Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased you can join me today for this lesson on Electrolysis: extracting aluminium.

During this lesson, we're going to learn about how we can extract aluminium from its ore.

I'm gonna guide you through this lesson and I'm going to be here to support you all the way through, especially the tricky parts.

A learning outcome for today is, "I can explain why "electrolysis is required for the extraction "of aluminium from its ore, and describe the process." We've got some key words in today's lesson.

Aluminium oxide, this is a compound consisting of two aluminium atoms and three oxygen atoms. And the formula is AL2O3.

Bauxite, a sedimentary rock with a high aluminium content.

And cryolite, a compound used in the smelting of aluminium to lower the melting point of the aluminium oxide.

In today's lesson, we have two learning cycles.

The first learning cycle is using electrolysis to extract metals, and the second learning cycle is the electrolysis of aluminium oxide.

So let's get started with our first learning cycle, using electrolysis to extract metals.

So first of all, aluminium is an important metal with many applications.

So you'll be used to having cans of drink in aluminium cans.

Some of you may have been to the hairdressers and had highlights, and you'll get aluminium foil is used as part of that process.

And aluminium is used to make trains.

So it's a very, very versatile metal.

Electrolysis is used to extract aluminium from molten aluminium oxide because aluminium is quite a reactive metal.

So thinking about aluminium for a little bit, why is it so important? Well, it's important due to its properties.

So aluminium is lightweight because it has a low density, this is why it's used in transport, for aircrafts as well as trains.

It's ductile and malleable so it can be pulled into wires and hit into different shapes.

It's a good thermal and electrical conductor.

It's protected by a layer of aluminium oxide so it doesn't corrode easily, which is why it's used for things that go outdoors quite a lot.

And it's not magnetic.

So because of its properties, aluminium is a really important metal.

So we need to have a good way of extracting it from its ore.

The reactivity series of metals determines the method by which they are extracted from their oars.

So the reactivity series of metals is something that you'll be familiar with from previous learning.

If we look at that series now, the top few metals, potassium, sodium, calcium, magnesium, and aluminium, all are above carbon in the series.

Carbon being a reference point.

This means that they are reactive metals and they are extracted by electrolysis.

Those below carbon are extracted by displacement with carbon.

So things like zinc, iron, tin, lead, copper can all be extracted by displacement with carbon.

And finally those metals right at the lower end are very un-reactive, such as gold and platinum.

And they are found in nature as the atom themselves.

So don't need to be extracted.

So it's important when we are trying to work out what extraction method to use to look at the reactivity series of metals.

Now electrolysis is used to extract metals from molten compounds when the metal is above carbon in the reactivity series.

As we've just said, aluminium is above carbon in reactivity series, so cannot be extracted with carbon.

When we use electrolysis, large amounts of energy are used in the electrolysis extraction process.

And this makes it an expensive way to extract metals.

Electrolysis must be used when there is no cheaper alternative.

So it's not our first choice, but we do need aluminium as we've already seen.

It is a metal with a lot of uses.

Okay, let's have a quick check for understanding.

Which metals are extracted from their ore by electrolysis? So hint, use the reactivity series at the side to work out your answer.

Is it sodium, gold, iron, or magnesium? Well done if you picked sodium.

Sodium's near the top of the reactivity series, above carbon, so that uses electrolysis, as does magnesium.

So well done if you've got both of those correct.

Now bauxite, so we've got an image of bauxite here.

It's a reddish-brown sedimentary rock.

And the key thing about bauxite is it has a high aluminium content.

And this is the rock which we use to extract aluminium from.

And it's the world's main source of aluminium.

So there are other minerals which contain aluminium, but you need to have a rock which has a high content to make it viable.

And Australia is the biggest producer of aluminium.

And you can see an image there of a bauxite mine in Australia, where you've got that lovely orangey-brown colour.

Bauxite contains a mixture of different aluminium minerals.

It has also gallium minerals and iron ore.

So before we can use the bauxite to extract the aluminium, the impurities must be removed from the bauxite.

So that's the first thing.

We start off with our rock.

We then remove the impurities, then the bauxite at that stage is converted to aluminium oxide.

And then the aluminium oxide is put into the electrolyzer and the aluminium is extracted by electrolysis.

So when we're mining, we mine the bauxite.

We remove the impurities, we get our aluminium oxide, and then we can do our electrolysis and get the aluminium extracted.

So quick check for understanding.

True or false, bauxite must be purified before aluminium can be extracted.

True or false? Well done if you chose true.

But why? Bauxite contains a mixture of minerals of different metals.

Bauxite contains only a mixture of aluminium minerals.

Okay, well done if you chose A.

Bauxite contains a mixture of minerals of different metals.

And that's why it needs to be purified before we can extract the aluminium.

Well done.

So let's think about aluminium oxide.

It's an ionic compound.

Its formula is AL2O3, so that means it contains some AL3+ ions and also O2- ions.

And as expected within an ionic compound, the ions are arranged in a lattice structure.

So you can see here that we've got our oxide ions and our aluminium ions arranged in irregular structure.

So for electrolysis to take place, the ions must be free to move.

