Lesson video

Hello, I am Mrs. Adcock, and welcome to today's lesson.

Today's lesson is on fractional distillation of crude oil.

We are going to be looking at what is crude oil and how can we use fractional distillation to separate crude oil into useful fractions.

Today's lesson outcome is, I can describe how crude oil can be separated into fractions using fractional distillation, and describe the differences in properties between fractions.

Some of the key words we will be using in today's lesson include crude oil, hydrocarbon, viscosity, fractional distillation, and fraction.

Here you can see each of those keywords written in a sentence.

It would be a good idea to pause the video now and read through those sentences.

You might like to make some notes so that you can refer back to them later in the lesson if needed.

Today's lesson on fractional distillation of crude oil is split into three main parts.

First of all, we're going to look at crude oil, then we're going to move on to look at some of the properties of hydrocarbons, and finally, we're going to finish today's lesson by looking at fractional distillation.

Let's get started on the first part of our lesson on crude oil.

Crude oil, natural gas, and coal are all fossil fuels.

And they are finite resources, that means that they cannot be regenerated or replaced in our lifetime.

They are also in limited quantities.

Here we can see an image of crude oil.

Crude oil is formed from dead plankton over millions of years.

Time for a question.

Which statements about crude oil are true? A, crude oil is formed from dead plankton, b, crude oil takes hundreds of years to form, c, crude oil is a renewable resource, or d, crude oil is a finite resource.

Choose any answers that you think are true.

The correct answers are crude oil is formed from dead plankton and crude oil is a finite resource.

B is incorrect because crude oil takes millions of years to form, not hundreds of years.

And C is incorrect because crude oil is a finite resource, not a renewable resource.

A renewable resource is one that is not used up or can be replaced in our lifetime.

How is crude oil formed? Dead plankton settled at the bottom of the oceans, and the plankton covered in layers of mud and other sediment.

High pressure from the sediments and rocks above, along with high temperatures from Earth, turned the dead plankton into crude oil.

Here we can see in the image we've got the ocean floor, we've got the dead plankton, and this is covered in sediments and mud.

We've got the water above, and the plankton, which was covered in layers of mud and other sediment, experienced high pressure and high temperature and over millions of years turned into crude oil.

The organic matter that formed crude oil decomposes into hydrocarbons.

Hydrocarbons are molecules that contain hydrogen and carbon atoms only.

Crude oil is a mixture of different hydrocarbons and some impurities.

Alkanes are the most commonly found hydrocarbon in crude oil.

Alkanes are a homologous series of compounds that contain single bonds between the carbon atoms. Here is the simplest alkane called methane.

We can see methane has one carbon atom and it has the molecular formula CH4.

Each carbon atom can form four covalent bonds, so we can see there are four covalent bonds around that carbon atom, and to each of those, a hydrogen atom is attached.

Time for a check for understanding.

Crude oil is a pure substance of the alkane methane.

Is that statement true or false? That statement is false.

Can you justify your answer? Why is that statement false? A, crude oil is a mixture of different hydrocarbons that all belong to the alkane homologous series, or b, crude oil is a mixture of different hydrocarbons and impurities.

The correct answer is b, crude oil is a mixture of different hydrocarbons and impurities.

We mentioned alkanes because the majority of these hydrocarbons are alkanes and methane is the simplest alkane.

The first four alkanes in the alkane homologous series are, methane, methane's got one carbon atom.

Then we have ethane that has two carbon atoms. You can see the molecular formula of ethane is C2H6.

After ethane, we have propane.

Propane has three carbon atoms. We can see the molecular formula is C3H8, and around each of those carbon atoms there are four covalent bonds.

After propane, we have butane.

And you can see in that molecule of butane we've got four carbon atoms and then we have hydrogens surrounding those.

And butane has the molecular formula C4H10.

So the first four alkanes in the alkane homologous series are methane, ethane, propane, and butane.

The general formula of alkanes is CnH2n+2, where n represents any whole number.

So if, for example, in butane, n is four, then if we wanted to work out how many hydrogen atoms we would have, we do two times n, that's two times four is eight, plus two, and that would be 10.

We can use that general formula to work out the molecular formula for any alkane if we know the number of carbon atoms. Time for another question.

