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 brine.

It's part of the Electrolysis unit.

We are going to work through this lesson together.

I'm here to support you all the way, especially through the tricky bits.

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

Here are our keywords.

Brine.

Brine is a concentrated aqueous solution containing between 3.

5% and 26% of sodium chloride.

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

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

Reduction.

Reduction is a type of reaction in which a substance loses oxygen or gains electrons.

Oxidation.

Oxidation is a type of reaction in which a substance gains oxygen or loses electrons.

You may wish to pause the video and jot 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 one, in which we'll look at the electrolysis of brine, and then at the second learning cycle, we look at the reactions at the electrodes.

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

Brine is a concentrated aqueous solution of sodium chloride.

And what's important is the actual amount of sodium chloride that is in the solution.

It has to be in the range between 3.

5% to 26% by mass to actually qualify to be brine.

Now, brine forms naturally due to the evaporation of saline water.

So, having salty water, the water actually evaporates off, the concentration of salt will then go up.

Sea water is salty, and we'll know that from when we've been to the sea, but it's not brine.

That's because the concentration of sodium chloride and sea water is below 3.

5%.

Brine is generated in the mining of sodium chloride.

Deposits of salt, sodium chloride, are found under layers of rock.

And during the mining, we need to get that out.

So, what happens is a shaft is drilled into rock, so that's like a big drilling going right the way through all the layers of rock.

Then, water and air are pumped into that shaft, and this dissolves the salt.

And as it dissolves, the brine is forced back up the shaft by the change in pressure.

Then, the brine is collected.

So, that's one of the ways in which brine is mined.

Brine has many uses.

It can be used for a food preservative, and you might have come across tinned tuna that is stored in brine.

And when you open it, it's sort of just like a watery solution that the tuna is in.

It's a source of chemicals.

So, table salt is sodium chloride crystals.

So, brine will basically evaporate all the water off and you'll be left with our sodium chloride.

But equally, raw materials reducing other chemicals such as chlorine gas comes from the chloride, and caustic soda, which is sodium hydroxide.

And both of these can be produced using brine.

So, which of the following is the most accurate definition of brine? A, is it salty water? B, Is it sodium chloride solution? C, an aqueous solution containing between 3.

5 and 26% sodium chloride.

Or D, a concentration of aqueous solution of sodium chloride.

So, we're wanting the most accurate definition of brine.

Well done if you chose C.

That's the correct answer.

So, the chemical formula of brine is NaCl.

And it's dissolved in water, so we have the aq in brackets.

So, the cations present are the Na+ and the H+ ions.

The anions present are the Cl- ions and the OH- ions.

Remember, the H+ and the OH- ions come from the water.

During the electrolysis of brine, there are two cations present.

We've just said that the Na+ and the H+, and they are in competition at the negative electrode or the cathode.

So, if we look at the animation, we can actually see the negative electrode is attracting two positive ions, the blue ones and the grey ones.

Two anions are present.

That's the chloride and the OH-, and they are in competition at the positive electrode or the anode.

Again, look at the animation.

At the positive electrode, we'll see the green, and the pale blue negatively charged ions are being attracted to them.

So, that's the first point to remember.

The ions are in competition at the electrodes.

So, during the electrolysis of brine, using inert electrodes, a gas is produced at both the anode and the cathode, and the two gases are different.

So, if we set up our electrolysis cell, we will actually expect to see bubbles of gas being formed at both the anode and the cathode.

At the cathode, hydrogen will be produced because sodium is more reactive than hydrogen in the reactivity series of metal, so that reactivity series comes into play again.

At the anode, chlorine gas will be produced because a halide is present.

So, it's really important to remember those rules that we learned about in the last lesson.

Sodium hydroxide solution forms in the electrolyte solution, making it alkaline.

So, at the end of the reaction, we will find that we have sodium hydroxide present as well.

Quick check for understanding.

True or false? During the electrolysis of brine, hydrogen is produced at the cathode.

Well done if you picked true.

That is correct.

Let's have a look at the reason why.

Is it A, sodium is more reactive than hydrogen, so sodium forms at the cathode? Or B, sodium is more reactive than hydrogen, so hydrogen forms at the cathode.

Well done if you chose B, that is correct.

And remember, we keep on having to go back to that reactivity series of metals to get these answers right.

So, testing for hydrogen gas.

To test a sample of gas to see if it's hydrogen, we want to hold a burning splint to the end of an inverted test tube.

