Lesson video

Hello, my name's Dr.

Warren, and I'm so pleased that you can join me today for this lesson on the electrolysis of molten salts.

It's part of the electrolysis unit.

I'm here to work with you, to teach you, and support you all the way through this lesson, especially the tricky parts.

Our learning outcome for today's lesson is, "I can explain why salts can be electrolyzed when in their liquid state, and describe the reactions taking place." We have some key words for this lesson.

The first is molten.

Molten is a liquid substance formed by heating ionic compounds and metal ores.

Anode, a positively charged electrode, in an electrolysis cell, to which anions are attracted.

Cathode, a negatively charged electrode, in an electrolysis cell, to which cations are attracted.

Anion, a negatively charged ion, and cation, a positively charged ion.

So you may wish to pause the video now, and jot down these keywords and their meanings so that you can refer to them later on in the lesson.

In this lesson, we have two learning cycles.

The first is on molten salts, and second is on electrode reactions.

So let's get started with our first learning cycle on molten salts.

So a solid ionic compound cannot conduct electricity.

And you'll remember from previous learning that we have a giant ionic lattice with strong ionic bonds between oppositely charged ions, and these bonds act in all directions.

You can see from the diagram that we have sodium plus ions, the purple ones, and chloride minus ions, the green ones, and they're held strongly in position.

They're fixed position in the arrangement of the solid.

The ions are therefore not free to move, so cannot act as charge carriers and conduct electricity so a solid ionic compound cannot conduct electricity.

But what happens when it melts? So an ionic compound when it melts, it changes from a solid state to a liquid state as the ions gain energy during melting.

So during melting, energy is transferred to the ionic compound by heating.

And the ions gain energy which breaks that strong electrostatic force between the oppositely charged ions in the lattice structure.

This means the ions are now free to move, and that's really important when it comes to electrolysis.

It means that they are now actually an electrolyte.

So salts are ionic compound which can be electrolyzed when in the molten state.

And the word molten is a term that's used to describe the liquid state formed by heating ionic compounds and ores until they melt.

And the ions are free to move, and once they're free to move, they can act as charge carriers.

So just as a little recap, an example of salts, we have sodium bromide, for example.

Sodium bromide consists of Na+ ions and Br- ions.

You'll have learned about this in the structure and bonding unit.

Calcium chloride, calcium's in group two of the periodic table so it produces Ca2+ ions, and then it will have two Cl- ions to make sure that the charge is balanced.

So when we're doing electrolysis, it is really important to remember back to our structural bonding learning and work, be able to work out what charges are on the ions.

So quick check for understanding, true or false.

"Solid sodium iodide cannot conduct electricity." True or false? Well done if you picked true.

Let's justify our answer.

So in a solid, the Na+ and I- ions act as charge carriers as they are free to move, or B, the Na+ ions and I- ions are not free to move because they're arranged in a giant ionic lattice, which is correct.

Well done if you chose B.

B is the correct answer, it's a solid.

We need to remember that solids cannot conduct electricity when they are ionic.

Salts are ionic compounds.

In the molten state, it's different.

The ions are free to move and act as charge carriers, and this means that they can conduct electricity as shown in the circuit where we've got the light bulb lit up.

The liquid can conduct electricity because the ions are turned back to their elemental form of atoms or molecules during electrolysis as they either gain or lose an electron.

So that's another key learning point that the ions have a chemical reaction taking place at the anode or the cathode.

So what actually happens? Well, let's think about it for a moment.

When a molten salt is put into electrolysis cell and the power is switched on, the free moving ions are attracted to the electrodes.

You can see in this little animation that we have a positive electrode and the green, and negative ions are attracted towards it.

We have a negative electrode, and the the grey positive ions are attracted towards that.

Opposites attract, and this is what happens as soon as the power supply is turned on in a electrolysis cell, the anion, or negatively charged ion is always attracted to the anode or positively charged electrode, and the same happens.

The cation or the positively charged ion is always attracted towards the cathode or negatively charged ion.

And in a molten salt, you have a positively charged ion and a negatively charged ion.

So quick check for understanding.

"What happens when molten calcium chloride is electrolyzed?" So the Cl- sign is attracted to the anode.

The Cl- is attracted to the cathode.

The Ca2+ is attracted to the anode.

The Ca2+ is attracted to the cathode.

So make sure we've got these definitions right.

Make your choices.

Well done if you chose A, the negative chloride ion is attracted to the positive anode.

And D, the positive met lion?? is attracted to the cathode, which is negatively charged.

Excellent work if you've got those correct.

All right, so this brings us to our first task, and we've got some questions here for you to have a go at.

So question one, calcium bromide is an ionic compound.

Name the anion present, and B, explain why calcium bromide is an electrolyte when molten but not in the solid state.

Question two, in electrolysis, electrodes are used.