What is the general formula of alkanes? Is it a, CnH2n, b, CnH2n+1, or c, CnH2n+2? The correct answer is c and it was CnH2n+2.

That is the general formula of alkanes.

Well done if you got that question correct.

Time for our first practise task of today's lesson, and for this task, you need to complete the table below.

You need to use the information you've been given on either the name of the alkane, the molecular formula, or the displayed formula to help you fill in the rest of that table.

When you are writing the molecular formula, it's good practise to do the carbon atoms first before you do your hydrogen atoms. And when you are drawing your displayed formula, remember to show all of the covalent bonds that are present in the molecule.

Pause the video now, have a go at answering this question, then come back when you're ready to go over the answers.

Here you can see the answers to this question.

If we start with methane, we can see methane has the molecular formula CH4, and then we have a molecule of methane there where we've got one carbon atom and that's surrounded by four hydrogen atoms, which are all bonded by covalent bonds.

In the next column, we've got ethane.

Ethan has two carbon atoms, so we knew from the molecular formula that this would be ethane.

And then we have our displayed formula showing the covalent bonds that are present in our molecule of ethane.

Next column is propane.

Propane has three carbon atoms, so that has the molecular formula C3H8.

If you remember that general formula for alkanes is CnH2n+2.

So if we know propane has three carbon atoms, we could do three times two, that's six, and add two and we get eight, so we know it's got eight hydrogen atoms. And below there we have the displayed formula for propane.

And in the final column, we were given the displayed formula of butane, and the molecular formula for butane is C4H10.

We have looked at crude oil and how crude oil is formed and learned that crude oil is a mixture of different hydrocarbons with some impurities.

Now we're going to move on to look at some of the properties of hydrocarbons.

When hydrocarbons combust in a plentiful supply of oxygen, so that will be complete combustion, the carbon and hydrogen atoms oxidise to produce carbon dioxide and water.

Here we can see an example of complete combustion of a hydrocarbon.

And we've used methane, so we've used an alkane.

This is the simplest alkane.

So when methane reacts in a plentiful supply of oxygen, the carbon and hydrogen atoms that are present in the methane react with oxygen atoms and we form the products carbon dioxide and water.

Here we can see a balanced symbol equation for that reaction, and you can see the carbon and hydrogen atoms present in the methane react with those oxygen atoms from the oxygen, and we form carbon dioxide and water.

During complete combustion, a large amount of energy is released.

For example, when a Bunsen burner is used, the combustion of methane gas releases significant amounts of heat and light energy into the surroundings.

Time for a question.

Which of the following are products from the complete combustion of propane? A, carbon dioxide, b, oxygen, c, water, d, hydrogen.

Propane is an alkane, so it is a hydrocarbon, and when our hydrocarbons combust in a plentiful supply of oxygen, we produce carbon dioxide and water.

So well done if you correctly identified those two products.

We have looked at how hydrocarbons can completely combust to produce carbon dioxide and water, now we're going to have a look at flammability.

As the molecular size of hydrocarbons increases, the hydrocarbons become less flammable, and flammability is the ease with which a substance will ignite.

For example, we have methane, then ethane, then propane, then butane, so we have the molecular size increasing down our list.

As the molecular size increases, flammability or that ease of ignition decreases.

Butane is a larger molecule than methane, and therefore butane is less flammable than methane.

Is this statement true or false? Butane is more flammable, so easier to ignite, than ethane.

Is that statement true or false? That statement is false.

Well done if you got that correct.

Can you now justify your answer? Is it a, as the molecular size of hydrocarbons increases, flammability increases, or b, as the molecular size of hydrocarbons increases, flammability decreases? The correct answer is b, as the molecular size of hydrocarbons increases, flammability decreases.

Well done if you got that correct.

Having looked at complete combustion of hydrocarbons and flammability, we're now going to look at viscosity.

As the molecular size of a hydrocarbon increases, the viscosity increases.

Viscosity is how easily a fluid flows, so the more viscous a fluid is, the less easily it flows.

Just to recap, as the hydrocarbon molecular size increases, viscosity increases, so the fluid flows less easily.

Here we can see that crude oil, which is a mixture of hydrocarbons, is highly viscous.

