Bitesize and Sounds revision podcasts | Overview
Revise GCSE Chemistry by listening to these GCSE podcasts from Bitesize and BBC Sounds.
Listen on the BBC Sounds app when you're out and about, or listen at home as part of your GCSE exam revision. BBC Sounds is where you can catch the latest music tracks, discover binge-worthy podcasts or listen to live radio stations, all in one place.
Let podcast presenters Dr Sunayana Bhargava and Tulela Pea prepare you for your Chemistry exam by guiding you through the key facts about chemical changes.
Episodes are roughly ten minutes long and there are eight episodes in the series.
Supercharge your revision with more podcasts for GCSE Physics and GCSE English Literature.
Making notes as you listen can help you remember the key points before the exam.
Episode 1 - The pH scale
In this episode, Dr Sunayana Bhargava and Tulela Pea define acids, alkalis and the pH scale. They look at the difference between strong, weak, concentrated and dilute acids.
Sunayana and Tulela define acids, alkalis and the pH scale.
TULELA: I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.
TULELA: And this is Bitesize Chemistry. This is the first episode in an eight-part series on chemical changes.
SUNAYANA: In this episode, we’ll be looking at what makes an acid an acid, and an alkali an alkali.
TULELA: As well as the pH scale, strength and concentration of acids.
SUNAYANA: We’ll place acids, alkalis and neutral solutions on the pH scale.
TULELA: Going from 0 to 14 real quick!
SUNAYANA: And as always, we’re aided by our AI chatbot with attitude, NNICK.
NNICK: Oh I love chemistry, I adore it! Divine chemistry!
TULELA: Feel free to hit pause along the way where you need to write things down and draw some diagrams, or rewind to listen again so that those key facts stick.
SUNAYANA: Acids and alkalis are everywhere around us – for example, there’s hydrochloric acid in our stomachs, and we put acid in the form of vinegar on our chips. Tulela, can you think of any other examples?
TULELA: Yeah, there are some alkalis in the cleaning products we use in our kitchen.
SUNAYANA: But what makes an acid an acid and what makes an alkali an alkali?
TULELA: A good definition is that acids are simply substances that generate hydrogen ions when dissolved in an aqueous solution, ie water.
SUNAYANA: Let’s unpack that a bit – if a chemical contains hydrogen, which can be released as ions in water, then it is an acid.
TULELA: So for example when hydrogen chloride – or HCl – is added to water, that’s the aqueous solution, it dissolves and the bonds between the hydrogen and chlorine break down and the hydrogen ions H-plus are formed. That means they dissociate, and so the solution is acidic.
SUNAYANA: So an acid is a substance that produce hydrogen ions when placed in aqueous solution.
TULELA: Other examples are H2SO4 or sulfuric acid, and HNO3 or nitric acid. These are all examples of strong acids because the bonds in the molecules break up completely to produce a high concentration of hydrogen ions in the solution. Some acids, like ethanoic acid, are weaker because the hydrogen ions only partially break away in water.
SUNAYANA: So the difference between a strong and weak acid depends on the extent of how they ionise, or break apart, to produce hydrogen ions in water. Those that completely ionise or dissociate are strong. Those which are only partially ionised are weaker.
TULELA: Imagine in a netball game in which I was a strong player – quite effective at passing the ball. Let’s say I was the chlorine in hydrochloric acid and the ball is the hydrogen that I was good at always getting rid of. I was like the strong acid which fully release its H-plus ions away in the water.
SUNAYANA: Well if I was a weak player, more like ethanoic acid, then I wasn’t that good at passing, making me a weak acid which mostly just holds onto those H-plus ions and releases fewer of them in the water.
TULELA: Let go of that ball, Sunayana!
SUNAYANA: So strong acids are completely ionised, which means the H-plus ions completely break away in water – and weaker acids only partially ionise. That’s acids. What about alkalis?
TULELA: Alkalis are substances that produce hydroxide ions – or OH minus ions – when dissolved in water.
SUNAYANA: So for example, when sodium hydroxide NaOH is dissolved in water, the bonds between the positive sodium ion and the negative hydroxide OH minus ion break away from each other.
TULELA: And so sodium hydroxide is an alkali. As is potassium hydroxide as both of these chemicals dissolve in water and produce hydroxide ions.
So quick recap: acids produce hydrogen ions in water. Alkalis produce hydroxide ions in water.
SUNAYANA: Stronger acids are those where the hydrogen ions dissociate more fully. Weaker acids are those where the hydrogen ions only partially dissociate in water.
TULELA: Sunayana, time for an acid test?
SUNAYANA: Ready, go for it.
TULELA: OK, I’ve got five food and drink substances here – a cola drink, a jar of peanut butter, mushrooms, a bottle of tap water and a grapefruit and I want you to guess where to place them from most acidic to most alkaline. Have a think for yourselves dear podcast listening friend – maybe hit pause before Sunayana gives the answer… 3,2,1…
TULELA: So that list again – a cola drink, a jar of peanut butter, mushrooms, a bottle of tap water and a grapefruit. Sunayana, over to you.
SUNAYANA: What if I told you that it was going from acidic to alkaline: cola, grapefruit, peanut butter, tap water, mushrooms.
TULELA: I’m amazed by your talents, Sunayana!
SUNAYANA: No need to be.
TULELA: And that’s right! Because we know that we can measure how acidic or alkaline a substance is.
SUNAYANA: And we’re talking about the pH scale, of course.
TULELA: Spot on! Hi NNICK – can you give us a quick summary of the pH scale?
NNICK: The pH scale, hmm, I've got it somewhere, I'll check my RAM. [BLEAT] Yes he’s fine. Let’s see – P P P P P P … pH.
A summary of the pH Scale. The pH Scale is a series of numbers which indicate how acidic or alkaline a substance is. It ranges from 0 to 14, where 0 is very acidic and 14 is very alkaline. The middle value, pH 7, shows a substance is neutral. In pH, the H is upper case and stands for hydrogen. The p is lower case and nobody really knows what it stands for. Though it could be thought of as power. So pH could be thought of as “power of hydrogen”. This is not to be confused with the “power of love”. Whatever that is. It's not that being a hyperintelligent electronic algorithm I shun love exactly… I simply don't have the time for it darling. But I am open to offers.
TULELA: Thanks NNICK!
SUNAYANA: So pH …
TULELA: little p, big H
SUNAYANA: Measures how acidic or alkaline a solution is from 0 to 14.
TULELA: So pure water, that’s pH 7 and neutral. Below 7, the more acidic…
SUNAYANA: …and above 7, the more alkaline. And you may well have seen small strips of coloured paper and dipped it in a solution and it watched it change colour.
TULELA: That’s universal indicator paper. The colours from yellow to red indicate a solution is acidic with pH below 7. So red indicates a strong acid – pH 1 or 2. Whereas the colours blue to violet indicate an alkaline solution with pH above 7 and a green colour indicates a solution is neutral at pH 7.
SUNAYANA: Let’s look at what’s actually going on with those hydrogen ions to make those numbers go up and down.
TULELA: You might want to grab a pen and paper for this bit. You remember that we talked about acids being able to release Hydrogen H-plus ions. Well, the more concentrated those H-plus ions are in the aqueous solution
SUNAYANA: …the water…
TULELA: …the lower the pH. Every time we multiply the number of Hydrogen ions by 10 – making it more concentrated by a factor of 10 – we reduce the pH by 1.
