Lesson video

Hello, my name's Dr.

Warren, and I'm so pleased that you can join me today for this electrolysis lesson on purifying copper, part of the Electrolysis Unit.

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

Our learning outcome for today's lesson is, I can investigate what happens when a solution of copper sulphate is electrolyzed with copper electrodes.

And here are the key words: pure, a single element or compound that is not mixed with any other substance.

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 the cations are attracted.

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

Electroplating, a process that uses electricity to deposit a thin layer of metal onto the surface of another conductive object.

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

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

The first is on purifying copper, and the second on electroplating.

So let's get started with our first learning cycle on purifying copper.

Copper is a versatile metal with many industrial applications.

So for example, you'll find copper in the electrical leads.

You'll find copper pots and pans.

And you'll also find that copper is used to make the still for producing alcoholic drinks, such as whiskey.

Pure copper only contains copper atoms and no other element.

And you will hopefully remember this diagram from previous learning on metallic bonding, in the structure and bonding unit.

We have a regular lattice structure in pure copper of metal ions of the copper(2+) metal ions, and a sea of delocalised electrons.

And because of this structure we have the properties of pure copper, including high thermal conductivity, high electrical conductivity, and they're due to the sea of delocalised electrons.

It's got good corrosion resistance.

It's very malleable and ductile, that's due to the regular pattern, and the metal ions being able to slide over each other.

And it is strong, that regular lattice structure in pure copper is a strong structure.

Impurities affect the properties of pure copper as they disrupt the regular lattice.

And again, you should be familiar with this diagram from previous learning, where we have a larger atom or iron disrupting the lattice, the regular lattice in the copper structure, so it now becomes irregular.

Metal alloys are designed to produce desirable properties suited to specific applications.

So an example, brass is an alloy of copper and zinc, which is stronger than pure copper.

For other applications such as where good electrical conductivity is needed, pure copper is required due to those delocalised electrons.

So quick check for understanding, true or false.

Impure copper has exactly the same properties as pure copper.

True or false? Well done if you picked false.

Now, let's have a look at the reason.

Is it A, pure copper is a better electrical and thermal conductor than impure copper.

Or B, impurities in copper do not affect the properties.

Well done if you picked A.

That's an example of how the properties are different.

Excellent work.

Electrolysis is used to purify copper and purification of copper involves using copper electrodes in an electrolyte containing copper ions.

So up to now we've only used graphite or inert electrodes, but this time we have an anode of impure copper and a cathode of pure copper.

And then in the electrolysis cell we have some copper(2+) ions.

So the electrolytes is something like copper(2) sulphate.

The ions in the solution must be the same as those produced by the electrolysis of the metal you are purifying.

So that means if we're purifying copper, we need copper ions in the electrolyte.

If we're purifying something like nickel, we would need nickel ions in the electrolyte.

So quick check for understanding.

Which of the following electrolytes could be used to purify copper? Is it A, copper sulphate.

B, copper nitrate.

C, zinc sulphate.

Or D, iron chloride.

Well done If you chose A and B, both A and B have copper(2+) ions, and that is what is needed to purify copper.

Well done if you got that correct.

So what happens during electrolysis? Well, we have our electrolysis cells set up, the impure copper anode, which is shown by the larger anode in this diagram, breaks down into copper(2+) ions, and the two electrons flow around the circuit.

And you can see that on this little animation.

The Cu2+ ions move into solution, and they are attracted towards the negative electrode, or the cathode.

So it's shown very, very clearly on this animation what happens.

So when the copper ions reach the cathode, they combine with the two electrons to make copper atoms, and then they cover the cathode.

So Cu2+, plus two electrons gives copper atoms. So after time, the cathode will get bigger, and the anode will get smaller.

So during the experiment, the mass of the impure anode decreases, which you can see by this electrode getting smaller, and the mass of the cathode increases, which you can see by the diagram where the cathode is getting larger.

At the end of the experiment, the cathode is plated with pure copper.

Cu2+ ions remain in the electrolytes, and that's really important.

Impurities in the anode do not transfer to the cathode, leading to the pure copper deposition.

