Lesson video

Hello, my name's Dr.

Warren, and I'm so pleased that you've decided to join me today for this lesson on the principles of electrolysis.

It's the first lesson in the electrolysis unit.

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

The outcome for today's lesson is: I can describe electrolysis, and explain how it works using scientific terms. And we have some keywords.

The first is electrolysis, and this is a process that uses electricity to break down an ionic compound.

Electrolyte, a liquid or solution that contains freely moving ions.

And electrode, a rod of metal or graphite through which an electric current flows in and out of electrolyte.

You may wish to pause the video now so that you can note down these keywords and their meanings so that you can refer to them later on in the lesson when we are using them.

In today's lesson we have two learning cycles.

The first is the process of electrolysis, and then we're gonna have a look at an electrolysis cell.

So let's get started with our first learning cycle, the process of electrolysis.

So electrolysis, as we've already mentioned, is the decomposition or the breakdown of an ionic compound using electricity.

And like a lot of scientific words, we can break down the word into its meaning.

So for example, we have electro, which comes from electrical or electricity, and lysis, which means to split or set free.

So key idea here is we're using electricity to split up a compound.

So electrolysis is used to obtain elements from ionic compounds by separating them into their elements.

So if you can remember what the word means, that gives you a good starting point.

Now, just before we get into what electrolysis is, let's have a little bit of recap about what an electric circuit is, an electric current.

So an electric current is a flow of electrical charge, and you can see this in the animation.

You have to have a complete circuit for the light bulb to light up, and you can see that the electrons are flowing through the wire.

The electrical charge must be able to flow around the circuit.

And in metal, the charge carriers are electrons.

So that's something that you will have learnt about previously in your physics, and it's important to have that in mind when we start to think about what electrolysis is and how it actually works.

So, just a quick check of understanding before we go any further.

Which statement describes how to light up a bulb in an electric circuit? Is it A, electric charge can flow around the circuit, B, the electric circuit must be complete, or C, an electrolyte is added into the circuit? Well done if you chose A.

That is correct.

And also B.

So well done if you got both of those right.

So, let's move on and think about what electrolysis is.

So, sodium reacts with chlorine gas to form sodium chloride, a stable ionic compound, and this will be a reaction you're probably already familiar with.

Sodium metal, which is a highly-reactive metal, and chlorine gas, which is that green, toxic, non-metal gas, they burn with an orange flame.

It's something that your teacher may have demonstrated in the past.

You kind of see a lot of white stuff being produced which is kind of like smoke, but actually it is very fine powder of sodium chloride, which is a white crystalline ionic salt, and it's what we use to put on our chips, table salt.

So that's a compound that is made that is very, very stable from sodium and chlorine.

And we have the equation here, sodium plus chlorine gives sodium chloride.

Now, electrolysis can be used to get those elements back again.

So if we want to obtain the sodium metal and the chlorine gas from sodium chloride, the only way we can do it is to force them apart using electricity.

But for electrolysis to work, the electrical charge must flow through what we call an electrolyte.

Again, a little bit of recap.

You'll have done in it the structure and bonding unit.

When sodium chloride is formed, it forms a giant 3D crystal lattice, and you can see the positive charges in purple for sodium and the negative charges for the chloride ions.

And these are held together by a strong electrostatic force of attraction.

So, sodium chloride cannot conduct electricity when it is a solid, and this is because it's not an electrolyte because all the ions are not able to carry any charge because they are in a fixed position.

So all our Na+ and Cl- ions are fixed in that lattice, which means they cannot move out of the lattice and act as charge carriers.

So that's the first point that we really need to make sure we understand, that electrolysis cannot be conducted on solid ionic compounds.

So, but what happens, how we set these ions free? Well, one thing we can do is we can dissolve sodium chloride or any other ionic compound in a solvent, in this case of sodium chloride with water.

And when sodium chloride is solution or it's molten, that means that it's melted, they can act as electrolytes.

And that is because both the Na+ ions and the Cl- ions are free to move.

Once they're free to move, they can act as charge carriers in the electrolyte.

So, again, a quick check for understanding.

Solid sodium chloride is an electrolyte.

True or false? Well done if you picked false.

Now, what's the reason for that? Well, is it A, the Na+ and Cl- ions act as charge carriers as they are free to move, or B, the Na+ and Cl- ions are not free to move because they are arranged in a ionic lattice? Well done if you chose B.