SUNAYANA: So that it becomes more acidic.
TULELA: And every time we decrease the number of hydrogen ions by a factor of 10, by diluting it – we increase the pH by 1.
SUNAYANA: It becomes less acidic.
TULELA: So, increase the concentration by a factor of 10, decrease the pH by 1.
SUNAYANA: And decrease the concentration by a factor of 10, increase the pH by 1.
SUNAYANA: Tulela, have you ever been stung by a bee?
TULELA: Yeah, it’s not very nice.
SUNAYANA: So bee stings are around pH 5, so slightly acidic and some people claim that if you rub on an alkali cream you can relieve some of the pain. Although it’s not really the acid causing the stinging pain, adding an alkali to an acid is a good example of a neutralisation reaction.
TULELA: In those neutralisation reactions where we add an acid to an alkali, the hydrogen ions…
SUNAYANA: …the H-pluses from the acid…
TULELA: …combine with the hydroxide ions…
SUNAYANA: …the OH-minus from the alkali…
TULELA: …to produce water –
SUNAYANA: H2O!
SUNAYANA: So a quick summary of acids and alkalis, Tulela?
TULELA: You bet.
SUNAYANA: Acids produce positive hydrogen ions (H-plus) in aqueous solution.
TULELA: Alkalis produce hydroxide ions (OH-minus) in aqueous solution.
SUNAYANA: Strong acids are those where the hydrogen ions completely dissociate.
TULELA: In weaker acids H-plus ions partially dissociate.
SUNAYANA: The pH…
TOGETHER: …little p, big H!
SUNAYANA: The pH scale from 0 to 14 measures how acidic or alkaline a solution is.
TULELA: pH 7 is neutral and the lower the number below 7 the more acidic.
SUNAYANA: The higher the number above 7 the more alkaline.
TULELA: If we increase the concentration of hydrogen ions by a factor of 10 we reduce the pH by 1, making it more acidic.
SUNAYANA: And if we dilute the concentration of hydrogen ions by a factor of 10 we increase the pH by 1, making it more alkaline.
TULELA: And in neutralisation reactions between acids and alkalis, the hydrogen ions from the acid combine with the hydroxide ions from the alkali to produce water.
SUNAYANA: You can listen on BBC Sounds for other episodes in this series as well as many more Bitesize podcasts.
TULELA: Thanks for listening!
TOGETHER: Little p, big H!
Listen on BBC Sounds
Question
What ions are produced by an acid in an aqueous solution?
Acids produce positive hydrogen ions (H+) in an aqueous solution.
Episode 2 - The reactivity series of metals
In this episode, Dr Sunayana Bhargava and Tulela Pea explore the reactivity series of metals and displacement of metals from a compound.
Sunayana and Tulela explore the reactivity series of metals and displacement of metals.
SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.
TULELA: And I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And this is Bitesize Chemistry. This is the second episode in an eight-part series on chemical changes.
TULELA: In this episode, we’re going to be looking at the reactivity series of metals, their reaction with acids and water and how a more reactive metal can replace a less reactive one from a compound.
SUNAYANA: As always it might be handy to write some notes or diagrams along the way, so hit pause where you need to. Don’t worry, we’ll still wait for you to hit play again.
TULELA: And also hit rewind if you need to go over some of those key facts. And remember to head over to BBC Bitesize on the web for more useful information and diagrams.
SUNAYANA: So before we delve a little deeper into what makes one metal more reactive than another, let’s look at why it’s so important to know about the reactivity of metals anyway.
TULELA: Well for one, knowledge of the reactivity series helps us to select appropriate materials for specific applications. For example, using a less reactive metal in a corrosive environment can extend the lifespan of a structure. And there are loads of other everyday examples of how it helps us to make informed decisions about what materials to use.
SUNAYANA: Let’s get a little background on this from our binary banter-bot NNICK. Hi NNICK, can you give a summary about the reactivity series of metals?
NNICK: The reactivity series of metals is a list of metals showing how reactive they are in comparison to one another. It's a bit like a football league table for metals – except without the football teams, the promotions, the relegations or the blubbing.
When metals react with other substances, they lose electrons to form positive ions. Reactive metals lose their electrons easily. Less reactive metals don't give up their electrons as easily, and so react with other elements more slowly, or not at all.
SONG
NNICK: Dear reactive metal, would you love to give your electrons away?
REACTIVE METAL: Wa hey!
NNICK: I'll take that as a yes.
Dear reactive metal, your chemical reactions are vigorous. Is that true?
REACTIVE METAL: Woo hoo!
NNICK: I'll take that as a yes.
Dear unreactive metal, would you love to let your electrons go?
UNREACTIVE METAL: No.
NNICK: I'll take that as a no.
Dear unreactive metal, do your chemical reactions go rapidly?
UNREACTIVE METAL: No.
NNICK: I'll take that as a no.
Dear reactive metal, are you at the top of the reactivity series?
REACTIVE METAL: Yippee!
NNICK: I'll take that as a yes.
Dear unreactive metal, are you at the top of the reactivity series?
UNREACTIVE METAL: No.
NNICK: Shall I take that as a no?
UNREACTIVE METAL: Er…y…yes?
TULELA: Thanks NNICK. So, the reactivity of a metal, or how vigorously it reacts, is related to its tendency to lose its outer shell electrons and form positive ions.
SUNAYANA: Cations.
TULELA: Correct – cations. The more reactive, the easier it forms cations and the higher in the series. The less reactive, the lower in the series.
SUNAYANA: Some versions of the reactivity series include more metals than others but the metals are always in the same place relative to each other. A typical one might include potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper.
TULELA: And sometimes hydrogen and carbon are included even though they are non-metals as it helps us compare just how reactive the metals are. And we’ll see why this is useful in the episodes on metal extraction and electrolysis.
SUNAYANA: It might be useful to press pause and look at the reactivity series on the Bitesize website for the next section.
SUNAYANA: So how do we go about creating the list, I hear you ask.
PAUSE
SUNAYANA: I said, I hear you ask.
TULELA: Sorry – how do we go about creating the list, Sunayana?
SUNAYANA: Thanks! Well, we can compare how each of the metals reacts with water or a dilute acid.
TULELA: Let’s take that list from before. And if we add a small piece of each metal in turn into a test tube with dilute hydrochloric acid we’ll see that those at the top of the list, the most reactive metals like potassium and sodium will react very vigorously, producing hydrogen.
SUNAYANA: And if we hold a burning splint to the test tube, we’ll hear a squeaky pop. [LOUD POP] The more reactive the metal, the more hydrogen has been made and so the louder the pop.
TULELA: Whereas those which are less reactive like the zinc and iron react very slowly with the acid, with a very quiet squeaky pop. [QUIET POP]
SUNAYANA: What about metals which are less reactive than hydrogen itself?
TULELA: Good point. So, metals such as copper won’t react at all in the dilute hydrochloric acid and so no squeaky pop at all – and so they are placed below hydrogen in the list.
SUNAYANA: That’s testing the metal with a dilute acid. We can also test to see how the metals react with water. And in the same way we’ll find that those which are most reactive – like potassium and sodium – react vigorously.