Now this is where it becomes expensive because pure aluminium oxide has a high melting point of over 2000 degrees centigrade.

So that means that it has to be heated to that temperature before those ions are free to move.

The other issue is it's insoluble in water and an aqueous solution cannot be made.

So we must go get the ions to move through the molten root.

To reduce the cost of electrolysis of aluminium oxide, the aluminium oxide is mixed with molten cryolite.

And you can see there it's like a white stone in the solid state.

So it's molten.

And when it's mixed with the aluminium oxide, this cryolite acts as a solvent.

So what actually happens is the aluminium oxide dissolves into the cryolite.

So the mixture is an electrolyte.

There is AL3+ and O2- ions, which are free to move and act as charge carriers.

And we have our electrolyte.

And you can now see that we have our mixture here.

We've got our aluminium 3+ ions, our oxide 2- ions, and cryolite, and the lattice structure has been broken down and those ions are free to move around.

But what is really important for the expense of this process is electrolysis can now take place at a lower temperature of about a thousand degrees Celsius rather than at 2000, which makes the whole process a lot more economically viable.

Okay, another check for understanding.

Which statement is the most correct? Cryolite is added to aluminium oxide, A, because it's an electrolyte.

B, to break the ionic bonds in the aluminium oxide.

C, to lower the melting point of the mixture, allowing electrolysis to take place at a lower temperature.

D, to save money.

Okay, what do you think? Well done if you chose C to lower the melting point of the mixture, allowing electrolysis to take place at a lower temperature.

That is the correct answer, so well done.

That brings us now to task A.

Using electrolysis to extract metals.

So we've got three questions that I'd like you to answer.

One, why can aluminium not be extracted by heating aluminium oxide with carbon? Two, explain why pure aluminium oxide is not added to the electrolysis cell.

And three, give three uses of aluminium and explain what properties of aluminium make it good for these uses.

So if you'd like to pause the video and answer the questions, then we'll look at the answer in a moment together.

So let's have a look at the answers to question one.

Why can aluminium not be extracted by heating aluminium oxide with carbon? Well first of all, aluminium is more reactive than carbon.

Or we could say carbon is less reactive than aluminium.

Or we might say that it will react with the carbon.

But either way we cannot extract aluminium by heating it, aluminium from aluminium oxide, by heating with carbon because it doesn't work.

Aluminium is more reactive than carbon.

Question two, explain why pure aluminium oxide is not added to the electrolysis cell.

Well, for electrolysis to work, the aluminium and the oxide ions must be free to move.

So it must be in a liquid state.

Since aluminium oxide has a high melting point and is expensive to melt, it is dissolved in cryolite to lower the melting point.

Therefore pure aluminium oxide is not used, it would just be far too expensive.

Now question three, give three uses of aluminium and explain what properties of aluminium make it good for these uses.

So really you could choose anything.

So whatever you've got, as long as it's aluminium, that's fine.

But the key thing is we need to link it to a property.

So here are some examples.

It could be used for a drinks can because it doesn't corrode, it's lightweight, it's not magnetic, so it can be sorted from steel cans when recycled.

Aluminium foil, again, doesn't corrode, especially if we're using it in cooking.

It's malleable, so it can be rolled into thin sheets.

Transport for example, aeroplanes , it's lightweight and it does not corrode.

Cooking pans, it's a good thermal conductor, it's lightweight and it does not corrode.

So if you've got these, well done.

And if you've got some things similar, that's fine as well.

Excellent work.

This now brings us to the end of our first learning cycle.

We're now going to move on to our second learning cycle and look at the electrolysis of aluminium oxide in much more detail.

So the electrolysis of aluminium oxide is carried out in a special electrolysis cell.

And you can see a diagram of it here.

We have our power supply linked to the graphite anode and cathode.

And if you look carefully, you'll see you've got these great big graphite anodes which have the positive charge.

Those are held into the aluminium oxide dissolved in cryolite, which is our electrolyte.

Around the outside of, or around the casing of the electrolysis cell is actually the graphite cathode that goes around the sides on the bottom.

And then at the bottom, we have a tap where the molten aluminium comes out.

So it's a bit different to some of the cells that we'll use in the lab because it's done on a massive industrial scale.

So during this electrolysis of aluminium oxide, aluminium is produced at the cathode.

So the cathode again is that negative lining of the steel just on the steel case.

The AL3+ ions are attracted to the negatively-charged cathode.

So it's the same principle as we've already met.

At the cathode, the aluminium 3+ ions receive three electrons, and they are reduced to form aluminium atoms that all then sink to the bottom where they can be tapped off.

There we have our molten aluminium.

All right, quick check for understanding.

True or false.

Aluminium is produced at the anode during the electrolysis of aluminium oxide.

Well done if you picked false.

That's the correct answer.

Now why? So, A, aluminium ions are positively charged and so are attracted to the cathode.

Or, B, aluminium ions are negatively charged and so attracted to the anode.

Well done if you chose A, that is the correct answer.

Aluminium ions are positively charged and so attracted to the cathode.

Right, now let's think about what else is produced during electrolysis.

Well, oxygen is produced at the anode and you can see in the diagram are three carbon anodes.