Time for a check for understanding.

As the molecular size of a hydrocarbon increases, the viscosity, a, decreases, b, stays the same, c, increases.

As the molecular size of a hydrocarbon increases, the viscosity also increases.

The longer that hydrocarbon chain, the less easily the fluid flows.

Let's have a look at how the molecular size affects boiling point.

The larger the molecular size of a hydrocarbon, the higher the boiling point.

Here we've got ethane, propane, butane, and then we've introduced another alkane here, which is pentane, and pentane has five carbon atoms. We can see the boiling point for each of these alkanes.

Ethane has a boiling point of negative 89 degrees Celsius.

Propane has a boiling point of negative 42 degrees Celsius.

Butane has a boiling point of negative one degree Celsius.

And pentane has a boiling point of 36 degrees Celsius.

We can see that as the molecular size of the hydrocarbons has increased, the boiling point has also increased.

Why is this? Well, this is because larger hydrocarbons are held together by more intermolecular forces, and therefore, they require more energy to overcome these intermolecular forces.

If we quickly recap, we have seen as the molecular size of the hydrocarbon increases, the flammability decreases, the viscosity increases, and the boiling point also increases.

Time for a question.

Which of the following alkanes has the highest boiling point? A, methane, b, propane, c, butane.

See if you can remember how many carbon atoms are present in each of those molecules and then use that information to help you answer the question.

The correct answer is c, butane.

Butane is the longest alkane out of those three options, and therefore butane will have more intermolecular forces, and therefore has a higher boiling point because it needs more energy to overcome those intermolecular forces.

Methane has one carbon atom, that has the lowest boiling point out of those three alkanes.

Propane has three carbon atoms and butane has four carbon atoms. Well done if you got that question correct.

Time for our second practise task of today's lesson.

Question one, the table below shows the boiling point of three alkanes.

You've been given the molecular formula for those three alkanes and you've been given the boiling point.

A, how do we know that the three substances in the table are alkanes? B, which alkane in the table is the most viscous? C, which alkane in the table is the most flammable? And d, explain the trend in boiling points of the alkanes.

For d, you're going to need to describe the trend and then give reasons for that trend.

Pause the video now, have a go at answering this question, then come back when you're ready to go over the answers.

Let's see how you got on.

Question 1-a, how do we know that the three substances in the table are alkanes? They are all hydrocarbons and they all have the general formula CnH2n+2.

That was a tricky question, so well done if you got that one correct.

1-b, which alkane in the table is the most viscous? C10H22 is the most viscous.

As the molecular size of the hydrocarbons increases, then so does viscosity.

Well done if you got that question correct.

For c, we needed to say which alkane in the table is the most flammable.

That would be C2H6.

That is the alkane in the table with the smallest molecular size, and the smaller the molecular size, the more flammable the alkane will be.

1-d, explain the trend in boiling points of the alkanes.

As the molecular size of the alkane increases, the boiling point increases, and this is because larger alkanes have more intermolecular forces, so more energy is required to overcome them.

Hopefully you got that question correct.

Well done for your hard work with those questions.

We have looked at crude oil and we've just looked at some of the properties of hydrocarbons, now we're going to move on to the final part of our lesson on fractional distillation.

Crude oil can be separated into more useful fractions by fractional distillation.

Here in the image we can see an oil distillery where we get the fractional distillation of crude oil.

Fractional distillation is really important because it separates those different hydrocarbons that we find in crude oil into fractions, and it separates them based on their boiling point.

Let's have a go at this question.

Fractional distillation is used to separate hydrocarbons based on which property? A, flammability, b, viscosity, or c, boiling point.

During fractional distillation, hydrocarbons are separated based on their boiling point.

Well done if you got that question correct, you're clearly listening well.

Fractional distillation involves heating crude oil until it vaporises.

The vaporised crude oil then enters a fractionating column, and we can see the fractionating column shown here.

The fractionating column is hottest at the bottom, the temperature decreases up the column, and it is cooler at the top.

The hydrocarbon vapours enter low down in the column and they rise up the column until they condense.

Hydrocarbons condense at different heights depending on their boiling point.