TULELA: Again, hydrogen is released in this reaction which we can show with our squeaky pop test. [LOUD POP]
SUNAYANA: And less reactive metals such as zinc and iron react only very slowly, or not all in the case of copper.
TULELA: So, we’ve reacted a metal with water and dilute acids to see how reactive it is. But now we can also react a metal with a solution of a different metal compound to see how or indeed if it reacts at all.
SUNAYANA: For example, if we put an iron nail into a solution of copper sulfate, the iron which is more reactive will displace or kick out the copper from the sulfate and we’ll be left with iron sulfate and copper metal. And this is an example of a displacement reaction.
TULELA: So it’s a bit like if you have a dancing couple on the dance floor giving it some nice choreographed moves. They’re the compound. But along comes a much more energetic dancer who displaces one of the couple and off they go together in a new compound leaving the poor displaced dancer on their own.
SUNAYANA: Story of my life. But if we try and react a less reactive metal into a solution of a more reactive compound then nothing will happen.
TULELA: That less reactive solo dancer cannot displace the more energetic one.
SUNAYANA: Just as with the experiments with water and dilute acid, we can use displacement reactions to work out where in the series a metal can be placed. And there are some examples of these on the Bitesize Chemistry web pages, so make sure you check them out.
TULELA: So if a metal reacts with a solution of another metal compound, then it is more reactive and so higher in the reactivity series. And if there is no reaction then it is less reactive and therefore lower in the reactivity series.
TULELA: Time for a quick-reaction quiz about quick reactions?
SUNAYANA: I see what you’ve done there, Tulela. OK go for it.
TULELA: OK podcast listening friends, three questions, five seconds each before we give you the answer. Or hit pause if you need a little more time to think about it. Here’s question 1. In the reactivity series, what does it mean if one metal is placed above another?
SUNAYANA: The metal above is more reactive.
TULELA: Question 2. Potassium is higher in than reactivity series than calcium. What can we say about the how easily potassium and calcium form ions compared to each other?
SUNAYANA: Potassium is more reactive, so it loses its outer shell electrons more easily than calcium to form a cation.
TULELA: And question 3. In a reaction between zinc metal and magnesium chloride, nothing happens. What does that mean?
SUNAYANA: That zinc is less reactive than magnesium and so is below it in the reactivity series. Hope you got all of those right. Here’s a quick summary of this episode – feel free to write these down if that helps you remember them.
TULELA: The reactivity of a metal is related to its tendency to form positive ions – cations.
SUNAYANA: We can put metals in order of their reactivity from their reactions with water and dilute acids. Remember the squeaky pop test for hydrogen. [LOUD POP]
TULELA: And a more reactive metal can displace a less reactive metal from a compound.
SUNAYANA: There’s loads more about reactivity series and displacement reactions on the Bitesize webpages, or you can listen on Sounds to other chemistry topics in this series.
TULELA: Bye!
SUNAYANA: See ya!
Listen on BBC Sounds
Question
In the reactivity series, what does it mean if one metal is placed above another?
If one metal is placed above another, it means the metal above is more reactive.
Episode 3 - Salt formation
In this episode, Dr Sunayana Bhargava and Tulela Pea explore chemical reactions that form salts and how they are named.
Sunayana and Tulela explore chemical reactions that form salts and how they are named.
TULELA: I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.
TULELA: And this is Bitesize Chemistry. This is the third episode in an eight-part series on chemical changes. In this episode we’re going to look at salt formation and naming. Not just sodium chloride, the salt you put on your chips, but all sorts of other salts nearly all of which you definitely do not want to put on your chips!
SUNAYANA: Will we look at how acids react with metals to produce salts?
TULELA: Yes, indeed we will! As well as how acids react with metal hydroxides, metal oxides and metal carbonates to produce salts.
SUNAYANA: And we’ll look at how we name those salts and how we can crystallise salt solutions to produce solid salts.
TULELA: And as always we’ll round it off with a quick quiz and summary of all the most important facts. Ready with your pen and paper to make notes?
SUNAYANA: And hit pause and rewind when you need to, to have a little more time to let those key facts really sink in.
TULELA: Before we get into the nitty gritty of salt formation, let’s look at why this is an important process in chemistry. Time to get some insight from our Neural Networked Intelligent Computer Knowledge-bank – NNICK. Hi NNICK, can you tell us a little bit about salts please?
NNICK: Well, let's see. Ah, well a salt is any compound formed by the neutralisation of an acid by … [FUNKY BASS] Yes, a base. Obviously that's the wrong kind of bass, but it is slightly funkier than a hydroxide ion. Salts have many many many uses, for example in water treatment, medical applications and the food industry. How important is salt formation in those fields? Well, on a scale of 1 to 2, where 1 is "not at all important", and 2 is "so very very important, you wouldn't believe how incredibly important", I would say 2.
SUNAYANA: Thanks NNICK! In episode 1 of this series we looked at acids and alkalis and these are the key to salt formation. Have a listen to that if you need a quick recap.
TULELA: And we begin by taking an acid and reacting it with certain metals. Always good to have an example up your sleeve – so let’s go with hydrochloric acid and magnesium. When we add these two together, we find that they react to form the salt magnesium chloride and hydrogen gas. We can confirm this by holding a burning splint near to the gas and if we hear a squeaky pop, [POP SOUND] that will indicate the presence of hydrogen.
SUNAYANA: Another example is zinc reacting with sulfuric acid. And in this case the product of the reaction will be the salt zinc sulfate and hydrogen gas.
TULELA: So in general, if a metal reacts with an acid then the products are a salt and hydrogen. Acid plus metal forms salt plus hydrogen.
SUNAYANA: If the acid is hydrochloric acid then the salt is a chloride. If the acid is sulfuric acid then the salt is a sulfate. And if the acid is nitric acid then the salt… Tulela?
TULELA: Nitrate.
SUNAYANA: OK. That’s one way to form a salt. Another is when we neutralise an acid with an alkali or base. Let’s start with alkalis.
TULELA: Whoa – hang on! Let’s summarise the difference between alkalis and bases.
SUNAYANA: Sure. So all bases can neutralise acids but alkali is the term we use if the base is soluble – that is, it can dissolve in water. Bases that are insoluble are still called bases.
TULELA: So all alkalis are bases, but not all bases are alkalis. You may continue…
SUNAYANA: Thanks. So, when we neutralise an acid with an alkali we produce a salt and water. For example, sodium hydroxide NaOH is an example of an alkali.
TULELA: And when we react this with hydrochloric acid, we produce sodium chloride – that is the salt, and water. So acid plus alkali forms salt plus water.
SUNAYANA: And the name of the salt produced has two parts. The first part comes from the name of the metal in the metal hydroxide, and the second part comes from the acid used. Remember how the naming works!
TULELA: Hydrochloric acid produces chlorides, sulfuric acid produces sulfates and nitric acid produces nitrates.
SUNAYANA: Sounds reasonable to me.
TULELA: Perhaps dear podcast listening friend you’d like to have a go at this example. What is the name of the salt produced when we neutralise sulfuric acid with potassium hydroxide? You have five seconds before my dear amigo Sunayana gives the answer… in 3,2,1.
SUNAYANA: OK, so take the metal from the hydroxide and add that to the type of acid used and we get potassium sulfate – hope you also got that.