Carbon is what they're made from.

Graphite is a form of carbon that can conduct.

So it's actually graphite anodes.

So what happens when we have our O2- ions, they are attracted to the positively charged anode.

Remember opposites attract.

At the anode, the O2- ions lose two electrons.

They are oxidised, produce oxygen atoms. The oxygen atoms are very reactive.

So they immediately combine to produce oxygen molecules, oxygen, gas, and that gas is then collected at the anodes.

So as the oxygen is produced at the anode, some of it will react with the graphite electrodes and form carbon dioxide gas.

So remember this is something because it's at high temperature, your carbon plus oxygen gives carbon dioxide.

If it was at low temperature, then it wouldn't be so much of a problem.

So just the symbol equation, C plus O2 gives CO2, and that is a gas as well.

So as a result of this, what will happen over time is the anodal gradually wear away where that carbon, that graphite has reacted with the oxygen forming CO2.

And eventually it will need to be replaced.

And this again adds to the cost of the process.

'Cause we did say the electrolysis extraction of aluminium oxide was expensive.

When we keep on having to replace the graphite anodes, that adds to the cost as well.

So to try and keep the cost down, there are a number of factors that need to be considered when choosing an extraction site.

Because there's some factors like having to replace the anodes is something that we can't do anything about.

We can't do anything about the temperature getting any lower than a thousand degrees melting point when it's mixed with the cryolite.

But there are things that we can do.

First of all, we can make sure the source of electricity needed for the electrolysis, we can make sure that it's a cheap source of electricity.

We can make sure the source of raw materials, the bauxite, the cryolite, and the carbon electrodes are relatively cheap as well.

We can look at the transport links to make sure that those raw materials will be brought and then the aluminium will be taken away to the markets very, very easily.

So if I was actually looking for a new site, I might be looking for it to be by a port.

So things can be imported by ship or by a rail link.

The consumers are encouraged to recycle aluminium to save money and natural resources because it's much cheaper to recycle aluminium than to extract it.

And of course that saves the natural resources as well.

So quick check for understanding, true or false.

Oxygen is produced at the anode during the electrolysis of aluminium oxide.

True or false? True, well done if you got that correct.

Let's have a look at the reason.

Oxygen ions are negatively charged and are attracted to the cathode.

Oxygen ions are negatively charged and are attracted to the anode.

Well done if you picked B.

Oxygen ions are negatively charged and attracted to the anode.

Okay, so that brings us to task B.

The electrolysis of aluminium oxide.

So aluminium is extracted using this electrolysis cell.

First question, the thing we'd like you to do is to label the diagram and just write down the labels for A to E.

Okay, let's have a look and see what you've got.

A is the graphite cathode.

So it's important to have the word cathode there.

And if you've got graphite as well, that's great, 'cause that's what it's made from.

B is the steel case.

C is the graphite anode.

D is the aluminium oxide dissolved in cryolite.

Or you could have just said electrolyte, that would've been acceptable.

And E is the molten aluminium that's tapped off ready to be shipped to wherever it's needed to go.

Right, let's move on to the next lot of questions.

So the formula for aluminium oxide is AL2O3.

What ions including charges are present in the molten aluminium oxide? Question three, describe what happens to the aluminium ion when the electrolysis cell is switched on.

Question four, explain why the anodes are regularly replaced.

And question five, give two reasons why the extraction of aluminium is an expensive process.

So pause the video while you have a go at these questions and then we'll have a look at the answers together.

Okay, so question two, well, the ions present are the AL3+ ion, the aluminium ion must have 3+ and the O2- ion.

So well done if you got those correct.

Question three, what happens to the aluminium ion? Well, the aluminium 3+ ions are attracted towards the cathode or the negative electrode.

The O2- ions are attracted towards the anode or positive electrode.

You don't actually need to include that bit in the answer to this question, which is why I see them brackets.

So here, the aluminium iron gains three electrons to become an aluminium atom.

So that's actually what happens at the cathode.

Question four, explain why the anodes are regularly replaced.

Well, oxygen gas forms at the anode and some of the oxygen reacts with the carbon or the graphite anode to form carbon dioxide.

And that means that gradually the anode wears away the electrode.

And so it needs to be replaced.

So well done if you got that answer.

Question five, give two reasons why the extraction of aluminium is an expensive process.

Well, you could have any from this.

A lot of electricity is needed to run the electrolysis cell.

Aluminium oxide is a high melting point and so must be dissolved in cryolite to lower it.

The cryolite is an additional cost.

And the anodes need regularly replacement.

So well done if you've got any of those answers or all of them, excellent work.

So this brings us to our summary of the key learning points.

So first of all, aluminium is above carbon in the reactivity series, and can therefore not be extracted with carbon.

Impurities are removed from the bauxite or at the start of the process.

Cryolite acts as a solvent and allows the electrolysis to take place at a lower temperature.

Carbon electrodes need to be continuously replaced as they react with oxygen as it's produced, forming carbon dioxide.

Electrolysis is an expensive process as it requires large amounts of energy.

I hope you have enjoyed this lesson today, and I look forward to learning with you soon, thank you.