In a solid, they are not able to move, so it cannot be an electrolyte.

Excellent work so far.

So, for electrolysis to work, we must have an electrolysis cell, and you can see a diagram here, but we're gonna learn more about that in the second learning cycle.

So the electrolyte, that's the liquid which has ions that are free to move, is put into the electrolysis cell, and you can see that now in the beaker.

And it's connected into the electric circuit, which again you can see done by the wires.

The electrical charge must be able to flow all the way around the circuit.

So you can see the electrode is connected to a bulb which connected to the power supply.

The power supply is connected to the other electrode.

And what you find is the electrolytes in those, they're our charge carriers, these are ions and they are able to complete the circuit.

So when you first look at that diagram, it almost looks like there is a gap because there is no wires there.

But it is that liquid, the electrolyte, that has the free moving ions that completes the circuit and allows electrolysis to work.

Now, during electrolysis, chemical reactions take place at the electrodes to change the positive and negative ions into atoms. So it's a chemical change.

So we are basically breaking up our ionic compound into its elements using a chemical method of separation, and it's something you probably haven't come across before.

So, let's just have another quick check for understanding.

Which of the following statements are true about electrolysis? Is it A, electrolysis is a physical method of separation, B, electrolysis is a chemical method of separation, or C, electrolysis is an electrical method of separation? Is it A, B, or C? Well done if you chose B.

Electrolysis is a chemical method of separation.

In a little bit we'll be able to see that at the electrodes, products are formed.

Now, using equipment, you've come across a lot of equipment during chemistry, which set of equipment is used to break down a compound chemically? Is it A, is it B, or is it C? So have a look at those diagrams and make your choice.

Well done if you chose A.

That is an electrolysis cell and that is used to break down a compound chemically.

B, you'll have come across that already in previous learning, is a distillation, and that basically separates compounds physically.

And C is a gas syringe which collects a gas that's formed during a reaction.

So it kind of separates out the product so you can then go and test it.

But A is the correct answer for electrolysis.

Now, in chemistry in the past, there's been some quite famous chemists.

Sir Humphry Davy was one of the first scientists to work on electrolysis, and in 1807 he discovered the reactive metals such as sodium and potassium.

Up to that point, we had Mendeleev's periodic table which had gaps in, and nobody had actually been able to extract these metals because they were so reactive.

But Davy discovered that by using electrolysis of their molten salts, he was able to do that.

And so today electrolysis is still used to extract reactive metals from their ores, so thanks to Sir Humphry Davy.

So Sir Humphry Davy discovered sodium and potassium by, was it A, electrolysis of molten salts, B, filtering the metal ores, or C, reducing metal ores with carbon? Well done if you chose A.

He discovered it by the electrolysis of molten salts, and that is the method of extraction still used today for reactive metals.

Okay, so that brings us to our first task, Task A.

So we've got some questions here.

The first one, which ions are present in each salt? So for A, B, C, and D, if you can write each ion as its chemical symbol and show the charges.

Now, this is something you have done in the structure and bonding unit, so it's a little bit of recap.

So we've got potassium chloride, sodium bromide, lithium iodide, and calcium chloride.

And then for question 2, explain why solid lithium iodide is not an electrolyte but lithium iodide solution is.

Pause the video while you have a go, and then when you're ready, restart and we'll check the answers.

Okay, so for 1 A, we have a K+ ion and a Cl- ion, potassium chloride.

Sodium bromide, we have an Na+ and a Br- ion.

For lithium iodide, we have a Li+ and a I- ion.

And then for calcium chloride, calcium's in group 2 of the periodic table, we have a Ca2+ and a Cl- ion.

So well done if you got those right.

Remember, the charge is really important.

So, for question 2, explain why solid lithium iodide is not an electrolyte but lithium iodide solution is.

Well, in solid lithium iodide, the Li+ and the I- ions cannot act as charge carriers because they are in fixed positions in that giant ionic lattice.

Okay, so remember, all ionic compounds and solids have that giant ionic lattice, so in fixed positions, so they can't conduct electricity.

But in lithium iodide solution, the lithium ions and the iodine ions are free to move and act as charge carriers.

So therefore, lithium iodide solution is an electrolyte.