TULELA: Let’s also have a look at some other bases – metal carbonates and metal oxides. In the case of oxides, the products are the same as for hydroxides – salt and water. For example copper oxide, which is insoluble so is simply a base, reacts with nitric acid to form copper nitrate and water. What about metal carbonates?
SUNAYANA: Acids plus metal carbonates react to form a salt, water and carbon dioxide. So, for example, hydrochloric acid plus copper carbonate form the salt copper chloride plus water plus carbon dioxide.
TULELA: Quick summary of all those reactions then. Acid plus metal form salt plus hydrogen.
SUNAYANA: Acid plus metal hydroxide form salt plus water.
TULELA: Acid plus metal oxide also form salt plus water.
SUNAYANA: And acid plus metal carbonate form salt plus water plus carbon dioxide.
TULELA: Whew!
SUNAYANA: One practical activity that you may be asked to describe is an experiment that forms crystals of a salt from acids and bases. Let’s use as an example our base copper oxide and react it with sulfuric acid.
TULELA: Remember that as copper oxide is an insoluble base, it’s a solid black powder. We slowly add this power to the acid to produce…
SUNAYANA: …acid plus metal oxide forms salt and water.
TULELA: Correct. So in this case the salt would be copper sulfate, which is soluble in water as a blue aqueous solution. Keep adding the copper oxide until there is no more reaction. We can tell this has happened because excess copper oxide, the black powder, will remain at the bottom. Now we need to filter off any remaining solids from the solution.
SUNAYANA: Funnel – filter paper – done. Collect the filtrate, which you’ll remember is the solution that has passed through the filter paper – in this case the blue solution.
TULELA: And finally to get crystals of copper sulfate out of the solution, we use one of the separation processes we talked about in series one – crystallisation.
SUNAYANA: Which you may recall is to gently heat our solution in an evaporating dish until most of the water evaporates away, and then leave the rest to evaporate at room temperature. The slower the evaporation, the larger the beautifully blue copper sulfate crystals that are formed.
TULELA: Time for a quick salt-related quiz. Three questions, 5 seconds each. If you need more time, hit pause and do write the answer down dear podcast listening friend.
SUNAYANA: Question 1. What is formed when zinc metal reacts with sulfuric acid?
TULELA: Acid plus metal forms salt plus hydrogen. And the salt is zinc sulfate.
SUNAYANA: Question 2. Let’s add calcium hydroxide to hydrochloric acid. What do we get?
TULELA: Acid plus metal hydroxide forms salt plus water. And the salt is calcium chloride.
SUNAYANA: And question 3. You add copper carbonate to nitric acid. What have you just formed?
TULELA: Acid plus metal carbonate forms salt, water and carbon dioxide. And the salt is copper nitrate.
SUNAYANA: Final summary, Tulela?
TULELA: Why not indeed.
SUNAYANA: Acids react with some metals to produce salt plus hydrogen.
TULELA: Acids react with metal hydroxides and metal oxides to produce salt plus water.
SUNAYANA: And acids react with metal carbonates to produce salt, water and carbon dioxide.
TULELA: To name the salt produced, take the name of the metal in the base and the type of acid used.
SUNAYANA: And to get crystals of a salt after a neutralisation reaction, filter and evaporate the resulting solution.
TULELA: Remember there’s loads more chemistry on the Bitesize website.
SUNAYANA: You can listen on BBC Sounds for other episodes in this series as well as many more Bitesize podcasts.
TULELA: Thanks for listening! See ya!
SUNAYANA: Bye!
Listen on BBC Sounds
Question
What is produced when an acid reacts with a metal carbonate?
When an acid reacts with a metal carbonate, salt, water and carbon dioxide are produced.
Episode 4 - Redox reactions
In this episode, Dr Sunayana Bhargava and Tulela Pea explore how metals react with oxygen to form metal oxides and introduce the idea of redox reactions.
Sunayana and Tulela explore how metals react with oxygen to form metal oxides.
SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.
TULELA: And I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And this is Bitesize Chemistry. This is the fourth episode in an eight-part series on chemical changes. In this episode, we’re going to look at processes known as redox reactions.
TULELA: Red-uction, which is the loss of oxygen.
SUNAYANA: And Ox-idation, which is the gain of oxygen.
TULELA: And we’ll look at how carbon can be used to extract pure metals from some metal oxides.
SUNAYANA: And we’ll have a quiz and summary of the main facts.
TULELA: Great, let’s go.
SUNAYANA: As always it might be handy to write some notes or diagrams along the way so hit pause when you need to. Don’t worry, we’ll wait for you to hit play again!
TULELA: And also hit rewind if you need to go over some of those key facts. And remember to head over to BBC Bitesize on the web for more useful information and diagrams.
SUNAYANA: Before we get into the details of the reaction between metals and oxygen, let’s get some background on why these reactions are relevant and important in the real world.
TULELA: Time for an overview from our AI chat-bot NNICK. Hi NNICK – can you tell us about the relevance of the reaction between metals and oxygen please?
NNICK: Ooh, I know the answer to this one! Pick me, pick me! Oh I see, you already picked me…
Many metals react with oxygen to make metal oxides. How enthusiastically metals react with oxygen depends on their reactivity. For example, magnesium is pretty keen, but gold can’t really be bothered. If the iron in your steel infrastructure reacts with oxygen and water it forms an iron oxide called rust. In summary, it’s like that old song…
SONG
You shouldn’t let that oxygen settle – on your metal.
Otherwise it’s no surprise – your metal may oxidise.
If building an infrastructure out of steel has appeal,
You won’t, I trust, want it to rust – and crumble into dust.
To protect your treasured metal creation from oxidation,
You’d need, I’d say, to find a way – to keep oxygen at bay.
Try coating, painting, galvanising and lubricating – or electroplating.
And as advised your metal prized, will stay unoxidised.
SUNAYANA: Thanks NNICK! So if something, in this case a metal, gains oxygen it is oxidised. A good example of this is iron. Take an iron nail and leave it in damp air for a while and it will rust. That rust is iron oxide. The iron has gained oxygen so it has been oxidised.
TULELA: Because iron rusts – or oxidises – so easily is the reason why pure iron is rarely found on Earth, but mostly in iron ores – those iron oxide compounds.
SUNAYANA: And why iron doesn’t rust in space, because of the lack of oxygen!
TULELA: You had to get that astronomy fact in there, didn’t you!
SUNAYANA: Always! But keeping on with that same idea is the reason why most metals are also found in the Earth’s crust as ores rather than in their pure metal form – because they oxidise easily.
TULELA: These ores are mainly oxide compounds, like iron oxide. But some are carbonates like calcium carbonate.
SUNAYANA: And an ore is simply a rock that contains enough of a metal to make it worth extracting that metal for profit.
TULELA: In episode 2 we looked at the idea of a reactivity series that lists metals in order of their reactivity. Have a listen back to that if you need a quick refresh. We saw that metals higher in the series are more reactive and metals lower in the series are less reactive.
SUNAYANA: In terms of oxidation this means that those higher in a series – like potassium, sodium and aluminium – oxidise much more easily than those lower like copper. So they form oxides, or corrode, much faster than those lower down.
TULELA: And the metals at the very bottom of the reactivity series – silver, gold and platinum – are all found in their pure form as they are so unreactive, they are resistant to oxidation.
