Lesson video

Hello.

I am Mrs. Adcock, and welcome to today's lesson on cracking fractions of crude oil.

We are going to be learning about how we can break down large fractions of crude oil into smaller, more useful molecules.

Today's lesson outcome is: I can describe the uses of different fractions of crude oil and describe what cracking is and how it is used to produce lighter fractions that are in greater demand.

Some of the key words that we will be using in today's lesson include cracking, thermal decomposition, catalyst, saturated and unsaturated.

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

Today's lesson on cracking fractions of crude oil is split into three main parts.

First of all, we are going to look at fractions of crude oil.

Then, we are going to move on to look at types of cracking.

And finally, we are going to finish the lesson by looking at products of cracking.

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

Crude oil is a finite resource.

That means that once it's been used, it cannot be regenerated or replaced in our lifetime.

And it forms over millions of years from dead plankton.

Here in the image, we can see some barrels of crude oil and we can see that crude oil is a black liquid.

Crude oil is a mixture of hydrocarbons.

Just a reminder that hydrocarbons are molecules that contain hydrogen and carbon atoms only.

So crude oil is a mixture of hydrocarbons, predominantly alkanes, along with some impurities.

Crude oil can be separated into different fractions by the process of fractional distillation.

Fractional distillation separates the different hydrocarbons based on their boiling point.

We can see some of the fractions that the crude oil has separated into.

If we start at the top of the fractionating column, we can see we collect some gases from the top, then the next fraction is petrol, then we've got naptha, then kerosene, then a fraction of diesel, a fraction of heavy fuel oil, and at the bottom, we can collect bitumen.

You will notice that each of these fractions has a range of boiling points and the hydrocarbons that are collected within those fractions will have similar boiling points.

Time for a question.

Which statements about crude oil are true? A: crude oil can be separated into fractions by simple distillation.

B: crude oil mainly consists of alkanes.

C: crude oil is separated into fractions based on the different viscosity of hydrocarbons.

Or D: crude oil is a renewable resource.

The statements that are true about crude oil are: B: crude oil mainly consists of alkanes.

Crude oil is a mixture of different hydrocarbons, which are mainly alkanes, and it also contains some impurities.

A is not correct because crude oil can be separated into fractions by fractional distillation, not simple distillation.

C is incorrect because crude oil is separated into fractions based on the different boiling points of the hydrocarbons.

And D is incorrect because crude oil is not a renewable resource.

It is a finite resource.

Well done if you got that question correct.

The table below shows some uses of fractions of crude oil.

So we can see we've got those different fractions and some of the uses of those fractions.

Liquid petroleum gases that are collected from the top of that fractionating column, they can be used for domestic heating and cooking.

Petrol, you might be familiar with.

Petrol is used as a fuel for cars.

Kerosene is also a fuel that's used as a fuel for aircraft.

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

And heavy fuel oil is used as a fuel for large ships and in some power stations.

Bitumen that is collected from the bottom of the fractionating column is used to surface roads and in roofing.

Time for another question.

Which fraction of crude oil is used to surface roads and roofs? A: kerosene, B: diesel oil, C: heavy fuel oil, or D: bitumen.

The correct answer is D: bitumen.

Well done if you got that question correct.

Smaller hydrocarbons are useful as fuels because they are more volatile, this means that they easily evaporate, they have lower boiling points, and they are more flammable, which means they are easier to ignite.

In the image here, we can see that petrol, which is a smaller alkane, is used as a fuel in cars.

There is often a higher demand for smaller hydrocarbons than the oil refineries can supply.

Meanwhile, oil refineries often have an excess supply of larger hydrocarbons that they struggle to sell.

So the smaller hydrocarbons that are useful as fuels are in a high demand and often the oil refineries have a excess of the larger hydrocarbons.

Oil refineries can overcome this supply and demand issue by using the process of cracking.

Cracking is a thermal decomposition reaction.

Thermal decomposition reactions are reactions that use heat to break down a reactant into two or more products.

So cracking is an example of a thermal decomposition reaction, and in the case of cracking, larger alkanes are broken down into more useful, smaller alkanes and alkenes.

For example, heavy fuel oil is a larger alkane, and we can see the molecular formula there of a hydrocarbon that we might find in this fraction.

This can undergo the process of cracking, which is where we break down this larger alkane into smaller molecules.

And we break it down into a smaller alkane, such as petrol, as well as additional shorter alkanes and alkenes.

Time for a question.

The process of breaking larger hydrocarbons into smaller hydrocarbons is known as: A; combustion, B: fractional distillation, or C: cracking? The correct answer is C: cracking.

So well done if you've got that question correct.

