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Fractional Distillation

Worksheet Overview

QUESTION 1 of 10

When we get crude oil out of the ground, it isn't very useful. It's a sticky mess, a bit like treacle- but not as nice to eat.


Crude oil is a mixture of hydrocarbons (substances made of carbon and hydrogen only). Each different hydrocarbon is much more useful- and much more valuable- if we can separate them out. In order to separate crude oil into its components it needs to be heated through a process called fractional distillation. Here is a diagram that shows how crude oil is separated:

 

An image of a fractional distillation column. Long chains are formed at the bottom and short chains are formed at the top. As you go up, the products become less viscous and more volatile. .

Crude oil is pumped into the bottom of the fractionating column, and heated to about 350 °C. This is hot enough for most of the liquid oil to turn to gas. As the gas rises up the fractionating column, it cools. When it gets cool enough, some of the gas condenses back into liquid. The liquid is collected where it forms, and pumped out. Each part of the fractionating column collects a different fraction; part of the oil with a certain boiling temperature.

Each of these fractions (refinery gas, petrol, naptha and so on) is still a mixture, but each fraction only contains a few, very similar, types of hydrocarbon. The molecules in each fraction are similar enough to be useful, even when mixed together.

What are the different fractions like?

Refinery gas contains molecules like methane, ethane and propane; very small molecules with a very low boiling temperature. That means they remain gases, even at the colder top of the refinery. This is the structure of ethane:

Petrol contains molecules like hexane, octane and decane. These molecules are larger, so the boiling temperature is a bit higher. This is the structure of hexane:

 

Diesel contains even larger hydrocarbon molecules, with between 15 and 20 carbon atoms in the backbone. These have even higher boiling points, such as pentadecane (15 carbon atoms):

How does chain length affect the properties of the hydrocarbons?

Longer molecules have higher boiling points
The component hydrocarbon molecules are long chains held together by intermolecular forces (inter means between). The forces are broken while boiling.

Lighter molecules with shorter chains need less energy for the forces to be broken, so they are easily separated. They have low boiling points.

Heavy molecules, like the ones in bitumen, have longer chains and more energy is needed to separate them. They have high boiling points.

Shorter molecules flow more easily, longer molecules are more viscous.

The smaller chains are also able to flow over each other better, meaning that they will be less viscous (viscous means sicky, like treacle). The longer the chains, the harder it is for them to flow over each other making them more viscous.

There's a lot to think about when you are learning about fractional distillation. Make sure you know the names of the fractions, and what they are used for. The patterns will help you; small molecules leave the refinery at the top of the column, and large ones at the bottom. Those patterns link to what you know about intermolecular forces, and the structures of alkanes.

What is the name of the process used to separate crude oil into its fractions?

Fractioning

Fractional dissolving

Fractional distillation

What type of forces hold hydrocarbon molecules together?

Intercarbonary

Intermolecular

Intermolecule

Is the following statement true or false?

 

 

Hydrocarbons with longer chains have higher viscosity.

True

False

Pick a word to complete this sentence.

 

True

False

Why do longer chain hydrocarbons have higher boiling points?

They need less energy to break.

They need more energy to break.

They do not need any energy to break.

What kind of substance is crude oil?

Pure element

Pure compound

Mixture

In fractional distillation, we inject crude oil at the bottom of the column. What happens to the temperature of the oil as you go up the column?

It heats up

It cools down

It stays the same temperature

What state change happens as the hot oil rises?

melting

boiling

condensation

Sort these fractions of crude oil into order, from the ones collected at the top to the ones collected at the bottom.

Column A

Column B

1 (top)
kerosene
2
diesel oil
3
residue
4
naptha
5
petrol
6 (bottom)
refinery gas

Which of these phrases describes naphtha? Tick all the correct answers.

pure substance

hydrocarbon

gas at room temperature

mixture

fraction of crude oil

  • Question 1

What is the name of the process used to separate crude oil into its fractions?

CORRECT ANSWER
Fractional distillation
EDDIE SAYS
The process used to separate crude oil into its fractions is called fractional distillation. It's distillation, because we are boiling and then cooling a liquid mixture to separate it out. We are separating the mixed-up oil into parts, which is why we call it "fractional". (Science names are full of clues, if you know where to look).
  • Question 2

What type of forces hold hydrocarbon molecules together?

CORRECT ANSWER
Intermolecular
EDDIE SAYS
Intermolecular forces are the ones between hydrocarbon molecules; a bit like "international" is between nations. The more/stronger intermolecular forces there are, the harder it is for the chains to move or boil.
  • Question 3

Is the following statement true or false?

 

 

Hydrocarbons with longer chains have higher viscosity.

CORRECT ANSWER
False
EDDIE SAYS
Viscosity is how something flows - the shorter the chain the easier it is for the chains to move over each other - the lower the viscosity.
  • Question 4

Pick a word to complete this sentence.

 

CORRECT ANSWER
EDDIE SAYS
Longer chains mean stronger intermolecular forces - so if you increase the chain length, you also increase the boiling point.
  • Question 5

Why do longer chain hydrocarbons have higher boiling points?

CORRECT ANSWER
They need more energy to break.
EDDIE SAYS
Longer chain hydrocarbons have higher boiling points, because they need more energy to break.
  • Question 6

What kind of substance is crude oil?

CORRECT ANSWER
Mixture
EDDIE SAYS
Crude oil is a mixture, with lots of different types of hydrocarbon molecule in it. That\'s because it forms naturally, but it makes it less useful than we would like.
  • Question 7

In fractional distillation, we inject crude oil at the bottom of the column. What happens to the temperature of the oil as you go up the column?

CORRECT ANSWER
It cools down
EDDIE SAYS
The oil starts off hot; about 350 °C, which is hot enough to boil most of it into a gas. As it rises, the temperature cools down.
  • Question 8

What state change happens as the hot oil rises?

CORRECT ANSWER
condensation
EDDIE SAYS
Most of the oil starts as a hot gas, but turns to liquid as it cools; this is called condensation.
  • Question 9

Sort these fractions of crude oil into order, from the ones collected at the top to the ones collected at the bottom.

CORRECT ANSWER

Column A

Column B

1 (top)
refinery gas
2
petrol
3
naptha
4
kerosene
5
diesel oil
6 (bottom)
residue
EDDIE SAYS
Because this question is about names, there aren't really any short cuts to learning them. Keep testing yourself on this order, until you get it right every time. It will happen!
  • Question 10

Which of these phrases describes naphtha? Tick all the correct answers.

CORRECT ANSWER
hydrocarbon
mixture
fraction of crude oil
EDDIE SAYS
Naphtha is a mixture of hydrocarbons (between about 8 and 12 carbon atoms per molecule), which is one of the fractions of crude oil. Its boiling point is about 110 °C, so it can't be a gas at room temperature (it's a liquid; it freezes at about -30 °C).
---- OR ----

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