SUNAYANA: So if you’re lucky to find some pure gold or silver then your job’s done. But if you want to extract a pure metal from one that does oxidise easily then you’ve got a little bit more work to do.
TULELA: This extra work is to remove the oxygen from the metal compound. And that’s the opposite of oxidation and is called reduction.
SUNAYANA: So oxidation is the gain of oxygen, and reduction is the loss of oxygen. And one way to reduce an oxidised metal is by using carbon – but only if the metal is below carbon in the reactivity series. That’s because carbon can only take away oxygen from metals less reactive than itself.
TULELA: That’s important to remember. If a metal is more reactive than carbon – ie higher in the reactivity series, then we need to use electrolysis to reduce it from its oxide. And we’ll talk about that in episode 7 of this series. For now, let’s think about metals that are less reactive than carbon.
SUNAYANA: Let’s keep with our iron oxide example and extract pure iron. Iron oxide, in this case Fe2O3 is added into a blast furnace – basically a huge and very hot container – to which is added carbon in the form of coke.
TULELA: And that coke is the carbon-based fuel – not the drink!
SUNAYANA: Absolutely. Because iron is lower in the reactivity series than carbon, the carbon reduces the iron ore to produce iron and carbon dioxide. You can find a diagram showing this and descriptions of intermediate steps in the process on the BBC Bitesize website.
TULELA: Hang on, Sunayana. If the iron oxide has been reduced by carbon and the product is iron and carbon dioxide, then has the carbon been oxidised?
SUNAYANA: Exactly! The carbon has gained oxygen which is the definition of oxidation. And this shows that every time a reduction reaction happens, an oxidation reaction also happens. Hence redox. You can’t have one without the other.
TULELA: Noted!
How about a quick redox quiz for our lovely podcast listening friends?
SUNAYANA: You took the words out of my mouth. Three questions, five seconds for you to ponder. Hit pause if you need extra time to write your wonderful answers down.
TULELA: I’m sure they’ll ace it. Question 1. Why is iron found in ores rather than as an uncombined element in the earth?
SUNAYANA: Because iron oxidises easily in air and moisture to form iron oxide – or rust.
TULELA: Question 2. Why can we use carbon to get pure iron from its ore?
SUNAYANA: Carbon is higher in the reactivity series than iron and so can reduce its ore by removing the oxygen.
TULELA: And question 3. Why can we find pure gold on earth?
SUNAYANA: Because it doesn’t oxidise to form ores as it is so unreactive.
TULELA: Hope you got those! A lump of gold if you did, but you have to find it yourselves.
SUNAYANA: And a rusty nail if you didn’t, which you can turn back to pure iron if you have a blast furnace handy.
SUNAYANA: Time for a summary, Tulela?
TULELA: Summarise away, Sunayana.
SUNAYANA: When something gains oxygen in a reaction it is oxidised.
TULELA: And when metal oxides lose oxygen they are reduced.
SUNAYANA: We can use carbon to reduce metal oxides only if the metal is less reactive than carbon in the reactivity series.
TULELA: In this case the metal oxide is reduced whilst the carbon is oxidised.
SUNAYANA: Oxidation and reduction always take place at the same time hence these are called redox reactions.
TULELA: In the next episode we’ll have more on oxidation and reduction but this time in terms of the exchange of electrons!
SUNAYANA: Exciting! See you then.
TULELA: Bye!
Listen on BBC Sounds
Question
What word is used to describe when a substance gains oxygen in a reaction?
When a substance gains oxygen in a reaction, it has been oxidised – this is called oxidation.
Episode 5 - Redox in terms of electrons
In this episode, Dr Sunayana Bhargava and Tulela Pea explore redox reactions in terms of gain or loss of electrons.
Tulela and Sunayana explore redox reactions in terms of gain or loss of electrons.
TULELA: I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.
TULELA: And this is Bitesize Chemistry. This is the fifth episode in an eight-part series on chemical changes.
SUNAYANA: In this episode, we’re going to look at redox reactions in terms of the gain or loss of electrons. And we’ll be saying “oil rig” over and over again. Because “oil rig” is a nice way to remember which is which.
TULELA: We’ll be looking at ionic equations in these displacement reactions.
SUNAYANA: We’ll be chatting about the use of half-equations to help us determine which of the substances involved are oxidised or reduced.
In the previous episode we looked at redox reactions – those in which oxidation and reduction was occurring – from the point of view of gain or loss of oxygen. Have a listen to that episode if you need a refresh.
TULELA: We defined oxidation as a gain of oxygen, and a reduction as the loss of oxygen. In many cases, we can indeed explain a reaction in terms of the change in oxygen content. But sometimes, especially when oxygen isn’t involved in the reaction, an explanation is needed in terms of electrons. Time to call our AI know-it-all for some background. Hi NNICK! Can you tell us why it’s useful to explain oxidation and reduction in terms of electron transfer please?
NNICK: Hmm – this sounds like a case for… Detective Inspector NNICK! I’ve gathered you here because you are all suspects in the case of missing oxygen.
CROWD: What?
NNICK: Yes. Last evening, magnesium was oxidised after reacting with chlorine to form magnesium chloride.
CROWD: [Gasps] No!
NNICK: And the question I put to you all, whoever you are, is – if magnesium was oxidised, where is the oxygen? One of you must have hidden it.
CROWD: No!
NNICK: What do you have to say for yourselves? Yes, you at the back with the squeaky voice.
SUSPECT: Oxidation is loss of electrons.
CROWD: [Gasps] What?
SUSPECT: Yes – magnesium lost electrons and was oxidised. While chlorine gained electrons and was reduced.
NNICK: So you’re saying that electron transfer is a useful way to understand oxidation, especially in cases like this where there’s no oxygen involved?
SUSPECT: Yes.
NNICK: Right, well, then you are all free to go.
CROWD: Hurray!
TULELA: Thanks NNICK. So if a substance loses electrons in a chemical reaction, then it has been oxidised. And if it gains electrons it has been reduced.
SUNAYANA: And this is where “Oil Rig” is a good way to remember – because “Oil Rig” O-I-L R-I-G stands for Oxidation Is Loss. Reduction is Gain. But only use “Oil Rig” when we are talking about electron loss and gain, NOT about oxygen.
TULELA: Got ya! OIL RIG only for loss and gain of electrons not oxygen!
SUNAYANA: Hi NNICK – can you give a summary about “oil rig”?
NNICK: Oil rig [CHORUS: O-I-L]. Oxidation is loss [CHORUS: of electrons]. Oil rig [CHORUS: R-I-G]. Reduction is gain [CHORUS: of electrons]. Oil rig.
SUNAYANA: Thanks NNICK – nice singing!
TULELA: Let’s have an example to illustrate this. How about describing the reaction between magnesium and copper sulfate solution in which the magnesium displaces the copper from the sulfate solution and we’re left with magnesium sulfate and copper.
SUNAYANA: So this is one of those displacement reactions that we talked about in episode 2 of this series – where a more reactive element displaces a less reactive one from a solution of its compound.
TULELA: Right – we start with the magnesium which is neutral. And end with magnesium sulfate. So in this case the magnesium has lost two electrons, becoming a magnesium 2+ ion in order to bond with the sulfate.
SUNAYANA: O-I-L – oxidation is loss of electrons.