And if the gas is hydrogen, it will burn with a squeaky pop.

And if you listen carefully now, you will hear that pop.

(gas popping) (gas hissing) That's something to look out for.

So, our test for chlorine gas.

Place a piece of paper, a piece of blue litmus paper in the gas.

If the gas is chlorine, the blue litmus paper will turn red and then white as it is bleached.

The remaining ions, that is those that don't get discharged at the electrode, form sodium hydroxide solution.

They remain in the electrolyte in the beaker.

And we can test for the alkaline solution to see that it will turn red litmus paper blue when we have an alkaline solution.

Okay, another quick check for understanding.

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

How would you confirm gas is chlorine? So, which test would you do? A, put some damp blue litmus paper into the gas and see if it turns red, then white.

B, place a glowing splint into the test tube and see if it re lights.

C, holds a burning splint at the end of the test tube and see if it pops.

A, B, or C? Well done if you chose A.

That is the correct test for chlorine gas.

So, now we have our first task.

Question one, identify where the following statements about electrolysis of brine are true or false.

So, on your worksheet, you basically need to put a tick in either column for each statement.

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

So, the first statement, brine contains only Na+ and Cl- ions.

Well, that is false.

It's a solution.

So, there is also water present.

So, that means that statement two, brine contains Na+, Cl-, H+ and OH- ions are true.

We must remember in any solution, there's H+ and OH- ions as well as the salt ions.

A gas is formed as each electrode.

That is true, because we have a reactive metal.

A silver deposit is observed on the cathode.

That is false because there is no metal below hydrogen in the reactivity series.

Sodium atoms are attracted to the cathode.

No, that is false.

Only ions are attracted, not atoms. The electrolytes becomes alkaline during the reaction.

That is correct.

As sodium hydroxide is left behind.

And Cl- ions are attracted to the anode.

That is also true.

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

That's brilliant.

So, we're gonna move on to question two now, and some pupils are discussing why hydrogen and chlorine are formed at the electrodes during the electrolysis of brine.

So, I want you to read these statements and decide which pupils are correct, and explain your answer.

So, let's just have a look at them together.

To start with, Aisha, she says, "Chloride ions are attracted to the positive electrode and form chlorine molecules." Andeep says, "The carbon electrodes react with hydroxide ions forming hydrogen." Lucas says, "The sodium ions sink to the bottom of the cell, so the hydrogen ions go to the negative electrode." Sofia says, "The hydrogen ions are attracted to the negative electrode." So, pause the video, have a go at the question, and then we'll look at the answer together.

All right, well, Aisha is correct.

The chloride ions, the Cl- ions, are attracted to the positive electrode and form chlorine molecules.

So, well done if you got that right.

Lucas, well, Lucas is incorrect here.

The sodium ions remain in the electrolysis cell.

They do not sink, okay? The sodium ions will be attracted towards the negative electrode, but they won't actually sink.

There's competition at the negative electrode between sodium ions and hydrogen ions.

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

Andeep, well, I'm afraid Andeep is incorrect as well.

The carbon electrodes do not take place in the reaction.

They are inert.

The reason they're there is to allow charge to flow around the circuit.

So, well done if you disagreed with Andeep.

And Sofia, she is correct, but we can also add that hydrogen molecules are formed as well.

So, at the hydrogen ions are attracted to the negative electrode, they're discharged to become hydrogen atoms, which then combine to form hydrogen molecules.

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

That was a tricky question.

So, that brings us to the end of our first learning cycle, and we're now going to move on to have a look at the reactions at the electrodes.

So, in the electrolysis of brine, hydrogen always forms at the cathode, that negative electrode, and chlorine always forms at the anode, the positive electrode, And at the electrodes, an either an oxidation or reduction reaction occurs.

So, just revisiting what these mean, oxidation is a type of reaction in which a substance either loses the electrons or gains oxygen.

Reduction is a type of reaction in which a substance either gains electrons or loses oxygen.

And when we're talking about electrolysis, we're gonna be thinking about the definition that has electrons in it.

A useful way to remember what happens to the electrons during oxidation and reduction is to use the OIL RIG, an acronym.

Oxidation Is a Loss of electrons, that's your OIL, RIG, Reduction Is Gain of electrons.

It's a really useful way to remember them, and that's something that I still use today.

A quick check of understanding.

Which of the following definitions of oxidation are correct? A, gain of oxygen, B, gain of electrons, C, loss of electrons, D, when a substance burns.

