly. It is more viscous than fraction Q.

Part (d): Why fractional distillation still works on a mixture

Petroleum contains many different hydrocarbon molecules, but each individual hydrocarbon still has a boiling point linked to its chain length (longer chains, higher boiling points).

As petroleum vapour rises through the column, the temperature falls with height. Hydrocarbons whose boiling points match the temperature at a given height condense there and are drawn off together as a fraction. Since chain length (and hence boiling point) varies continuously through the mixture, fractional distillation groups the hydrocarbons into fractions covering similar, overlapping ranges of chain length and boiling point. It does not need to separate the mixture into single pure compounds to work.

Final answers

(a) Fraction Q. It has the lowest boiling point range and shortest chain length, so it is most volatile.
(b) Q, S, R, P (increasing chain length).
(c) Fraction R’s larger molecules have stronger intermolecular forces, so they flow less easily than fraction Q’s smaller molecules.
(d) Each fraction collects hydrocarbons with a similar range of chain length and boiling point, so the mixture separates by height in the column even though no fraction is a single pure compound.