Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you can join me today for this lesson on the electrolysis of aqueous solutions.

It's part of the electrolysis unit.

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

The learning outcome for today's lesson is, I can describe the reactions of electrolysis of an aqueous solution, including the tests for identifying the products.

And here are the key words: aqueous solution, this is formed when a substance is dissolved in water; cathode, a negatively charged electrode in an electrolysis cell to which the cation are attracted; anode, a positively charged electrode in an electrolysis cell to which the anions are attracted; electrolytes, a liquid or solution that contains freely moving ions.

You might want to stop the video and jot down these keywords and their meanings so that you can refer to them later on in the lesson.

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

The first one is about aqueous solutions.

Then we're gonna have a look at the electrolysis rules, and finally, how to test the products in electrolysis.

So let's get started with our first learning cycle, aqueous solutions.

Many ionic compounds will dissolve in water.

Water is the solvent.

The ionic compound is the solute.

We say they are soluble in water.

And here we have an example of sodium chloride, which is that white crystalline solid, being put into some water, have a stir, and we can't see it anymore.

The solid cannot be seen because an aqueous solution has been formed.

An ionic substance dissolves when its ions are separate, and they spread out through the particles of the water, which is a solvent.

So you can see in this animation, we have our sodium chloride as particle clumps all together.

They go into the water, and they separate out.

The ions are now free to move throughout the aqueous solution, and because of that, they can act as charge carriers and will be an electrolyte.

So let's have a quick check for understanding.

Copper chloride solution can conduct electricity.

True or false? Well done if you picked true.

That is correct.

Now let's have a look why.

So is it A, in an aqueous solution, the ions are not in a fixed position and cannot act as charge carriers, or B, in an aqueous solution, the ions are free to move and act as charge carriers? Is it A or B? Well done if you chose B.

That is correct.

In solid sodium chloride, the ions are in a fixed lattice, but when dissolved in water, they're free to move.

So you remember from your learning of structure and bonding, in sodium chloride, we have an Na+ ion and we have a Cl- ion, and both of them have full outer shells of electrons and are drawn with their charges.

When they are in solution, they are free to move around within the water, but a water molecule can also ionise or dissociate.

And when this happens, it forms a hydrogen ion and a hydroxide ion.

As you remember that a water molecule is covalently bonded.

We have an oxygen atom sharing electrons with hydrogen atoms. Well, when it ionises, first of all, it can form a hydrogen ion.

So one of those covalent bonds breaks.

And the other ion it forms is a hydroxide ion, which is an OH-.

And it's really important that you remember this when we are looking at the electrolysis of aqueous solutions.

Pure distilled water does not conduct electricity because there are only H2O molecules present, and this means there are no charge carriers.

But in an aqueous solution of salt, there is always some H+ and OH- ions presence because some of the water molecules ionise or dissociate.

And so we do have some H+'s and we do have some OH-'s, and we have to take this into account.

So if we look at some examples, if we have an aqueous solution, for example, of calcium chloride, the cations present, the positive ions are Ca2+, the calcium and H+ from some water.

The anions present are Cl- ions, the chloride ions, plus some OH- ions from the water.

And this is the same with all the aqueous solutions, so potassium nitrate has K+ and H+, and then NO3- and OH-.

Zinc sulphate has Zn2+ and H+ ions present.

And then the ions are SO4 2- and OH-.

This is a really important point that you need to remember when we're dealing with aqueous solutions.

So for example, if we have a aqueous solution of copper sulphate, remember, this is formed when copper sulphate crystals are dissolved in water, and you can see a solution being made there.

The cations present are Cu2+ and H+.

The anions present are SO4 2- and OH-.

And these are things that you have to learn.

So quick check for understanding, which ions are present in an aqueous solution of sodium nitrate? Have a look at those ions carefully and make your choice of A, B, C, or D.

It's an aqueous solution of sodium nitrate.

Well done if you chose C.

We have an Na+ and an NO3- from the sodium nitrate, and we have an H+ and an OH- from the water.

So well done if you got that correct.

Now, during the electrolysis of an aqueous solution, there's a few things going on.

First of all, we've just talked about the fact there's more than one type of positive ion or cations, and these are in competition at the negative electrode or the cathode.

So if we have a look at these animation, what we can see is at the negative electrode, we have both the blue positive ions and the grey positive ions being attracted towards it, okay? So all the positive ions get attracted towards the negative cathode.

And equally, at the positive electrode or the anode, all the negative ions, the green ones and the very pale blue ones, are being attracted towards it.

And there is this competition that takes place at the electrodes during the electrolysis of an aqueous solution.

