Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you can join me today for this electrolysis lesson.

It's a practical lesson where you're going to be testing some aqueous solutions.

I'm gonna show you two different setups, and your teacher will tell you which one you are going to do.

I'm here to talk you through the lesson and support you all the way.

Our learning outcome for today's lesson is I can investigate what happens when aqueous solutions are electrolyzed with inert electrodes.

And here are our key words.

Aqueous solution, this is formed when a substance is dissolved in water.

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

Electrolyzed, when a substance has been broken down by electrolysis.

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

The first is the practical, and the second is interpreting the results.

So let's get started with thinking about the practical.

Electrolysis involves the decomposition or the breakdown of a substance by passing an electric current.

So electrolysis is used to separate ionic compounds into their elements.

For example, if we have lead iodide, we can separate it into lead and iodine using an electrolysis cell with a power supply and two electrodes, a positive anode and an negative cathode.

For electrolysis to work, an electrolyte, a liquid or aqueous salt solution that contains freely moving ions must be present.

So a quick check for understanding.

An electrolyte is either a liquid, a liquid that contains freely moving ions, a solution or an aqueous solution.

Well done if you chose B, a liquid that contains free moving ions, or D, an aqueous solution, which of course also contains free moving ions.

So well done if you got that right.

So here is the equipment needed to set up a simple electrolysis cell, and I'm just gonna go through it.

We have some electrodes.

These are inert electrodes, so they're made from graphite.

We are going to use a beaker as part of our electrolysis cell.

Then we have a Petri dish lid, which has electrode holes already drilled into it.

We need our electrolyte, and at the moment, that's in a conical flask.

And also a measuring cylinder to measure out the right amount.

We will either using a power pack or some batteries.

We need some electric leads, and it's always a good idea if you've got a red one and a black one, then it's easier to distinguish between the positive and the negative electrode.

And some crocodile clips.

So if you're going to use the simple electrolysis cell, you will need to gather together all of this equipment at the start.

And now I'm going to talk you through the method for setting up a simple electrolysis cell.

So it's really important that when you're doing practical work to remember to wear your safety glasses.

We don't want to get anything in our eyes.

So the first thing we need to do is pour 50 centimetres cubed of electrolytes into the beaker and use a measuring cylinder to measure out that amount.

Then take the Petri dish, the one with the two holes in, and place it on top of the beaker, so it's got a lid.

Put the Petri dish lid on top, and then put the electrodes through the holes in the dish, so they are standing in the electrolyte.

Then we need to connect our electrodes to our power supply, so that's either a power pack or batteries.

And you want to use the red lead to connect it to the positive terminal, and the black lead to connect it to the negative terminal, because that will help you to remember which way round the anode and the cathode is.

Once you've done that and you switch on the power supply, you are ready to observe.

So you need to observe the positive electrode and record any changes in a table.

We then observe what's happening at the negative electrode, and again, record any changes in your table.

We're gonna do a test for chlorine.

We can just hold some damp blue litmus paper near the positive electrode and see if it has a colour change.

Remember, if it's chlorine, it will go from blue to red to bleached.

So in this simple electrolysis cell, we're not actually collecting any gases.

So just a quick check before we get on with the practical.

Which of the following pieces of equipment will you use to measure 50 centimetres cubed of electrolyte? Is it A, a conical flask, B, a measuring cylinder, or C, a beaker? Well done if you chose B.

That will give you the most accurate measurement.

So here we have an alternative method for setting up electrolysis cell to include gas collection at the electrodes.

And I'm gonna talk you through a couple of video clips.

The first one is electrolysis of sodium chloride solution, and the second one, the electrolysis of copper sulphate solution, so you can actually see this method if that's the one your teacher asks you to do.

So here we have the equipment to set up the electrolysis of sodium chloride, collecting gases.

We're gonna put a light bulb into the circuit, just to show that electrolysis is happening and the electric current is flowing.

We have some sodium chloride and we're gonna pour it into this electrolysis cell where it covers the electrodes.

And then we're going to fill up a test tube with the electrolyte, the sodium chloride solution, and then we are gonna place that on top of each electrode so we can collect any gases that are formed.

So when you do that, you'll notice that we have to actually put our finger on the end of the test tube and submerge it under the electrolyte to make sure we don't lose anything.