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

Question one.

The table below shows the supply and demand of fractions of crude oil.

We can see we've got three different fractions, petrol, kerosene, and heavy fuel oil.

We've got the carbon chain length for the molecules that we will find within these fractions, the relative percentage supply, and the relative percentage demand.

1A: state a use for each of the fractions in the table, and then B: the supply of petrol does not meet the demand.

How can oil refineries overcome this problem? Pause the video now.

Have a go at answering this question.

Then when you come back, we'll go over the answers.

Let's see how you got on.

Question 1A: state use for each of the fractions in the table.

Petrol is used as a fuel in cars, kerosene is used as a fuel in aircrafts, and heavy fuel oil is used as a fuel in large ships and in some power stations.

Hopefully you were able to remember the uses of each of those fractions.

Part B: the supply of petrol does not meet the demand.

How can oil refineries overcome this problem? Your answer may include that the oil refineries can crack, so that means to break down, larger molecules, such as heavy fuel oil, where the supply is greater than the demand into smaller molecules.

To help meet the demand of those smaller molecules, they can break or crack larger molecules into smaller molecules.

Well done if you remembered to talk about the process of cracking in your answer.

We've had a look at the fractions of crude oil and some of the uses of these different fractions.

Sometimes we can meet the demand for those smaller molecules by using the process of cracking.

Now we're going to move on to have a look at the types of cracking.

There are many methods for cracking, including steam cracking and catalytic cracking.

We can see an image there of some steam that's used in steam cracking and a zeolite catalyst that can be used in catalytic cracking.

Cracking is an example of a thermal decomposition reaction as it involves using heat to break down larger alkanes into smaller molecules.

Let's have a look at steam cracking first of all.

Steam cracking involves heating larger alkanes to vaporise them.

They mix these vapours with steam and heat to high temperatures of over 800 degrees Celsius.

Here we can see going into our furnace, we've got the larger alkanes and we've got steam and we are going to mix these together, heat them, vaporise the larger alkanes, and then the larger alkanes will crack into smaller, more useful molecules.

So we can see coming out of our furnace, we've got our cracked products.

Another type of cracking that we're going to learn about is catalytic cracking.

Catalytic cracking involves, first of all, heating larger alkanes to vaporise them, so this is similar to the process of steam cracking, but then, rather than mixing these vapours with steam, we are going to pass these vapours over a hot catalyst.

And just a reminder that a catalyst is a chemical that speeds up the rate of a reaction without being used up itself.

We can perform catalytic cracking in the laboratory using the following apparatus.

Here we have some ceramic wool soaked with our larger alkane that we are hoping to break down.

We've got some broken pottery that we can use as our catalyst.

We will heat the ceramic wool and that larger alkane will vaporise and then pass over our catalyst.

And when it does this, it will crack, so it will be broken down into smaller molecules.

These will pass through the delivery tube and up into our inverted test tube where we collect our cracked products.

And as these bubble up through the water, they will displace the water and we will collect our cracked products in the test tube.

Which of the statements about cracking is or are true? A: cracking is a thermal decomposition reaction.

B: two methods of cracking are steam and catalytic cracking.

C: catalytic cracking involves mixing alkane vapours with steam.

The statements that are true are cracking is a thermal decomposition reaction.

Cracking involves using heat to break down those larger molecules into smaller, more useful molecules.

And B is also correct.

There are two methods of cracking that we've learned about today.

They are steam cracking and catalytic cracking.

C is not correct because catalytic cracking, as its name suggests, involves passing those vapours over a hot catalyst.

It's steam cracking, and again, the clue's in the name, steam cracking involves mixing alkane vapours with steam.

Hopefully you chose answers A and B and got that question correct.

It's time for our second practise task of today's lesson, and for this task, you need to describe two ways in which kerosene can be broken down into smaller, more useful hydrocarbon molecules.

So have a think about what we've just learned and try and include as much detail as possible in your answer.

Pause the video now, answer this question, and then come back when you're ready to go over the answers.

Let's see how you got on.

Kerosene can be broken down into smaller, more useful alkanes by catalytic cracking or steam cracking.

Steam cracking involves heating to produce kerosene vapours, then mixing with steam and heating to temperatures over 800 degrees Celsius.

So hopefully you included lots of those details in your answer about steam cracking.

Catalytic cracking involves heating the kerosene to produce vapours, which are then passed over a hot catalyst.

Well done if you included lots of details about steam and catalytic cracking.

We have had a look at the fractions of crude oil and we've just learned about steam and catalytic cracking and how these can be used to break those larger alkanes into smaller, more useful products.