What is the difference between an anode and a cathode? Pause the video and have a go at these answers, and then when you're ready, press play, and we'll look at the answers together.

So the answer to 1a, the anion present is bromide.

So well done if you've got that.

And now remember, the answer for part B, "An electrolyte contains ions that are free to move and act as charge carriers.

In solid calcium bromide, the ions are at a fixed position in the lattice structure and cannot move.

Molten calcium bromide is in the liquid state.

The ions are free to move." Question two, the electrolysis electrodes are used.

What is the difference between an anode and a cathode? The anode is positively charged and the cathode is negatively charged.

That's the difference.

So very well done if you got those correct.

All right.

We move on to question three.

So a teacher set up a microscale.

So a small scale electrolysis experiment.

So looks a little bit different to the ones we've seen.

The electrodes are made from nichrome wire, which you can see labelled, and an iron nail, and it's connected to a power supply.

Okay, so the first thing we want you to do is identify the anode cathode.

Then B, explain how you came up with your answer to part A.

C, for electrolysis to work, zinc chloride must be heated, why? And D, what will be produced at the anode and the cathode? So pause the video while you have a go at answering this question.

Let's have a look at the answer for this question.

So identify the anode and the cathode.

Well, the nichrome wire is the anode, and the iron nail is the cathode.

So well done if you've got that the right way round.

So let's think about how do we know? Well, the nichrome wire is attached to the positive terminal of the battery and the iron nail is attached to the negative terminal of the battery.

So that's how we know which way round they are.

So excellent work if you've got the reason right as well.

C, for electrolysis to work, the zinc chloride must be heated, why? Well, the zinc chloride needs to melt when it's heated, and that means that the ions will be free to move and act as charge carriers.

Part D, what will be produced at the anode and the cathode? Well at the anode, we will see chlorine gas being produced because the anode is positive, and the chloride ions will be attracted towards it.

And the cathode, we will see a deposit of zinc metal.

So very, very well done if you've got that answer correct.

Excellent work.

So that brings us to the end of our first learning cycle.

We're now going to move on to our second learning cycle and think about what happens at the electrodes in more detail, so electrode reactions.

So in the electrolysis of molten salts, remember we have a metal and that always forms at the negative electrode, and a non-metal always forms at the positive electrode.

So at the electrodes, we'll either get any oxidation or a reduction reaction occurring.

So just as a reminder from previous learning, oxidation is a type of reaction in which a substance losers electrons, and reduction is a type of reaction in which a substance gains electrons.

And through this losing and gaining of electrons, we will be able to complete the circuit, as electrical charge, we'll be able to flow around the circuit.

So a useful way to remember what is happening to the electrons during oxidation and reduction is again to use this idea of an acronym, oil rig.

So oxidation is a loss of electrons, and reduction is gain of electrons.

And I still use that today when I quickly want to try and remember which way round it is.

So quick check for understanding.

"Which statements are correct about the electrolysis of molten salts?" Is it A, metals form at the positive electrode? B, non-metals form at the positive electrode.

C, ions are in a fixed position, or D, oxidation and reduction reactions occur at the electrodes.

Okay, let's make your choices.

Well done if you chose B.

Non-metals form at the positive electrodes.

Remember, non-metals form negative ions.

Opposites attract.

Excellent work.

And D, we get oxidation and reduction reactions occurring at the electrodes.

And the way to remember that is they involve the exchange of electrons and that's what we are looking for to complete the electric circuit.

So we're going to take a closer look at the electrolysis of molten sodium chloride.

Now during the electrolysis of molten sodium chloride, oxidation occurs at the anode or positively charged electrode.

So let's have a look at this diagram and what we can actually see is the chloride ions.

Those are the green ones are being attracted towards the anode, and there they are being oxidised.

They are losing an electron, and chlorine gas is being formed at the anode when two chlorine atoms combine to form a chlorine molecule, and this animation shows that quite nicely.

The chlorine molecule goes off and we can see the electrons that they have lost being sent round the wire.

Okay, so what is happening at the cathode? Well, this time the cathode is a negatively charged electrode.

So we are looking at the sodium cation.

So the sodium ions which are showed in grey, they are attracted towards the cathode and they gain an electron, and it is reduced to produce a sodium atom.

And what you'll find is sodium metal is formed at the cathode and you can see this animation with the electrons going round the circuit and then they are picked up by the sodium ions, and that circuit has been completed.

Let's have a quick check for understanding.

"What is formed at the anode during the electrolysis of molten sodium chloride?" Is it sodium metal, chlorine gas, or sodium chloride? Well done if you chose B.

It's chlorine gas.

Okay, so we're gonna look at these electrode reactions a little bit more in detail and we can write them as word equations to help us understand what's going on.

So at the cathode, we have a sodium ion.

It takes an electron and it produces a sodium atom.

And these sodium atoms are basically deposited on the electrode.