Next, we're gonna connect up the electrodes, so the red electrode is positive, so that will be our anode, and then the black lead, we're gonna connect to the negative terminal, which will be our cathode.

Once we've got everything set up, we turn on the power supply, and you will notice straight away that some gases are being formed, or that fizzing is happening at each of the electrodes.

After some time, you'll notice that one of the electrodes has much more gas in the test tube than the other, which suggests we're collecting two different gases.

Once we've got a good amount of gas into those test tubes, the next thing we're going to do is test the gases to find out what is there.

So to do that, simply lift up the test tube and let the electrolytes off, and that was the test for hydrogen.

And it has a positive test, as you could hear there was a squeaky pop.

Again, lift up the test tube, let out the electrolyte, and then put the indicator paper into the test tube and you'll see it's starting to turn red, which suggests that it might be chlorine present.

And you can see it's getting a little bit paler as well as it starts to bleach.

In this experiment, we're using the similar setup as we did for the sodium chloride, but this time we're going to test and electrolyze some copper sulphate solution.

So you can see that the cell is connected up, the power's been switched on, and the light bulb is lit.

One of the electrodes, the one connected to the red lead, so the positive electrode is producing gas, but there doesn't seem to be any gas being produced at the negative electrode, the one that's connected via the black lead.

So after some time, we can see clearly that we have at the anode a gas being produced but not at the cathode.

And what we can do now we've collected a whole test tube of gas is we can test it to see if it's oxygen.

So if we just get a glowing spill and hold it into the test tube, we can see it relight.

But no gas has been produced at the cathode, so let's take off the test tube and see if anything else has been produced.

So we can disconnect the electrolysis cell and pour out the electrolyte and have a good look and see if there's any changes taking place at the negative electrode.

And what we can clearly see is there is an orange brown deposit all over it that if we gently put our finger on it, you can see that it starts to come off a little bit.

Right, now it's your turn, task A.

What we'd like you to do is set up and run an electrolysis cell using the following electrolytes, and then complete the table with your observations.

So we're basically going to use three different aqueous salt solutions, so you're gonna do this experiment three times, with copper II chloride solution, then sodium chloride solution, then copper II sulphate solution.

And in between each test, you'll need to get rid of all the electrolyte and rinse it out with some water and then set it up again.

So have a go at the practical and we'll look at the results in a little bit.

If you are using the simple electrolysis cell, the setup is gonna look something like this.

If you're collecting gases, the setup is gonna look something like this.

And at both times you need to make sure at the end of the first run, you throw away the electrolyte and wash it out the beaker before doing the next test.

Now we'll have a look at your observations.

So, for copper II chloride, at the positive anode, we should see some fizzing or bubbles of gas.

Blue litmus turns red then white.

At the cathode, the negative electrode, we will see a orange brown solid appearing.

For sodium chloride, again, at the positive anode, we'll see fizzing, bubbles of gas.

The blue litmus turns red, then white.

And this time at the cathode, we'll see fizzing, bubbles of gas.

For the copper II sulphate solution, we'll see some fizzing and bubbles of gas.

And the blue litmus paper this time will stay the same at the positive electrode.

At the negative electrode, at the cathode, we will see an orange brown solid.

So I'm really hoping that those are the results that you got.

So really well done if you've got those results.

So that brings us to the end of the practical.

Now we're gonna move on to our second learning cycle and have a look at interpreting these results.

Okay, so let's have a look at the results in more detail, but a bit background first.

So in an aqueous salt solution, there are always the following ions.

There is an H+ and an OH- sign from the water.

And then there is a metal cation, which is a positive, and a non-metal anion, which is a negative that comes from the salt, and that's gonna be true for every aqueous salt solution.

So we can work out the ions in the electrolyte from the salt.

Calcium chloride, we have Ca2+ from the salt and H+ from the water, and the anions present, CL- ion from the salt and OH- from the water.

Potassium nitrate, K+ and H+ are the cations present, K+ from the salt, H+ from the water.

And then you've got the nitrate ion NO3- from the salt and OH- from the water.

So for zinc sulphate, we have the Zn2+ ion from the salt and H+ from the water.

And then the anion SO42- from the salt and OH- from the water.

So whenever you are using the electrolytes from an aqueous solution, you've always got the H+ and OH-, and then you can work out the other ones.

So quick check for understanding, which of the following are present in an aqueous solution of sodium sulphate? Is it A, B, C, or D? Well done if you got D.