Now we are going to move on to have a look at some of those products of cracking.

Alkenes are a homologous series of compounds that contain the carbon to carbon double bond functional group.

Here are examples of alkenes containing that carbon to carbon double bond functional group.

We can see we've got ethene.

Ethene has two carbon atoms. Each carbon atom can form four covalent bonds.

In ethene, we have a double covalent bond between the two carbon atoms. Here we have a molecule of propene.

Propene contains three carbon atoms and it has a carbon to carbon double bond.

Here we can see that carbon to carbon double bond that is the functional group of alkenes and we can see it present here in ethene and propene.

What is the functional group present in alkenes? Is it A: carbon to carbon double bond, B: an OH group, or C: a COO group? The correct answer is carbon to carbon double bond is the functional group that's present in alkenes.

Well done if you got that question correct.

Alkenes are used as the starting material in the production of many chemical products, such as polymers.

Here are examples of plastic products made from alkenes.

So we have a polymer here called polyvinyl chloride, or PVC, and this is used for insulation.

And here we can see it used insulating those electrical wires.

So this polymer was made from alkenes.

Polypropene is another polymer made from alkenes and it's used in carpets.

And poly ethene, which we often shorten to polythene, is another polymer made from alkenes and this is used in plastic bags.

Cracking breaks down large alkane molecules into shorter alkanes and alkenes.

Here's an example of cracking.

We have a longer alkane molecule here, which has the molecular formula C10H22, and then we are going to crack this product.

So we are going to use heat to break it down into a shorter alkane and an alkene.

And we can see that our shorter alkane has the molecular formula C8H18 and our alkene has the molecular formula C2H4.

Hopefully you can see that our alkanes do not contain a carbon to carbon double bond.

They've all got single bonds, but our alkene has a carbon to carbon double bond.

Another thing that you will hopefully notice is that we haven't gained or lost any atoms here.

We had 10 carbon atoms in our alkane and we still have 10 carbon atoms in our products, and we have 22 hydrogen atoms in our longer alkane, and in our products, we've still got 22 hydrogen atoms. We've got 18 in the alkane and four in the alkene.

This is an example of cracking.

This longer alkane could have been cracked, it could've been broken, at any point, and in which case, you would've produced different products.

But in this example, we've produced an alkane with eight carbon atoms and an alkene with two carbon atoms. We are going to practise now writing balanced equations for cracking reactions.

So I will have a go first and then it'll be your turn to have a go.

Write a balanced equation for a cracking reaction.

An alkane with the molecular formula C14H30 is cracked to produce an alkane containing eight carbons and one other product.

First of all, identify the starting alkane to be cracked.

So in this case, we are starting with a molecule that was C14H30.

Then work out the formula of the alkane produced.

In our question, it told us that we were producing an alkane containing eight carbons.

Now the general formula for alkanes is CNH2N plus two.

So if our alkane contains eight carbon atoms, we can work out the number of hydrogen atoms. It'll be eight times two, that's 16, plus two.

So the molecular formula of our alkane that we produce, our shorter alkane, is C8H18.

Finally, we are going to work out the formula of the alkene produced by balancing the carbon and hydrogen atoms. When we perform cracking, a larger alkane is broken down into a shorter alkane and an alkene.

We were told in the question that we produced an alkane and one of the product, so that other product will be an alkene.

We can work out how many carbon atoms will be present in our alkene by looking at the number of carbon atoms present in our reactor.

In this case, we started with a alkane, which contained 14 carbon atoms. There are eight carbon atoms in the alkane in the products.

That means there are six carbon atoms, which must be present in our alkene.

We can do the same for the hydrogen atoms. There are 30 hydrogen atoms in the alkane that we cracked, 18 hydrogen atoms are in the alkane in the product, which means there are 12 hydrogen atoms that are unaccounted for, and these will be present in our alkene.

Your turn to have a go.

What you need to do is write a balanced equation for a cracking reaction.

You have an alkane with the molecular formula C11H24 and that is cracked to produce an alkane containing six carbons and one other product.

Pause the video now and have a go at answering this question.

And then when you come back, we'll go over the answer.

Let's go through the answer.

First of all, identify the starting alkane to be cracked.

In this question, the starting alkane had the molecular formula C11H24.

Then we work out the formula of the alkane produced.

We know it contained six carbon atoms. Using the general formula CNH2M plus two, we can work out the number of hydrogen atoms, so it will have the molecular formula C6H14.

And finally, work out the formula of the alkene produced by balancing the carbon and hydrogen atoms. If we look in our products, we now have a total of 11 carbon atoms and there's 11 carbon atoms in the reactant.