So another quick check for understanding, during the electrolysis of copper sulphate, aqueous, which ions are in competition at the cathode? Is it A, B, C, or D? So it's the cathode.

Okay, well done if you've got B.

Cu2+ and H+ are both in competition at the cathode.

Remember, the cathode is the negative electrode, and positive ions are attracted to it.

So well done if you got that correct.

Okay, that brings us to our first task.

For the first question, have a look at this diagram.

Some students set up this experiment, and we've got some questions.

So A, will the current flow through the distilled water? B, when potassium chloride crystals are added into the water, what will be observed at the electrodes? And C, why do you think this will happen? So pause the video and have a go at this question, and then we'll look at the answer together.

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

Will the current flow through the distilled water? Well, the answer is no.

Water molecules are covalently bonded.

There are no charge carriers.

Part B, when potassium chloride crystals are added into the water, we will see bubbles at the electrode.

So a chemical reaction will be taking place.

So why do we see those bubbles? The solid potassium chloride will dissolve in the water, that's the first thing, forming an aqueous solution, containing ions that are free to move acting as charge carriers.

A current can now flow.

And remember, from our previous lessons in electrolysis, when we've got a current flowing, we've got our charge carriers.

Chemical reactions take place at the electrodes.

So really well done if you got that right.

Let's move on to question two.

Some students set up an electrolysis experiment with sodium chloride solution.

Which row in the table shows the ions present in the solution? And give a reason for your answer.

And then which ions will be attracted towards the anode? Pause the video and have a go at this question.

Okay, so which row in the table shows the ions present in the solution? Row C, we have sodium-plus and Cl- ions from the sodium chloride, and we have H+ and OH- ions from the water.

It's really important that you got all four ions correct.

So well done if you did.

Which ions will be attracted towards the anode? The anode is positively charged, so the negatively charged ions will be attracted towards it.

That's the Cl- ions and the OH- ions.

So well done if you've got that right as well.

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

So we're gonna move on and have a look at the electrolysis rules.

During the electrolysis of an aqueous solution using inert graphite electrodes, we might find metal produced at the cathode, and we might see a gas being produced at the anode.

Now, one word I used there was inert.

Inert means that the electrodes don't take part in the reaction themselves.

They just act as a material which will allow the charge to be conducted through it.

During the electrolysis of an aqueous solution using inert graphite electrodes, a gas might be produced at the anode, and hydrogen might be produced at the cathode.

So depending on what compound we have, we will sometimes see hydrogen.

We will sometimes see gases, and we may sometimes see solids.

So there are rules for which substances are discharged at the anode and cathode to help us to be able to predict what is going to happen.

So at the cathode, we're either gonna see a hydrogen or a metal.

And this this is dependent upon the position in the reactivity series of the metals.

So we've got our reactivity series here, just to remind yourselves about it.

We've got our increasing reactivity, and the rule says the metal will be produced at the cathode if it is less reactive than hydrogen.

So for example, if the metal iron is copper or silver or gold or platinum, we will see that metal produced at the cathode.

Rule two, hydrogen will be produced at the cathode if the metal is more reactive than hydrogen.

So for example, if we have potassium or sodium or zinc or iron or any of those other metals above hydrogen reactivity series, then hydrogen will be produced.

So applying those rules at the cathode, we'll give you another example, during the electrolysis of copper sulphate solution, copper will be produced at the cathode because it is less reactive than hydrogen.

During the electrolysis of sodium sulphate solution, hydrogen will be produced at the cathode because sodium is more reactive than hydrogen.

So knowing that reactivity series is really important for predicting.

Quick check for understanding, true or false.

Hydrogen will be formed at the cathode during the electrolysis of silver nitrate solution.

Well done if you picked false.

Now, what is the reason? Silver is less reactive than hydrogen.

So the silver ion will be discharged, or the hydrogen ion is always discharged as hydrogen is A reactive gas, A or B? Well done if you chose A.

That is the correct answer.

And remember from the reactivity series, silver is way down below hydrogen.

So very well done if you got that right.

So what happens at the anode? Well, we have some rules as well.

Rule one, a halogen, so remember, halogens are found in group seven of the periodic table, so that's chlorine, bromine, or iodine.

A halogen will always be produced at the anode if the electrolyte contains a halide ion.

Rule two, oxygen is produced at the anode if the electrolyte does not contain a halide.

So if the electrolyte is sulphate or nitrate, then it'll be oxygen that is produced.

So here is just a table, a little bit summary of some of the different ions.

So negative ions from electrolyte, chloride, bromide, and iodide, they are your halide ions, sulphate and nitrate.

And it tells you what will be produced.

So sulphate and nitrate will always produce oxygen, and the halides will produce the halogen.