You've always got the H+ and the OH- from the water, so that means it must be C or D, and then you've got your sodium plus and it's sulphate, so that's SO42-, so well done if you got that right.

So let's just recap from previous learning what happens during electrolysis.

So during electrolysis, we have two cations in competition at the negative electrode.

So if we look at this little animation, we can see that the cations, the positive ions are being attracted to the negative electrodes, so the blue pluses and the grey pluses.

And then two anions are in competition at the positive electrode or the anode.

And again, looking at the animation, you can see the green minus and the pale blue minus ions are being attracted to the electrode.

So there's this competition at the electrodes during the electrolysis of an aqueous solution, only one of the ions will get discharged at each electrode.

So to find out which one, there are rules for each substance being discharged at the anode and the cathode.

So at the cathode, it's gonna be hydrogen or a metal.

It depends on the position in the reactivity series.

So the first rule is the metal will be produced if it is less reactive than hydrogen.

So if it's copper, silver, gold, or platinum, then you'll get the metal.

Hydrogen will be produced if the metal is more reactive than hydrogen, so if it's potassium, sodium, calcium, or any above hydrogen, you will get hydrogen being produced.

So reactivity series is really important to remember.

So let's look at applying the rules at the cathode.

So during the electrolysis of copper chloride solution, copper will be produced at the cathode because it is less reactive than hydrogen.

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

So what about the anode? We've got rules for those as well.

So at the anode, it's either gonna be oxygen or a halogen that will be discharged.

Remember, halogens are chlorine, bromine, or iodine.

They are those group seven non-metal elements.

So our first rule is a halogen will be produced if the electrolyte contains a halide ion.

Oxygen is produced if the electrolyte does not contain a halide ion.

So here we've got a table.

We've got our negative ions from the electrolytes, so chloride, bromide, iodide sulphate, or nitrate.

Those are the most common ones that you're gonna come across.

And the element produced at the anode is gonna be chlorine, bromine, iodine, or oxygen.

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

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

There's no halide present, so it's oxygen.

And we can test for that relighting a glowing splint.

During electrolysis of sodium chloride or copper chloride solution, chlorine will be produced at the anode because halide is present, and we can test that using blue litmus paper, which goes red and then is bleached.

So quick check for understanding, hydrogen will be formed at the cathode during the electrolysis of copper sulphate solution, true or false? Well done if you chose false.

Now let's have a think why.

Is it because copper is less reactive than hydrogen? So the copper ion will be discharged.

Or the hydrogen ion is always discharged as hydrogen is a reactive gas? Well done if you chose A, that is the correct answer.

Okay, so now we're going to have a look at the results you got from the first part of the lesson.

So complete the table by naming the products, and you might use your results from task A to help.

So we have our electrolytes, copper II chloride solution, sodium chloride solution, and then sodium sulphate solution.

So once you've worked out what's produced at the anode and the cathode in question two, go on to describe how you could test your product for hydrogen gas, oxygen gas, or chlorine gas.

Pause the video while you have a go at these questions, then we'll look at the answers together.

Okay, so products at the anode, for copper II chloride, it's chlorine because it's a halogen.

Sodium chloride, it's chlorine, again, it's a halogen.

Sodium sulphate, it's oxygen because it is a sulphate.

Products at the cathode.

Well, the first one is copper for copper II chloride, because copper is below hydrogen in the reactivity series.

And for the other two, we've got a reactive metal layer, so hydrogen gas is produced.

So very well done if you've got those correct.

For question two, describing the test for hydrogen gas, hold a lit splint to the test tube.

It'll burn with a squeaky pop if hydrogen is present.

B for oxygen gas, hold a glowing splint to the test tube, it will relight if oxygen is present.

C, chlorine gas, place a damp blue litmus in the gas.

The litmus paper will turn red and then be bleached white if chlorine is present.

So very, very well done if you got all of those correct.

So that brings us to the end of our practical lesson, so let's have a quick recap of the key learning points.

Electrolysis involves the decomposition or breakdown of a substance bypassing an electric current.

There are two types of positive ions in competition at the negative electrode, two types of negative ions in competition at the positive electrode.

There are rules for which substances are discharged at the anode and cathode.

The gases produced during electrolysis can be collected and tested.

I hope that you have enjoyed today's practical lesson, and I look forward to learning with you again very soon.