Well done if you chose A.

it's gain of oxygen, And well done if you chose C, loss of electrons.

Both of these are correct definitions.

So, during electrolysis of brine, oxidation occurs at the anode.

The chloride ion, that's Cl-, is an anion.

So, it's attracted to the positive electrode.

And you can see this in the animation, the green chlorides are attracted to the positive electrode.

There, it loses one electron, it's oxidised to form a chlorine atom.

The chlorine atoms then join together and form a chlorine molecule.

And again, in this animation, if you watch it carefully, you can see the electrons moving around the wire and the chlorine molecule being formed.

So, what happens at the cathode? We have a reduction reaction.

The hydrogen ion is a cation, so it's attracted to the cathode.

You can see that on the animation in blue.

It gains one electron.

It's reduced to form a hydrogen atom.

And then the hydrogen gas is formed at the cathode when two hydrogen atoms combine to form a hydrogen molecule.

So, it's important that to notice that in this animation, you can see it quite clearly.

The H+ ions and the Na+ ions are attracted towards the negative electrode, but the H+ ions win the competition.

They take the electrons and we get hydrogen gas formed.

Quick check for understanding.

What is formed at the anode during the electrolysis of brine? Is it sodium metal, chlorine gas, or sodium hydroxide solution? Well done if you picked B, it's chlorine gas.

Excellent.

So, the electrode reactions can be written as word equations, and this sometimes helps us to understand and think through what is actually happening.

So, at the anode, the chloride ion, becomes the chlorine atom plus an electron.

Then, two chlorine atoms join together to form a chlorine molecule.

The word equation helps us to show that oxidation reaction has taken place.

Remember, oxidation is a loss of electrons.

At the cathode, this time we have a hydrogen ion.

It collects or gains an electron to make hydrogen atom.

So, hydrogen ion plus electron gives a hydrogen atom.

Then, two hydrogen atoms react together to make a hydrogen molecule.

The word equation helps us to show that this is a reduction reaction.

Remember, reduction is gain of electrons.

So, quick check for understanding.

What is left in the electrolysis cell at the end? Is it A, water, B, hydrogen, C, sodium hydroxide solution, or D, chlorine? Well done if you chose C.

It's sodium hydroxide solution.

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

And for the first question, what we'd like to do is fill in the gaps.

So, complete the description of reduction and oxidation reactions taking place during the electrolysis of brine using the words, atom, ion, molecules, and electrons.

So, pause the video and have a go at the question.

And then when you're ready, we'll answer it together.

Reduction is the gain of electrons.

At the cathode, each positive hydrogen ion gains one electron to become a hydrogen atom.

Hydrogen atoms join to become diatomic hydrogen molecules.

Oxidation is the loss of electrons.

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

Chlorine atoms joined to become diatomic chlorine molecules.

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

It's really important that you get these right, the words right for different types of particles that are involved.

Excellent work.

All right, question two, which of the following statements describe what happens during the electrolysis of brine? Put a tick by the correct answer and amend the incorrect ones.

Pause the video and have a go, and then we'll look at the answer together.

Sodium ions are reduced to produce sodium atoms. No, they're not.

Sodium ions remain in the electrolysis cell at the end of the reaction.

So, that was incorrect.

Chloride ions are oxidised to produce chlorine atoms. Yep, that is correct.

So, well done if you've got that.

Hydrogen ions are reduced at the cathode.

Yes, they are.

Give that one a tick.

Chloride ions are oxidised at the cathode.

No, chloride ions are oxidised at the anode.

It's important that we get those the right way around.

OH- ions remain in the electrolysis cell at the end of the reaction.

Yeah, that is correct.

They do.

Hydrogen atoms combine with chlorine atoms to produce hydrogen chloride molecules? No.

What happens is hydrogen atoms form hydrogen molecules and chlorine atoms form chlorine molecules.

So, well done if you got those right, and especially well done if you got the explanations and were able to amend those incorrect statements.

So, that brings us to the end of our lesson.

So, we'll just summarise the key learning points about the electrolysis of brine.

First of all, there are four ions present in a solution of brine, H+, OH-, Na+, and Cl-.

The product at the cathode, the negative electrode, is hydrogen gas.

The product at the anode, which is a positive electrode, is chlorine gas.

Sodium hydroxide solution forms in the electrolyte solution, making its alkaline.

Two types of ions are in competition at each electrode.

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