TULELA: So the magnesium has been oxidised. And the copper, which was originally an ion in the sulfate solution has been displaced from the solution by gaining two electrons to become neutral copper.
SUNAYANA: R-I-G – reduction is gain of electrons.
TULELA: So the copper ions have been reduced. Maybe our dear podcast listening friends might want to try one which we can work through.
SUNAYANA: How about the reaction between aluminium and iron oxide which produces aluminium oxide and iron? Press pause whilst you think about what has been oxidised and what has been reduced in terms of electron transfer. But remember to press play again or we’ll be waiting for ages.
PAUSE
TULELA: Ah, there you are. We started with iron (or Fe) as a positive ion bonded to oxygen in the compound iron oxide and finished with neutral iron – so the iron ions have gained electrons…
SUNAYANA: R-I-G – reduction is gain of electrons.
TULELA: …and have been reduced. And so the aluminium which started as neutral but ended up losing electrons to form a positive aluminium ion in aluminium oxide and so…
SUNAYANA: O-I-L – oxidation is loss of electrons.
TULELA: …aluminium has been oxidised.
TULELA: Sometimes in these redox reactions it’s useful to look at what’s happening at the oxidation and reduction parts individually. And this is where this idea of half-equations come in.
SUNAYANA: The oxidation half-equation represents the reaction that involves the loss of electrons. And it shows the atom or ion that has been oxidized, the electrons lost, and the products formed.
TULELA: The reduction half-equation represents the reaction that involves the gain of electrons. And it shows the atom or ion that has been reduced, the electrons gained, and the products formed. It's important to ensure that the number of electrons lost in the oxidation half-equation equals the number of electrons gained in the reduction half-equation to maintain charge neutrality.
SUNAYANA: So let’s go back to our example from earlier with the reaction between magnesium and copper sulfate. For the oxidation half-equation we concentrate only on the magnesium as this is what has been oxidised to magnesium 2 plus ion. And our half-equation would be written as Mg (for magnesium) goes to Mg 2 plus, to show that it’s become an ion, plus 2 e minus showing the two electrons that have been lost.
TULELA: And in the reduction half-equation, we concentrate on only on the copper. This started off as copper ion which were reduced by gaining electrons to become neutral copper. And so we’d write this half equation as Cu (for copper) 2 plus, as it was an ion in the sulfate, plus 2 e minus (those two electrons) goes to Cu.
SUNAYANA: If you didn’t quite follow that, you can rewind and relisten or find it written down on the Bitesize website. Remember, half-equations show only the relevant atom or ion which either lose electrons in the oxidation half-equation or gain electrons in the reduction half-equation.
TULELA: If we do want to include all the ions in an equation – and sometimes that’s also useful to check whether all the charged balance out nicely – we use an ionic equation.
SUNAYANA: In an ionic equation, we combine the two half-equations but don’t include anything else that hasn’t lost or gained an electron. And these are called spectator ions.
TULELA: So remember the difference. Ionic equations show all the ions that change in terms of their charge in a reaction. And half-equations show what is happening to one atom or ion, whether it is being oxidised or reduced.
SUNAYANA: How about an electron transfer-based redox quiz, Tulela? Three questions, five seconds each, press pause and write down the answers.
TULELA: Question 1. In terms of electron transfers, what is meant by oxidation?
SUNAYANA: O-I-L – oxidation is loss of electrons.
TULELA: Question 2. In a reaction between copper sulfate and magnesium, why have the copper ions been reduced to copper atoms?
SUNAYANA: Because magnesium is more reactive, copper has been displaced. The copper ions have each gained two electrons and are now neutral. R-I-G – reduction is gain of electrons.
TULELA: And question 3. In an oxidation half-equation an atom has lost two electrons to form an ion. How many electrons have been gained in the reduction half-equation of the same reaction?
SUNAYANA: That would be two. Electrons lost in one half-equation will always balance out electrons gained in the other half-equation.
TULELA: Time for a summary, Sunayana?
SUNAYANA: We can define redox reactions in terms of electron transfers.
TULELA: Oxidation is the loss of electrons.
SUNAYANA: Reduction is the gain of electrons.
TULELA: These can be shown in half-equations. The oxidation half showing the atom or ion which has lost electrons and so has been oxidised.
SUNAYANA: And the reduction half, showing the atom or ion that has gained electrons and so has been reduced.
TULELA: Ionic equations show all the ions that are changing but miss out on any so-called spectator ions that don’t change.
SUNAYANA: You can listen on BBC Sounds for other episodes in this series as well as many more Bitesize podcasts.
TULELA: Thanks for listening!
SUNAYANA: Bye!
NNICK: Oil rig [CHORUS: O-I-L]. Oxidation is loss [CHORUS: of electrons]. Oil rig [CHORUS: R-I-G]. Reduction is gain [CHORUS: of electrons]. Oil rig.
Listen on BBC Sounds
Question
In a reaction between copper sulfate and magnesium, what is the half-equation for copper? Has the copper been reduced or oxidised?
Cu²+ + 2e- → Cu. Copper has been reduced – reduction is gain of electrons.
Episode 6 - Electrolysis of molten solutions
In the first of three episodes on electrolysis, Dr Sunayana Bhargava and Tulela Pea explore electrolysis of molten ionic compounds and define electrodes and electrolytes.
Tulela and Sunayana explore electrolysis of molten ionic compounds.
SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.
TULELA: And I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And this is Bitesize Chemistry.
TULELA: And this is Bitesize Chemistry. This is episode six in an eight-part series on chemical changes. In this episode, we’re going to look at electrolysis of molten solutions.
SUNAYANA: We’ll define what electrolysis is, on what molten compounds it can be used and what’s going on in terms of oxidation and reduction of the ions involved.
TULELA: As always it might be handy to write down some notes or diagrams along the way. So hit pause where you need to. Don’t worry, we’ll wait for you to hit play again!
SUNAYANA: And remember to head over to BBC Bitesize on the web for more useful information and diagrams.
TULELA: OK, we’re going to be spending the next three episodes getting to know electrolysis so that by the end of the series electrolysis will be your new best friend.
SUNAYANA: No one will replace you, Tulela.
TULELA: Aw, that’s nice. In episode 5 we looked at oxidation and reduction from the point of view of electron transfers in a reaction. And this is also very relevant to electrolysis. If you need a quick refresh of that, have a relisten to that episode and you’ll be fully up to speed.
SUNAYANA: Before we welcome electrolysis into our lives, let’s get some background on what it is and why it’s useful. Hi NNICK! Can you give us a brief introduction to electrolysis please?
NNICK: Electrolysis involves the decomposition of electrolytes – ionic compounds or solutions – into their constituent elements, or ions.
Imagine a molten ionic compound containing positive and negative ions. An electric current is passed through it using a pair of electrodes. Because the compound is molten, its ions are free to move.
Let's represent a negative ion, or anion, with this sound [BOING]
And let's represent a positive ion, or cation, with this sound [MIAOW]
If I go all the way over to one side [GOES LEFT]:
[MISERABLE VOICE] I'm the negative electrode, or cathode. And positive ions are attracted to me. [LOTS OF MIAOWS] I give them electrons and they are reduced. [MIAOW?]
And if I go over to the other side [GOES RIGHT]:
[HAPPY VOICE] I'm the positive electrode, or anode, and negative ions are attracted to me. [LOTS OF BOINGS]. They give me electrons and are oxidised. [BOING?]