At the anode, we have a chloride ion, which forms a chlorine atom as one electron has given up, so chlorine atom plus an electron.

These chlorine atoms then join together, so two chlorine atoms form a chlorine molecule, which then is released as gas.

So 2Cl gives Cl2.

So actually writing out these equations at the cathode and anode will help us remember and understand that chemical reactions are taking place at the electrodes.

A quick check for understanding.

"During the electrolysis of molten zinc chloride, so we have a Zn2+ ion, so the Zn2+ ion is reduced at the cathode." True or false? Well done if you chose true.

That's the correct answer.

But why? So is it the zinc iron adds two electrons to form the zinc atom, or is it the zinc atom forms a zinc ion and gives up two electrons? Well done if you chose A.

That's the correct answer.

The zinc ion takes two electrons, and the zinc atom is formed.

Excellent work.

So that brings us to our second task, task B.

So for the first question, what we'd like you to do is to complete the sentences.

"So complete the description of reduction and oxidation reactions that take place during the electrolysis of molten zinc chloride." And to do that, use words, atom or atoms, ion and electron or electrons.

Pause the video while you have a go at this question, then we'll look at the answer together.

So reduction is the gain of electrons.

At the cathode, each zinc ion gains two electrons to become a zinc atom.

Oxidation is the loss of electrons.

At the anode, each negative chloride ion loses one electron to become a chlorine atom.

Chlorine atoms join to become diatomic chlorine molecules.

So really well done if you've got all of those answers correct.

It's really important to get to use the right words when we're talking about these different particles.

All right.

Question two.

"Which statement describes what's happening to the electrolysis of molten lithium bromide?" So tick the correct answers and then give a reason.

So Li+ signs are reduced to produce Li atoms. Li+ signs are oxidised to produce Li atoms. Bromide ions are oxidised to produce bromine atoms, or bromine atoms combined to produce bromine molecules.

Pause the video while you have a go at this question.

Okay, let's have a look at the answers.

So first of all, lithium ions are reduced to produce lithium atoms. So the lithium ion gains in electron to produce lithium atoms. Reduction is gain of electrons.

So the second statement is incorrect.

Lithium ions gain an electron to produce lithium atoms. Oxidation is a loss of electrons, so that is incorrect.

Bromide ions are oxidised to produce bromine atoms. That is correct because the bromide ions lose an electron to produce bromine atoms. Oxidation is a loss of electrons.

The final statement, bromine atoms combined to produce bromine molecules.

Well, bromine acids are very active because they're not stable.

They combine to form a diatomic bromine molecule.

So that is correct.

So really well done if you've got those correct and got the explanations right too.

Excellent work.

All right, question three.

We've got some students here discussing why zinc and chlorine are formed at the electrodes during the molten electrolysis of zinc chloride.

So I want you to read these statements and then decide who you agree with or disagree with and give a reason.

So let's have a look.

We have Aisha.

She says, "The chloride ions are oxidised at the anode and form chlorine molecules." Andeep says, "The carbon electrodes react with zinc chloride forming the elements zinc and chlorine." Lucas says, "The zinc chloride is split up into its elements, zinc and chlorine, which then move to the electrodes." And Sofia says, "The zinc ions are reduced at the cathode forming zinc atoms." So pause the video while you have a go at this question.

Okay, let's have a look at the answers together.

So Aisha said the chloride ions are oxide at the anode and form chlorine molecules.

That is what happens.

So well done if you agreed with Aisha.

That was correct.

Excellent work.

All right, Andeep.

Well, I'm afraid we can't agree with Andeep.

The carbon electrodes react with zinc chloride forming the elements zinc and chlorine.

I'm afraid that doesn't happen.

So if you disagreed with Andeep, well done.

The carbon electrodes are inert.

They do not take part in the reaction.

They allow charge to carry around the circuit, but there is no chemical reaction and that's really important.

So if you disagreed with Andeep, and you've got that answer, well done.

All right.

What about Lucas? I'm afraid Lucas is incorrect as well.

He thought that zinc chloride ion split up into its elements which then moved towards the electrodes.

Unfortunately, that is incorrect.

What happens is the zinc and chloride ions moved the electrodes, then the zinc ions are oxidised and the chloride ions are reduced.

So if you agreed with Lucas, I'm afraid you were wrong, but if you disagreed and you got that answer, very, very well done.

And finally, what about Sofia? She said that zinc ions are reduced at the cathode forming zinc atoms, and that, of course, is correct.

So well done if you've got that answer.

Excellent work.

Okay, so now let's have a look at the key learning points to this lesson.

First of all, a solid ionic compound cannot conduct electricity.

Salts are ionic compounds that can be electrolyzed when they are in the molten state.

Freely moving ions can reach the electrodes and undergo oxidation or reduction.

Gases may be formed at the electrodes.

A metal always forms that the negative electrode and a non-metal always forms at the positive electrode.

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