So let's have a go at applying the rules at the anode.

So we've got an example.

During the electrolysis of copper sulphate solution, oxygen will be produced at the anode.

During the electrolysis of sodium chloride solution, chlorine will be produced at the anode.

So we need to remember those rules and be able to apply them.

So let's have a check for understanding.

Iodine will be formed at the anode during the electrolysis of aqueous potassium iodide.

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

Let's justify your answer.

Is it A, the electrolyte contains hydroxide ions because it is an aqueous solution, so oxygen will be produced, or B, the electrolyte contains iodide ions, which is a halide, so iodine will be formed? Well done if you chose B.

That is correct answer.

A halogen was there, so the halogen had to be produced following the rule.

Excellent work.

Right, this brings us to task B.

So here we have four aqueous salt solutions, and they're all electrolyzed using inert electrodes.

So what we'd like you to do is to complete the table to show the product being formed at each electrode.

Pause the video, and then when you've had a chance to do the question, we'll look at the answer together.

So our first solution is silver nitrate.

Oxygen will be formed at the positive electrode because there's a nitrate present.

Silver will be produced at the negative electrode because silver is way down below hydrogen in the reactivity series.

Lithium iodide, iodine will be produced at the positive electrode because it's a halogen.

And hydrogen gas will be produced at the negative electrode because lithium is way up at the top of the reactivity series for metals above hydrogen.

Copper sulphate, we are gonna find oxygen gas produced at the positive electrode because the sulphate is something that always produces oxygen.

Copper metal will be produced at the negative electrode because copper is found at the bottom of the reactivity series below hydrogen.

Magnesium chloride, we will get chlorine gas produced at the positive electrode because there's a halogen present, and hydrogen gas produced at the negative electrode because magnesium is a reactive metal.

So if you've got those right, very well done.

Question two, a sample of sodium nitrate solution was poured into an electrolysis cell, and the power was switched on.

So what you need to do is work through the parts of this question.

A, what was the electrolyte used in the experiment? B, name the ions present in the electrolysis cell.

C, what charge is on the anode and cathode? D, after about five minutes, what would you expect to see? What would you expect to observe at the electrodes? E, predict what is formed at the anode.

And F, predict what is formed at the cathode.

So pause the video and have a go at these questions.

Right, so we'll have a look at the answers.

So part A, the electrolyte was a sodium nitrate solution, or if you put sodium nitrate with aq behind it, that's fine as well.

The ions present in the electrolysis cell are Na+, H+, NO3-, and OH-.

So well done if you've got those, but you do need to get all four ions.

The charge on the anode, or the anode is a positively charged inert electrode, and the cathode is the negatively charged inert electrode.

After about five minutes, I would expect to see bubbles of colourless gas being formed at both electrodes.

And E, I predict that oxygen will be formed at the anode because it's a nitrate present, and I predict that hydrogen will be formed at the cathode because sodium is a reactive metal.

So very well done if you've got all the answers to those questions.

So that brings us to the end of learning cycle two, which was on electrolysis rules.

We're now going to move on and have a look at how we can test the products of electrolysis.

So the gases produced during electrolysis can be collected and tested, and if you want to actually collect them and test them, you need to have a setup like this that you can see in the diagram with some test tubes full of the electrolyte being put on top of the electrodes so that they can collect the gas as it's formed.

When the test tube is full of gas, what we need to do next is turn off the power supply.

When we've done that, remove the test tube.

Place your finger or thumb over the end to make sure the gas doesn't escape, because if we lose it all, we're not gonna be able to test it.

Then we will need to carry out a gas test to see what our products actually are.

In practise, it might be difficult to get enough gas to carry out the tests because we do need to have quite a bit to actually get the right result.

So that's something we need to look out for.

So quick check for understanding.

How would you collect a sample of gas produced at the cathode during the electrolysis of an aqueous solution? A, place an empty test tube on the cathode, B, place a test tube full of the electrolyte on the cathode, C, place A test tube full of water on the cathode.

B is the correct answer.

So well done if you chose that.

You always fill the test tube up with the electrolyte and then invert it and put it on top of the cathode.

Okay, so this is something you'll have learned from further down the school, but we'll just check them.

Testing for hydrogen gas.

So we've collected our gas, we wanna see if it's hydrogen.

What do we do? Well, we do the squeaky pop test.

To test for a sample of gas to see if it's hydrogen, we hold a burning splint to the end of the inverted test tube, and we should hear a pop, a loud squeaky pop if the gas is hydrogen.

And you can listen carefully, you'll hear at the pop now.

(gas pops) And finally, to test the sample of gas to see if it's chlorine, we use damp blue litmus paper.