Right, let's move on to the next questions in this task.

Question 3: during electrolysis the electrolyte is separated into different elements.

A, does a physical or chemical change take place? And B, can you give a reason for your answer? For question 4: to light up a bulb, an electric current must flow around an electric circuit.

A, what is an electric current, and B, in electrolysis, how is that circuit completed? So, if you'd like to pause the video while you answer the questions and then press play when you're ready to go again and we'll look at the answers together.

So for question 3 A, it is a chemical change.

Electrolysis is about a chemical change.

And B, the answer is because chemical reactions occur at the electrodes.

So very well done if you got that correct.

Question 4 A.

An electric current is a flow of electrical charge.

Remember, in metals that's electrons and in electrolysis it is an electrolyte.

So the ions in the electrolyte can act as charge carriers, which is how the circuit is completed.

So excellent work if you got that right.

Well done.

So this brings us to the end of our first learning cycle on the process of electrolysis.

So let's move on to our second learning cycle where we'll think about electrolysis cells.

So the first point is that electrolysis is performed by setting up a cell using a direct power supply.

So if you've got a power pack, you need to make sure that you go for DC and not AC.

If you're using normal batteries and cells, then you are okay because that is direct current.

Once we've got all our equipment, the anode is connected to the positive terminal.

And if you look at the diagram, you can see that via the red leads.

So the anode is positively charged because it's connected to the positive terminal of the power pack, and the cathode is negatively charged because it's connected to the negative terminal, and you can see that by the black lead.

So negative terminal of the power pack, and it's really important that you get those the right way round.

So, quick check for understanding.

The power supply to an electrolysis cell is always DC, direct current.

Is that true or false? Well done if you chose true.

So let's justify your answer.

Is it because when DC is used, the electrodes keep the same polarity? That means they're always either positive or negative.

Or when DC is used, the electrodes keep changing polarity, i.

e.

swapping from positive to negative? So that will be choice B.

So A or B? Well done if you chose A.

That is the correct answer.

It's really important that DC is used so that the anode is always positive and the cathode is always negative, otherwise the particles might get a little bit confused because they'll be swapping the direction in which they are moving.

So, in an electrolysis cell, the next thing we need to think about is the electrolyte, 'cause that contains free moving positive and negative ions, and these are really important because they act as the charge carriers.

And these ions are attracted to the oppositely charged electrodes.

Now, electrode is always metal or graphite, and it has to be a material that does conduct electricity, otherwise we'd have some problems. So if you have a little look at this animation, you can see our anode that is positive, and the negative ions shown in green are attracted towards it because opposites attract.

And if you look at the cathode, which is negative, the grey positive ions in this little animation are being attracted towards it.

So, key point here is the electrolysis cell has the electrolyte, it has free moving positive and negative ions that are attracted to the oppositely charged electrode.

During electrolysis, electric charge flows around the circuit.

So the negative ions in the electrolyte, well they are attracted towards the anode.

Remember, opposites attract.

And at the anode they lose electrons.

And those electrons then go round the wire and come out at the cathode.

At the cathode, this is where the positive ions go.

So positive ions are attracted to the cathode.

They gain those electrons at the cathode, and this completes the circuit.

The products are deposited at the electrodes, and you can often see those by bubbles of gas or a solid being formed on one of the electrodes.

Sometimes people find it difficult to remember which way round, and it's key that you do get it right.

So we can use the acronym PANIC.

So don't panic if you forget or you think you've forgotten, because what we can say, PANIC: Positive Anode Negative Is Cathode.

So just think of the word panic.

P for positive and A for anode, the first two letters.

And then N-I-C, negative is cathode.

And of course, if you can remember one, you get the other one right automatically.

So don't panic when you can't remember.

Just remember, PANIC: Positive Anode Negative Is Cathode.

Now let's have a quick check for understanding.

What happens to the Cu2+ ions in an electrolysis cell when the DC power supply is switched on? A, they move towards the anode, B, they move towards the cathode, C, they lose electrons at the electrode, D, they gain electrons at the electrode.

Make your choices.

Okay, well done if you got B, they move towards the cathode.

They are positive ions, they are attracted to the negative cathode.

At the negative cathode, they gain electrons.

Okay, so very well done if you got B and D correct.

Oh, we've got another scientist here that played a big role in electrolysis, and this time it's Michael Faraday.