So the table we've got here shows results obtained from an electrolysis experiment where copper sulphate was electrolyzed with an impure copper anode, and pure copper cathode for 10 minutes.

So we have our anode and cathode, they're both copper.

So at the start, we weighed the anode and cathode, recorded the mass, and at the end we weighed them again.

And what we can find out and see from this data is, mass was lost at the anode, <v ->0.

91 grammes.

</v> Mass was gained at the cathode, + 0.

85 grammes.

And overall, more mass was lost than gained due to the presence of impurities at the anode.

So they're about the same, but they're not totally the same, and that's because there was some impurities that just would have dropped off.

So let's have a quick check for understanding.

During the purification of copper, the mass of the cathode increases.

Is that true or false? True.

Well done if you pick that.

Let's have a look at the reason, the cathode dissolves during the reaction, the cathode is placed in pure copper.

Well done if you chose B.

During the reaction, the cathode is plated with pure copper.

Well done if you got that correct.

So that brings us to our first task.

First question, give three uses of copper, and explain which properties of copper make it suitable for these uses.

So it's uses of copper, link it to the properties.

Question two, you need to look at the diagram, which shows the electrolysis cell needs to purify copper metal.

First of all, label the diagram.

Then tell us what ion must be present in the electrolyte, and C, what materials are the electrodes made from.

Question three, describe how the electrodes change during the purification process, and give some reasons for your answer.

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

So for question one, you can give any three uses, as long as they are linked correctly to a property.

So here's some examples.

Electrical wires, they're good electrical conductors and ductile.

Cooking pots, they're good thermal conductors.

And a still or perhaps a statue, it's malleable and strong.

So well done if you've got those, and if you've got any other ones that are linked correctly to the properties, that's fine as well.

So for our diagram, A, is the anode, B, is the electrolyte, and C, is the cathode.

So well done if you got those labels right.

The iron that must be present in the electrolyte is copper(2+), because we're purifying copper metal.

And both electrodes have to be made from copper.

So well done if you got that correct.

Moving on to question three, describe how the electrodes change during the purification process, and give some reasons for your answers.

Right, well first of all, the mass and the size of the anode decreases.

The copper anode dissolves into the electrolyte, it produces Cu2+ ions, which go into the electrolyte and are attracted to the cathode.

So well done if you've got that, and you do need to get all the different bits of it.

The mass and size of the cathode increases, because it is plated in a layer of pure copper atoms. The Cu2+ ions are discharged at the cathode, and copper atoms are formed.

And you may wish to write an equation as well.

Cu2+, plus two electrons, gives Cu.

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

You're doing really well.

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

We're going to now move on to have a look at electroplating.

So the electrolysis technique used to purify copper has other uses including something called electroplating, where multiple metals are used.

So electroplating is a process that uses electricity to deposit a thin layer of metal onto the surface of another conductive object.

And we use electroplating for several reasons.

For example, it might be to enhance appearance.

So here we have some electroplated gold rings.

Now pure gold is expensive, but if we take a cheaper metal, we can cover it with a thin layer of gold, and it looks much better, and it is cheaper to buy.

Electroplating is also used to improve resistance to corrosion, and improve durability.

So another example is some electroplated nails.

Now iron nails rust quite easily, they corrode quite easily, but if they're electroplated and covered with another metal it can improve the resistance to corrosion, and so they will last longer.

So two really good reasons why electroplating is used.

So how does it all happen? Well again, we need an electrolysis cell to be set up, and the metal that is to be plated is going to be the cathode.

And you can see this diagram, we've just got Me for the metal, that is the cathode and it is the one that's gonna be covered.

The plating metal is the anode.

So in this case we're gonna use copper, so we're gonna try and cover that metal with copper.

And the metal salt solution containing the ions of the plating metal, so in this case it's copper(2+) ions.

So that's really important to think about which, the metal that is going to be plating needs to be the anode, and the electrolyte needs to have those ions in place.

An electric current causes the metal to be deposited on the object.

So those are our basic principles of electroplating, and the very similar to what we just looked at in terms of purification of copper.

A quick check for understanding.

To electroplate a bracelet using silver, the cathode is, A, made from silver metal.