Of course, all this is going on simultaneously, which would be more like this: [CACOPHONY OF BOINGS AND MIAOWS]
SUNAYANA: Thanks NNICK. So electrolysis is the process where an ionic substance is broken down using electricity. In fact, the word electrolysis comes from ‘electron’ meaning ‘from electricity’ and ‘lysis’ meaning ‘to split’. So literally means ‘to split using electricity’.
TULELA: Nice trivia, Sunayana.
SUNAYANA: That ionic substance is called an electrolyte and can be a molten substance…
TULELA: …remembering that a molten ionic compound is simply one that is melted into its liquid state.
SUNAYANA: Or an aqueous solution – that is, one where an ionic compound has been dissolved in water.
TULELA: The compounds have to be in this liquid form so that the ions are free to move and carry the charge. And in this episode we’re looking at the first type – molten.
SUNAYANA: Electrolysis also involves two electrodes – the positive anode and the negative cathode. You can remember which is which if you don’t PANIC! Because PANIC, P-A-N-I-C, stands for “Positive Anode, Negative Is Cathode”. And when these are connected to a power supply, an electric current flows from one to the other through the electrolyte.
TULELA: The negatively charged ions in the electrolyte are attracted to the positive anode where they lose their electrons. And so they are oxidised.
SUNAYANA: Remember “OIL RIG” from the previous episode. O-I-L stands for oxidation is loss of electrons.
TULELA: And the positive ions in the electrolyte are attracted to the negative cathode. Here, they gain electrons and are reduced.
SUNAYANA: R-I-G. Reduction is gain of electrons.
TULELA: And as the ions lose or gain electrons, they form the uncharged substance and are discharged from the electrolyte.
SUNAYANA: So the main thing to remember is that the negative ions flow to the positive electrode, the anode. And the positive ions flow to the negative electrode, the cathode. Remember that and you’ll have no problem.
TULELA: And another good tip to remember is that electrolysis only works on ionic compounds and not covalently bonded ones. As, if you remember, the vast majority of covalent compounds don’t conduct electricity but ionic ones do.
SUNAYANA: You know what I also find is useful? Examples. Real-life examples always bring the science to life and help me remember the facts.
TULELA: How about the electrolysis of lead bromide?
SUNAYANA: Music to my ears.
TULELA: Lead bromide it is then. Its chemical formula is PbBr2. And we can use electrolysis to separate the lead and the bromine out.
SUNAYANA: We heat the solid compound in a container for heating hot substances called a crucible until it melts and then insert two carbon electrodes into the now molten electrolyte. We attach the electrodes to a power supply to allow a current to pass from one electrode to the other through the liquid electrolyte.
TULELA: And why do we choose carbon for our electrodes?
SUNAYANA: Carbon is chosen because it has a high melting point and doesn’t react with the reactants or products during electrolysis.
TULELA: So let’s look at what we see at those electrodes. Anode first.
SUNAYANA: At the positive anode we see bubbles of reddish-brown gas. This is the bromine vapour which has been produced as the bromide negative ions lose electrons – they are oxidised. O-I-L. Oxidation is loss.
TULELA: And at the negative cathode we see silvery liquid metal forming. This is the lead which is produced as the positive lead ions in the electrolyte gain electrons – they are reduced. R-I-G. Reduction is gain.
SUNAYANA: So in general the metal is formed at the cathode, the negative electrode, because that is where the positive metal ions are attracted. And the non-metal element is formed at the anode, the positive electrode, where the negative non-metal ions are attracted.
TULELA: How about you dear podcast listening friends try one for yourselves? And we’ll walk you through the answer together. Let’s go with the electrolysis of molten zinc chloride – and you identify what is the product at the anode and cathode. Press pause whilst you have a think. Write your answer down and we’ll be back as soon as you press play again.
SUNAYANA: But do hit play or we’ll miss you!
TULELA: So, zinc chloride was our molten substance. It’s an ionic compound so we can use electrolysis to separate it. Once we’ve added in two electrodes and connected a power supply, what will we find at the anode and cathode?
SUNAYANA: The negative chloride ions will be attracted to the positive anode, where bubbles of chlorine gas will form. The chlorine ions have lost their electrons and so have been oxidised.
TULELA: And the positive zinc ions will be attracted to the negative cathode where zinc metal will form. The zinc ions have gained electrons so have been reduced. Hope you got both correct!
SUNAYANA: You may be asked to write half-equations describing these redox changes – and there’s a reminder on the previous episode as well as on the Bitesize website.
TULELA: Quiz time for electrolysis of molten compounds, anyone?
SUNAYANA: Yes please!
TULELA: OK, usual rules, three questions, five seconds each – no prizes or we’ll run out of budget. Remember to write your answers down. Here we go.
SUNAYANA: Question 1. What are the names and electric charges of the two electrodes used in electrolysis?
TULELA: That’ll be the positive anode and the negative cathode.
SUNAYANA: Question 2. A molten solution of aluminium oxide is used as an electrolyte. What elements will be formed at the anode and cathode?
TULELA: The metal always forms at the cathode so in this case aluminium. And so oxygen gas will form at the anode.
SUNAYANA: And question 3. In that same electrolysis, what has been oxidised and reduced?
TULELA: Aluminium has been reduced as it has gained electrons, and oxygen has been oxidised because it has lost electrons.
SUNAYANA: Fab – you all did brilliantly. But any problems, have a look over at the Bitesize website for more.
TULELA: Quick summary before we end this episode off, Sunayana?
SUNAYANA: Let’s blitz it, Tulela.
TULELA: Electrolysis is a process of splitting ionic compounds by passing electricity through them.
SUNAYANA: The liquid is called the electrolyte, and the electrodes are the anode…
TULELA: …which is positive…
SUNAYANA: …and cathode…
TULELA: …which is negative.
SUNAYANA: The metal ions in the electrolyte are reduced to form metal atoms at the cathode.
TULELA: The non-metal ions in the electrolyte are oxidised to form non-metal atoms at the anode.
SUNAYANA: And remember that electrolysis only works on ionic compounds and not covalent ones.
TULELA: In the next episode we’ll look at more electrolysis, and how we use it to extract pure metals from their ores, specifically aluminium. And in episode 8 we’ll look at electrolysis of aqueous solutions – that is those dissolved in water.
SUNAYANA: Can’t wait!
TULELA: Bye!
SUNAYANA: Thanks for listening!
Listen on BBC Sounds
Question
What are the names and electric charges of the two electrodes used in electrolysis?
The electrodes used in electrolysis are the anode (which is positive) and the cathode (which is negative).
Episode 7 - Extracting metals using electrolysis
In this episode, Dr Sunayana and Tulela Pea explore electrolysis to extract metals from molten compounds, showing how pure aluminium is extracted from its ore.
Tulela and Sunayana explore electrolysis to extract metals from molten compounds.
TULELA: I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.
TULELA: And this is Bitesize Chemistry.
SUNAYANA: This is the seventh episode in an eight-part series on chemical changes.
TULELA: In this episode, we’ll be looking at how we extract metals from their ores using electrolysis.
SUNAYANA: Including aluminium?
TULELA: Especially aluminium.
SUNAYANA: Great – I’d better unwrap this sandwich from its aluminium foil and we can have a closer look.
