Fractional Distillation

Question 1

What is a fraction?

Answer 1

a mixture of hydrocarbons with similar boiling points and numbers of carbon atoms

Question 2

What are alkanes?

Answer 2

most of the hydrocarbons from crude oil are alkanes

alkanes are a homologous series of compounds

Question 3

How do hydrocarbons in different fractions differ from each other?

Answer 3

a) number of carbon/hydrogen atoms (size)
b) boiling points
c) ease of ignition
d) viscosity
e) most are alkanes

Question 4

What is fractional distillation used for?

Answer 4

separating a mixture of liquids

separating crude oil into simpler/more useful mixtures/fractions

Question 5

What process is used to separate crude oil?

Answer 5

fractional distillation

Question 6

Why can crude oil be separated by fractional distillation?

Answer 6

because the different hydrocarbons have different boiling points

Question 7

What happens during fractional distillation?

Answer 7

• crude oil is heated to evaporate it
• the vapours rise up a fractionating column
• the column is hotter at the bottom and colder at the top, it has a temperature gradient
• the fractions condense at different points because they have different boiling points
• fractions with larger hydrocarbons exit first
• smaller hydrocarbons exit nearer the top
• this is because larger hydrocarbons have higher boiling points than smaller ones
• the fractions condense back to a liquid at different points when the column becomes cooler than their boiling points
• the fractions are piped out the column

Question 8

What happens to the gases fraction?

Answer 8

it does not condense and leaves the column at the top

Question 9

What happens to the bitumen fraction?

Answer 9

it does not evaporate and leaves at the bottom

Question 10

What order do the fractions leave in from top to bottom?

Answer 10

(petroleum) gases
petrol
kerosene
diesel
fuel oil
bitumen

Question 11

Why does petrol leave the column higher up than fuel oil?

Answer 11

it has a lower boiling point

so it takes longer before the column reaches a point that is cold enough for it to condense

Question 12

What difference in the number of C and H atoms is there in the column?

Answer 12

there are more/the hydrocarbons are bigger at the bottom and smaller at the top

Question 13

What difference in boiling point is there in the column?

Answer 13

the boiling point is lower at the top and higher at the bottom

Question 14

What difference in ease of ignition is there in the column?

Answer 14

the most flammable fractions are at the top, and the least flammable are at the bottom

Question 15

What difference in viscosity is there in the column?

Answer 15

the least viscous substances are at the top, and the most viscous are at the bottom

Question 16

What does viscosity mean?

Answer 16

how runny/thick a substance is

Question 17

Why does the boiling point increase as the number of carbon and hydrogen atoms increases?

Answer 17

• the strength of the intermolecular forces increases
• more energy is needed to overcome the forces
• this results in a higher boiling point
• the boiling point increases

Question 18

Crude oil is a complex mixture of hydrocarbons.
Most belong to the alkane homologous series.
Diesel oil is a fuel produced from crude oil.

Explain how diesel oil is separated from crude oil.

(6 marks)

Answer 18

Crude oil is separated into different fractions by fractional distillation.
The crude oil is heated to around 350°C to evaporate it.
Some of the hydrocarbons stay in the liquid state and leave at the bottom of the column.
The other hydrocarbons rise inside the fractionating column in the gas state.

(state clearly which separation method is involved, and outline what happens to the crude oil after heating it)

The fractionating column has a temperature gradient inside it.
The column becomes cooler towards the top, so the hydrocarbons cool down as they rise.
They condense to a liquid when they reach a part of the column that is cool enough.

(the temperature gradient is important, as well as the hydrocarbons condensing)

The gases do not condense and they leave at the top. Diesel oil leaves through a pipe as a liquid fraction when it condenses.

(could also explain that smaller molecules have weaker intermolecular forces and lower boiling points)

Question 19

What are gases used for?

Answer 19

domestic heating and cooking

Question 20

What is petrol used for?

Answer 20

fuel for cars

Question 21

What is kerosene used for?

Answer 21

fuel for aircraft

Question 22

What is diesel oil used for?

Answer 22

fuel for some cars and trains

Question 23

What is fuel oil used for?

Answer 23

fuel for large ships and some power stations

Question 24

What is bitumen used for?

Answer 24

surfacing roads and roofs

Question 25

Suggest two reasons to explain why bitumen is not suitable for use as a fuel

Answer 25

it is difficult to ignite/not very flammable

it has a high viscosity/is very viscous/does not flow easily

Question 26

Describe how crude oil is separated into fractions by fractional distillation

Answer 26

• oil is evaporated and passed into a column
• the column is hotter at the bottom and cooler at the top
• the different hydrocarbons rise up the column, cool, and condense at different heights