B, the bracelet.

Or C, contains silver ions, Ag+.

Well done if you picked B, the bracelet.

We want to cover the bracelet in silver, so therefore it must be the cathode.

Before electroplating the object is cleaned, and submerged in a metal salt solution to make sure there's no dirt on it.

The benefits of electroplating include a shiny decorative finish as you can see on these silver plated copper lions.

And also protecting the base metal from corrosion.

It's expensive, because it requires special equipment.

And remember, all things to do with electrolysis require lots of energy, because of the electrolysis cell.

So we have expensive metals, we have electricity making it an expensive process.

Okay, let's have a look at an example here.

An object is electroplated with zinc, and here is what happens to the mass over a couple of minutes.

So after one minute, the mass is increased by 0.

2 of a gramme, to 1.

5-0.

3 of a gramme.

After two minutes it's gone up by 0.

4 of a gramme.

So the data tells us that the mass increases by 0.

1 gramme every no 0.

5 minutes, or every 30 seconds.

To increase the mass of the object by 0.

5 grammes, we need to increase the time to 2.

5 minutes.

And that's quite important when we're thinking about electroplating, the thickness of the metal, so we can calculate how much is actually going on to that electroplated object.

Okay, so see if you can work this one out.

The experiment was then repeated using the same conditions and the object was left this time for five minutes.

What will the mass increase of the object be? See if you can have a go at doing that.

So pause a video, and have a quick check.

After five minutes, what will the mass increase of the object be? Well after 0.

5 of a minute, we're told it goes up by 0.

1 of a gramme.

So let's multiply that by 10 for five minutes, and we'll find it goes up by one gramme.

So well done if you've got that calculation correct.

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

A scientist is investigating electroplating using the electrolysis cell set up below.

We have a gold electrode and a spoon, and electrolyte.

So what we want you to do is circle the correct answers, which object was she electroplating, which iron must be in the electrolyte, and how will the mass of the gold electrode change? Just circle the correct answers.

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

Well, the object that was being electroplated was the spoon.

The iron that must be in the electrolyte is gold, 'cause that's what we're gonna cover it with.

And the mass will decrease on the gold electrode.

So well done if you got those answers correct.

Right, some pupils are investigating electrolysis and have set up two cells shown below.

Which pupils prediction is incorrect, and give a reason for your answer.

So we've got cell A, where we have got two copper electrodes.

And we've got cell B, where we've got two carbon electrodes.

Everything else is the same, and we've got copper sulphate for the electrolyte.

So let's have a look and see what the students are saying.

Andeep says, "Cell B has inert electrodes so oxygen will form at the anode, and copper on the cathode." Jacob says, "Both A and B will have the same results as they have the same electrode." Izzy says, "In cell A the anode will get smaller and the cathode will get bigger." So pause the video while you have a go at this question and when you're ready, we'll look at the answers together.

Right, so who was correct? Let's have a look.

Right, Andeep was correct.

Cell B has inert electrodes.

So we are going to get oxygen forming at the anode because it's a sulphate, and copper on the cathode because it is below hydrogen in the reactivity series.

Jacob is incorrect.

We'll come back to him in a moment.

Izzy says in cell A, because we have got the electrodes that are both the same, they're both copper, the anode will get smaller and the cathode will get bigger.

That is correct.

So Jacob, he says, "Both will have the same result because they have the same electrolytes." So no, Jacob is incorrect.

Both cells will produce copper at the cathode.

Pure copper for cell A, but cell B will produce oxygen at the anode, which is basically what Andeep was saying.

So very, very well done if you've got all those answers correct, and the meaning.

Excellent work.

So that brings us to the end of this lesson on the electrolysis of purifying copper, and we'll have a look at our key learning points.

Purification of copper involves using copper electrodes in an electrolyte.

The electrolyte must contain the same ions in the solution as the metal being purified by electrolysis.

Measuring the mass change in the copper electrodes demonstrates the transfer of copper.

Impurities in the anode do not transfer to the cathode, leading to pure copper deposition.

Other uses for this technique include electroplating where you would use multiple metals, for example, gold.

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