Now, Michael Faraday became Humphry Davy's secretary in 1813.

So remember we talked about Sir Humphry Davy early on.

But he went on to make his own scientific discoveries.

So he started off like an assistant and then he got into it himself.

And what he came up was with the Faraday's law of electrolysis.

And this states that the mass of a substance deposited at an electrode is directly proportional to the amount of charged passed through it.

And this was really important discovery, because it means that if we want to say, for example, cover something in a thin layer of metal, just by running the charge for a bit longer, we can make that layer a bit thicker, and it's a law that is still used today.

So Michael Faraday is another of those key chemists that helped move on the ideas in electrolysis.

Okay, another quick check for understanding.

During electrolysis metals are deposited at the anode.

Is that true or false? Think about the charge of a metal.

False.

Now, let's justify your answer.

Is it A, metals form positive ions which are attracted to the cathode, or metals form negative ions which are attracted to the anode? And this is what I mean by you've got to get your positives and negatives the right way round.

A or B? Well done if you chose A.

Metals form positive ions, so they are attracted to the cathode, which of course is negatively charged.

Excellent work.

Right, this brings us to our second task, Task B.

And in the diagram you have got some electrolysis cells of molten ionic compounds and they've been set up.

Can you identify the anode in each diagram, the cathode in each diagram, and what type of particles are found in the electrolyte? And finally, describe what happens when the power supply is switched on.

So pause the video while you have a go at that question, and then when you're ready, we'll have a look at the answers together.

Okay, so, let's have a look at the anode.

Well, the anode is the positively charged electrode, so that is W because it's connected to the plus side and Z.

So it doesn't matter which order the anode and cathode go.

What is important is which side of the power supply, the positive terminal, where that is.

So W and Z are the anode.

That means that X and Y must be the cathode, because you can trace that around and the electrode is connected to the negative terminal of the power supply.

And the types of particles found in the electrolytes are ions or charged particles.

So very well done if you got that right.

These are the fundamental ideas in electrolysis.

So, what happens when the power supply is switched on? Well, the ions are attracted towards the oppositely charged electrodes where they either gain or lose electrons.

So, in our second question, what we want you to do to start with is label the diagram.

So you've got these words: anode, cathode, power supply, electrolyte, and products.

If you can label that diagram.

And then in part B, why are ions often referred to as charge carriers? So once again, pause the video while you have a go at that question.

So, let's have a look at the answer.

The power supply is that bit of equipment up at the top.

Then our anode, what's key here? It's the one that is positively charged.

The cathode is the negatively charged one.

We have the electrolyte, which is that liquid we can see.

And then the products are either the bubbles or the solid being deposited at the electrodes.

So really well done if you got all of those labels correct.

Part B, why are irons often referred to as charge carriers? Well, they carry the electrical charge through the electrolyte, enabling electrical charge to travel around the completed circuit.

So that's what they do, so very, very well done if you got that answer correct.

Right, question 3.

This time we've got some words and definitions.

I want you to draw a line to connect the keyword to its meaning.

So our keywords are electrolysis and electric current, electrolyte, and electrode.

Then just draw a line to the correct meaning.

Again, pause the video while you have a go, and then we'll have a look at the answer together.

Okay, so electrolysis is the process that uses electricity to break down a compound.

So well done if you got that one right.

And electric current is a flow of electrical charge.

And that might be electrons if it's through a metal or it might be ions if it's in electrolysis.

An electrolyte is a liquid or a solution that contains freely moving ions, and these are attracted to the charged electrodes.

And finally, the electrode is a rod of metal or graphite through which an electric current flows in and out of an electrolyte.

So really, really well done if you got all of those right.

These definitions are really important for what's gonna follow in some of the next lessons about electrolysis.

This now brings us to the end of our lesson, so we're just gonna go through our key learning points together.

First of all, electrolysis is the process in which an electrolyte is broken down by electricity; it's not a physical method of separation.

The cathode is the negative electrode, where positive particles gain electrons.

The anode is the positive electrode, where negative particles lose electrons.

An electrolyte contains freely moving ions, and these are attracted to the charged electrodes.

Electrolysis allowed Davy and Faraday to make their scientific discoveries.

Well, I hope you enjoyed this lesson and I look forward to learning with me soon.