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Understand Fractional Distillation

Worksheet Overview

QUESTION 1 of 10

Ever wanted to take a bath in oil? Well, in Azerbaijan oil is so important to their economy that you literally could take a bath in it! Crude oil is one of the most important natural resources, making everything from medicine to the petrol you put in your car. It is finite, which means it will eventually run out, and non-renewable as it cannot be made again. It is a fossil fuel, like coal and natural gas. 

Fossil fuels were made over millions of years from the remains of dead organisms that were compressed together while trapped in the Earth. Dead animals made oil and dead plants made coal.

 


Crude oil is a mixture of hydrocarbons (substances made from carbon and hydrogen only). In order to separate it 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. .

 

 

 

You need to know this diagram and be able to talk about how the properties of the hydrocarbons change over the different fractions.


The component hydrocarbon molecules are long chains held together by intermolecular forces. 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. One example is petrol.

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


As the heat in the fractional distillation tower increases, small molecules evaporate first and rise to the top, while heavier ones stay at the bottom.

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

 

Have another look at the diagram above until you are sure you understand it, and then move on to give the questions a try.

How can crude oil be best described?

Finite

Renewable

Non-renewable

Hydroelectric

What type of fuel is crude oil?

Engine fuel

Fossil fuel

Crude fuel

Is the following statement true or false?

 

Oil is made from dead plants.

True

False

Select the elements that make hydrocarbons.

Hydrogen

Oxygen

Nitrogen

Carbon

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

What happens to the boiling point of hydrocarbons as you increase the chain length?

Increase

Decrease

Stay the same

Is the following statement true or false?

 

 

Hydrocarbons with longer chains have higher viscosity.

True

False

Why do longer hydrocarbon chains have higher boiling points?

They need less energy to break

They need more energy to break

They do not need any energy to break

Do fuels generally have long or short chains? 

Long

Short

  • Question 1

How can crude oil be best described?

CORRECT ANSWER
Finite
Non-renewable
EDDIE SAYS
How was this first question? The two words that were needed here were mentioned in the Introduction, so remember that you can always go back and have another look if you're not feeling too confident about these questions. Crude oil is finite, which means it will run out eventually, and non-renewable, which means it cannot be renewed - once it's gone, it's gone!
  • Question 2

What type of fuel is crude oil?

CORRECT ANSWER
Fossil fuel
EDDIE SAYS
A bit easier this one, didn't you think? Oil is a fossil fuel, as are coal and gas. It is a mixture of lots of different types of fuels and needs to be separated out before it can be burnt. Crude oil itself doesn't burn that well at all.
  • Question 3

Is the following statement true or false?

 

Oil is made from dead plants.

CORRECT ANSWER
False
EDDIE SAYS
Did you fall into the trap and get the wrong answer?! The statement was half right - it is made from dead organisms but it is not dead plants. Oil is made from dead animals, such as tiny shelled sea creatures, while coal is made from the remains of dead plants. Why don't you do a bit of research into how fossils were made if this is something you're interested in.
  • Question 4

Select the elements that make hydrocarbons.

CORRECT ANSWER
Hydrogen
Carbon
EDDIE SAYS
Hydrocarbons are made of only hydrogen and carbon - hence the name. We are good at naming stuff in science.
  • Question 5

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. We are distilling the oil into different fractions. Again, amazing at naming stuff in science... At least it helps us to remember it!
  • Question 6

What type of forces hold hydrocarbon molecules together?

CORRECT ANSWER
Intermolecular
EDDIE SAYS
Tricky words - it was hard to distinguish between the second and third options, wasn't it? Intermolecular forces hold hydrocarbon molecules together - the more intermolecular forces there are, the harder it is for the chains to move or the molecules to boil.
  • Question 7

What happens to the boiling point of hydrocarbons as you increase the chain length?

CORRECT ANSWER
Increase
EDDIE SAYS
Remember that useful chart in the Introduction if you're getting stuck with any of this. Longer chains mean a higher boiling point - so if you increase the chain length, you also increase the boiling point.
  • Question 8

Is the following statement true or false?

 

 

Hydrocarbons with longer chains have higher viscosity.

CORRECT ANSWER
True
EDDIE SAYS
Now this really is a tricky word! Viscosity is how something flows: the easier it flows, the less viscous it is - so water is not viscous but melted toffee is! With the different fractions of crude oil, the shorter the chain the easier it is for the chains to move over each other - the lower the viscosity. Longer chains mean that it is harder for them to move - they have a greater viscosity. So the statement was true.
  • Question 9

Why do longer hydrocarbon chains have higher boiling points?

CORRECT ANSWER
They need more energy to break
EDDIE SAYS
Longer hydrocarbon chains have higher boiling points because they need more energy to separate them. Those with shorter chains can separate more easily and without requiring so much energy.
  • Question 10

Do fuels generally have long or short chains? 

CORRECT ANSWER
Short
EDDIE SAYS
You could have used the diagram to help you with this one or maybe it was just a case of common sense. Fuels need to be volatile, which means that they can easily vapourise and become gas. So, they need to come off the top of the fractional distillation column - this means that they have short chains. That's the end of this set of questions. Do you feel more on top of this topic now? If you're still finding some of it a bit tricky, you can always reread the Introduction and have another go at the activity.
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