And that will turn red, and then it will go white as it's bleached.

So you might start off with your blue litmus paper and just hold this into the gas.

Part of it will go red, and then it will go white as it is bleached.

You'll also be able to smell that familiar swimming pool smell as the chlorine gas is released.

Right, quick test for understanding.

During electrolysis, a colourless gas is collected at the cathode.

How would you confirm that the gas is hydrogen? A, would you use damp blue litmus paper and see if it turns red, then white, B, place a glowing splint in the test tube to see if it relights, or C, hold the burning splint at the end of the test tube and see if it pops? We are looking for hydrogen gas.

Well done if you chose C.

That is correct.

That is the test for hydrogen.

Right, that brings us to our third task, task C, testing for products.

So we've got a little scenario here.

During the electrolysis of copper chloride solution, copper metal forms that the cathode, and a gas is produced at the anode.

I want you to read these statements and decide which are true.

So just put a ticks in the true column if you think it's true.

And then describe how to identify the gas.

Pause the video, and when you're ready, restart it, and we'll look at the answer together.

Let's have a look at the answer and see which ones are true.

So electrolysis splits the copper chloride, and copper is seen at the cathode.

Copper ions are attracted to the cathode lose electrons and form ions.

Well, neither of those are true, but copper ions are attracted to the cathode, and these gain electrons and form atoms. So well done if you chose that third statement.

Also, you will see a brown orange deposit observed at the cathode, and that is the copper that's being formed.

So well done if you chose that statement.

Copper atoms attracted to the cathode gain electrons.

It's not the copper atoms that are attracted.

It's the copper ions.

So that final statement is also incorrect.

So well done if you pick those two.

How are we going to identify the gas? Well, the gas could either be chlorine or oxygen 'cause we've got a copper chloride solution and we have got the water there as well.

So to test for chlorine, place a piece of damp blue litmus paper in the gas.

If the paper changes from blue to red to white, chlorine is present.

So that's a positive test for chlorine.

But to test for oxygen, place a glowing splint in the gas, if splint relights, oxygen gas is present.

So sometimes when we're not sure, we have to do more than one test.

So if you've got those right, very well done.

Excellent work.

Right, moving on to question two.

So this time, we're going to use what is probably a bit more of an unfamiliar solution, the electrolysis of copper chromate solution.

And it's been carried out in some slightly different equipment.

You can see our anode and cathode, but it's being carried out in a U tube.

And we're told some information here that copper chromate solution is green, so we have our green electrolyte.

We're also told that copper chromate contains blue copper ions, Cu2+, and yellow coloured chromate ions, CrO4 2- ions.

We are also given some information about what happens at the anode and cathode.

So when this is switched on, we find at the anode, the solution turned yellow, and we saw bubbles being formed, so bubbles of gas.

We found at the cathode, the solution turned blue, and a solid was formed.

So before you attempt to answer part A to B, C and D of this question, what you need to do is just make sure that you understand all that information that is given in the question.

And then for part A, we want to try and explain the colour changes, part B, we want to name that solid, part C, we want to name the gas at the anode, which relights a glowing splint, and part D, why is that gas formed? Now, to answer this question, a lot of the information is there in the text.

So pause the video and have a go.

Let's have a look at the answer to part A, explain the colour changes at the electrodes.

So the yellow chromate ions, CrO4 2- were attracted to the positively charged anode and so moved towards us.

So this explains why the solution at the anode turned yellow.

The blue Cu2+ ions were attracted to the negatively charged cathode and moved towards it.

So that explains why the solution turned blue.

So again, it's simply answering it with opposites attract, and that is one of the keys to understanding electrolysis.

So if you've got those right, very, very well done, excellent work.

Right, so B, what solid formed at the cathode? Copper, copper metal formed at the cathode.

Copper is low down the reactivity series below hydrogen, so the metal forms when the copper ions are discharged.

C, oxygen is the gas that produced at the anode, which relights a glowing splint.

That is one of our gas tests that we talked about.

And D, why is oxygen formed? And that is, following our rules, there are no halide ions present.

So although you weren't familiar with the chromate ion, you know it's not chlorine or bromine or iodine.

Therefore, oxygen must be formed.

So very, very well done if you've got the answers right, 'cause that's quite a tricky question.

So let's summarise our key learning points for this lesson.

First of all, we need to understand that multiple types of positive ions are in competition at the negative electrode, which is the cathode.

Multiple types of negative ions are in competition at the positive electrode, the anode, when we are looking at the electrolysis of aqueous solutions.

And because of this, there are rules for which substances are discharged at the anode and cathode.

The gases produced during electrolysis can be collected and tested, hydrogen may form at the cathode.

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