And in our products, we have a total of 24 hydrogen atoms and there was 24 hydrogen atoms in that starting alkane.

So the formula of that alkene will be C5H10.

At the bottom you can see our balanced equation for this cracking reaction.

Well done if you got that correct.

Alkenes are unsaturated, whereas alkanes are saturated molecules.

And an unsaturated molecule contains one or more double carbon to carbon covalent bonds.

Alkenes contain at least one carbon to carbon double bond, whereas alkanes, because they are saturated, they only contain single bonds between the carbon atoms. Here we can see an image of an unsaturated alkene.

This alkene has two carbon atoms, so this is ethene.

Here we have an image of a saturated alkane.

This alkane contains two carbon atoms. This is ethane.

It is saturated because it contains single bonds between the carbon atoms. Let's check that we've understood about saturated and unsaturated molecules.

Which of the following shows an unsaturated molecule? B shows an unsaturated molecule because the molecule in B contains a carbon to carbon double bond.

Hopefully you got that one correct.

Alkenes are more reactive than alkanes and alkenes react with bromine water.

Here we can see an alkene and we've got our bromine and these react together to form a bromoalkane.

We can see here the carbon to carbon double bond can open up and the bromine atoms can bond to the carbons on either side of that carbon-carbon double bond.

Bromine water can therefore be used as a test for alkenes.

Bromine water itself is an orange colour and the solution turns colourless when it is added to an alkene as the dissolved bromine molecules can react with the alkene.

And we saw how those bromine molecules can bond to the carbons on either side of that carbon to carbon double bond.

Here we can see the bromine water turns from orange to colourless when added to an alkene, showing a chemical reaction has happened.

When bromine water is added to an alkane it does not react as alkanes do not contain a carbon to carbon double bond.

The orange colour of bromine water remains.

Here we can see the bromine water stays orange when added to an alkane as there is no chemical reaction.

So we can use bromine water as a test for unsaturation because the bromine water will turn from orange to colourless when added to an alkene, but it will remain orange when added to an alkane.

Time for a question.

What colour change occurs when an alkene is added to bromine water? A: the bromine water turns from orange to clear.

B: the bromine water turns from orange to colourless.

C: the bromine water remains orange.

If you add an alkene to bromine water, the bromine water turns from orange to colourless because the bromine can react with an alkene and a chemical reaction occurs.

Well done if you got that question correct.

Be careful if you choose answer A because the bromine water is itself already a clear orange solution.

So be careful using the word clear.

It changes from orange to no colour, so it changes from orange to colourless.

And if you chose C, the bromine water remains orange.

That's what would happen if we added an alkane to the bromine water.

It's time for our final practise task of today's lesson, and there's three questions to complete in this task.

First of all, how are the products of cracking used? The second question is, describe a chemical test which can be used to distinguish between ethene, that's an alkene, and ethane, an alkane.

And the third question is to complete the following equations for cracking.

Pause the video now, have a go at answering these questions, and I'll see you in a moment.

Let's go over the answers.

Question one, how are the products of cracking used? Shorter alkanes are useful as fuels and alkenes are used to produce polymers and other chemicals.

Question two, describe a chemical test which can be used to distinguish between ethene and ethane.

We add bromine water to the ethene and to the ethane.

The bromine water will turn from orange to colourless with ethene, but the bromine water will remain orange with the ethane.

Hopefully you got that question correct.

Question three, complete the following equations for cracking.

Remember to make sure that your carbon and hydrogen atoms are balanced on both sides.

So in 3A, we will have the other product, which is an alkene, and that'll be C8H16.

In 3B, we have an alkane that was missing with the molecular formula C16H34.

In C, we have the alkene C8H16 that we needed to add.

And in D, we can see that we have produced three products from our cracking of this larger alkane, and the missing product has the molecular formula C4H8.

Well done if you've got those all correct.

We have reached the end of today's lesson on cracking fractions of crude oil.

Before we go, let's just summarise some of the key points we have covered in today's lesson.

Fractions of crude oil have different properties and uses, affecting the demand for these molecules.

Larger hydrocarbons can be broken down or cracked to produce smaller, more useful molecules.

Cracking can be done by various methods, including catalytic cracking that involves using a catalyst, and steam cracking that involves using steam.

The products of cracking include alkanes and alkenes.

And alkenes have a double bond between the two carbon atoms. So we describe alkenes as unsaturated.

Bromine water changes from an orange or orange-brown colour to colourless in the presence of an alkene, but the bromine water remains orange in the presence of an alkane.

Well done for all your hard work throughout today's lesson.

I hope that you've learned lots and I hope that you're able to join me for another lesson soon.