Where is a fractionating column hottest? Is it hottest at the bottom, in the middle, or at the top? A fractionating column is hottest at the bottom and then the temperature gradually decreases up the column.

Well done if you've got that question correct.

Smaller hydrocarbons with low boiling points do not condense, but they leave the top of the fractionating column as gases.

And then at the bottom we have our larger hydrocarbons with really high boiling points, and these are collected from the bottom of the fractionating column and some may not even have vaporised.

As we move up the fractionating column, the molecules that condense at each fraction have, a smaller molecular size, a lower boiling point, higher flammability, and lower viscosity.

Time for another question.

How will hydrocarbons collected from a fraction near the top of the fractionating column differ to those from a lower fraction? So think about those molecules that we are collecting from fractions near the top of the fractionating column.

Will they be, a, a larger molecular size, b, be less flammable, or c, have a lower boiling point? The correct answer is c, they will have a lower boiling point.

If we have a look at the other options, a is incorrect because those hydrocarbons collected from fractions near the top will be a smaller molecular size.

And b is incorrect because the smaller the molecular size, the more flammable those molecules will be.

Well done if you chose c and got that question correct.

Here we can see some of the fractions that we get from the fractionating column when we perform fractional distillation of crude oil.

At the top of the fractionating column, we collect our gases.

Then the next fraction down is petrol, then we get naphtha, then we get kerosene, diesel, a fraction of heavy fuel oil, and at the bottom we have a fraction of bitumen.

You will notice that each of these fractions has got in brackets the boiling point for that fraction.

And again, you will notice that these boiling points spread over a range, and that is because each fraction of crude oil is a mixture of hydrocarbons with a similar boiling point.

So those fractions are not pure, they contain a mixture of hydrocarbons that have a similar boiling point as seen in the image.

And the different fractions of crude oil have different uses.

If we start at the bottom, we've got bitumen, which is used for road surfaces and roofing.

Heavy fuel oil is used as fuel for large ships and some power stations.

Diesel oil is used as fuel in some cars, lorries, and trains.

Kerosene is used as fuel for aircraft.

Naphtha is used for making chemicals.

Petrol is used as a fuel in cars.

And gases, such as liquified petroleum gas, is used for domestic heating and cooking.

Which of the following fractions can be used as fuel? A, bitumen, b, petrol, c, kerosene, d, diesel.

The correct answer is b, petrol, c, kerosene, and d, diesel.

So each of those fractions can be used as fuels.

Bitumen we use for roofing and road surfaces.

Well done if you identified petrol, kerosene, and diesel, all as fractions that we use as fuels.

Time for our final practise task of today's lesson, and you need to describe how crude oil is separated by fractional distillation.

And we want lots of detail in this answer, so make sure you include the following key terms in your answer, vapour, fractionating column, and boiling point.

Pause the video now, have a go at answering this question, and then when you come back, we'll go over the answer.

Let's see how you got on.

We want to describe how crude oil is separated by fractional distillation.

The crude oil is heated to vaporise it and then the crude oil vapours enter the fractionating column.

And the column is hottest at the bottom with the temperature decreasing up the column.

The vapours rise up the column and they condense to form liquids at different points based on their boiling points.

Larger hydrocarbons have higher boiling points, so they collect near the bottom of the fractionating column.

Hopefully you've included all the key details in your answer.

You may have answered some of this the other way round and talked about smaller hydrocarbons with lower boiling points collecting near the top of the column, for example.

If you've missed any of the key points, it would be a good idea to just pause the video now and add some of those key details to your answer.

We've reached the end of today's lesson on fractional distillation of crude oil.

You've worked really hard throughout the lesson, so well done.

Let's just summarise some of the key points that we've covered in today's lesson.

Crude oil is a non-renewable mixture of different hydrocarbons and impurities.

Crude oil can be separated into groups of hydrocarbon molecules of similar size called fractions.

Most of the hydrocarbon molecules in crude oil are alkanes.

Smaller hydrocarbon molecules have lower boiling points, are more flammable, and are less viscous.

And fractions of crude oil include petroleum gases, petrol, kerosene, diesel oil, heavy fuel oil, and bitumen.

Well done again for all your hard work in the lesson.

I hope you've enjoyed the lesson and I hope you're able to join me for another lesson soon.