TULELA: As always, it might be handy to write some notes or diagrams along the way. So hit pause where you need to. Don’t worry, we’ll wait for you to hit play again!
SUNAYANA: And also hit rewind if you need to go over some of those key facts. And remember to head over to BBC Bitesize on the web for more useful information and diagrams.
TULELA: On the previous episode we introduced electrolysis, the process of how we can separate ionic compounds into their elements by passing an electric current through the compound when it is molten, or melted. If you need a quick reminder, have a relisten to that episode as we’ll be continuing with that idea in this episode which is about how we can extract pure metals from their ores by using electrolysis.
SUNAYANA: And we talk about extracting metals from their ores using carbon in redox reactions in episode 4 of this series.
TULELA: We did indeed. And if you also remember, we said that these redox reactions using carbon could only be used if the metal in question was less reactive than carbon. However, where the metal is more reactive, we need another method. And that is electrolysis.
SUNAYANA: Gotcha. And one of those metals is aluminium. How about a quick background on extracting aluminium from its ore from NNICK, our AI chat bot.
TULELA: He’s more like Old McDonald’s chat bot – AI AI Oh.
SUNAYANA: Know what you mean, Tulela. Hi NNICK! Can you give some useful background on extracting aluminium from its ore.
NNICK: Aluminium ore is called bauxite. It is purified to alumina using the Bayer process, which was invented by Carl Josef Bayer. The alumina is then dissolved in another aluminium compound with a lower melting point called cryolite. From this molten solution, aluminium is extracted using electrolysis.
SONG
NNICK: Bauxite is taken out of the ground
CHORUS: Y?
NNICK: Because it's aluminium ore
CHORUS: O.K.
NNICK: Then it's purified to alumina, an aluminium compound
CHORUS: O, Y?
NNICK: Because alumina's more useful than the raw ore before
CHORUS: O. O.K.
NNICK: Next the alumina is dissolved in molten cryolite.
CHORUS: Y, O, Y?
NNICK: Because its ions are freed to move by doing this.
CHORUS: O. O.K. I.C.
NNICK: And electricity's passed through this molten electrolyte
CHORUS: Y?
NNICK: So aluminium can be released by electrolysis
CHORUS: E, L, E, C, T, R, O, L, Y, S, I, S
NNICK: Yes
SUNAYANA: Thanks NNICK. Lots going on in that process to extract aluminium so let’s break them down one by one.
TULELA: Since aluminium doesn’t occur naturally in nature, it has to be extracted from its ore, aluminium oxide Al2O3 – also called bauxite. But, problem 1. Bauxite has many impurities – at best it’s around 50% pure.
SUNAYANA: Solution! The bauxite is purified by the Bayer process that NNICK mentioned and from that we get the pure aluminium oxide in a white powder called alumina. You won’t need to know any details about this for GCSE, and although Bayer invented this process over 140 years ago, it’s still used today.
TULELA: But before we can use the alumina for electrolysis, it needs to be molten. Remember we can only use electrolysis on ionic compounds in their liquid state – so we need to melt the alumina first.
SUNAYANA: But, problem 2. Aluminium oxide has a really high melting point – around 2000 degrees Celsius. And that means it would take loads of energy and therefore a high cost to melt it.
TULELA: Solution – instead we first dissolve it in molten cryolite which is another aluminium compound. And this reduces the melting point of the mixture to around 900 degrees Celsius which takes less energy to achieve and is therefore less expensive. And we can now use it as an electrolyte.
SUNAYANA: Which is the name of the ionic liquid used in electrolysis.
TULELA: We now have our molten ionic aluminium electrolyte Al2O3 which we contain in a lined steel case, called the cell. Into this we immerse carbon electrodes, connect to a power supply and allow an electric current to flow.
SUNAYANA: So, I suppose at this point we could continue to explain what happens next. Or perhaps dear podcast listening friend, why don’t you have a think about what happens to the ions in the molten liquid? Which ions migrate to the cathode, and which ions to the anode?
TULELA: Press pause and that will give us a chance to finish off that sandwich and you a chance to write down the answer. And see how you did when you press play again.
SUNAYANA: Still got a couple of bites left, which I’ll leave for later. But let’s continue with that process. As with all electrolysis of ionic compounds, the positive metal ions, in this case aluminium 3 plus, migrate to the negative cathode where they combine with electrons and so are reduced to aluminium atoms.
TULELA: Remember OIL RIG, where R-I-G stands for reduction is gain of electrons.
SUNAYANA: The molten aluminium sinks to the bottom of the cell, where it is tapped off.
TULELA: And the oxygen ions are attracted to the positive anode where they lose electrons – so are oxidised – to form oxygen atoms.
SUNAYANA: O-I-L – oxidation is loss of electrons.
TULELA: And this is where it gets more interesting as this oxygen reacts with the carbon electrode forming carbon dioxide and in doing so burning the electrode away. And this means that we need to continually replace this electrode. That’s an important thing to remember. There’s a diagram of the whole process on the Bitesize website for extra revision help.
SUNAYANA: Noted. And now that pure aluminium has been extracted it can be used in many processes in industry – from construction, electricity power lines, cans for packaging and foil to wrap up the last bits of my sandwich lunch. Hey, where’s it gone?
TULELA: Sorry Sunayana, it was really tasty.
The uses of aluminium are hugely important in our world today and so the process of extracting it from its ore bauxite is vital. But as with any process in chemistry, it has its pros and cons.
SUNAYANA: The pro is that aluminium extracted in this way is over 99% pure. What about cons?
TULELA: The cons include the huge amount of heat needed in the process to melt the ore and in trying to generate this from environmentally-friendly energy rather than from burning fossil fuels.
SUNAYANA: Talking of which, the carbon dioxide produced as oxygen combines with the carbon anode adds to greenhouse gas, as well adding to costs as anodes are constantly replaced.
TULELA: Time for a quick aluminium extraction-related quiz then.
SUNAYANA: Three questions, five seconds…hit pause if you need to and write those answers down. Here goes.
TULELA: Question 1. Why can’t we simply use carbon to extract aluminium from its ore as we can with some other metals?
SUNAYANA: Because aluminium is more reactive than carbon, and we can only use carbon to extract metals from their ores when the metals less reactive than carbon.
TULELA: Question 2. Why must we first dissolve the purified aluminium oxide, alumina, with cryolite?
SUNAYANA: Because pure alumina has a melting point around 2000 degrees Celsius and that would take huge amounts of energy. Dissolving in cryolite reduces this to 900 degrees Celsius.
TULELA: And question 3. Why does the carbon anode burn away and need to be replaced continually?
SUNAYANA: Because it reacts with the oxygen produced to form carbon dioxide.
TULELA: How did you do? Everyone’s a winner and you all get an aluminium can and Sunayana’s now crumpled up lunchbox sandwich foil.
SUNAYANA: Hey – I still want to recycle that, Tulela.
TULELA: Summary from today’s episode first.
SUNAYANA: We can extract aluminium from its ore aluminium oxide, also known as bauxite, using electrolysis.
TULELA: Its high melting point means that we dissolve it first in cryolite to make an electrolyte with a lower boiling point.
SUNAYANA: Pure aluminium is tapped off at the cathode and oxygen forms at the anode…
TULELA: …which reacts with the carbon to form carbon dioxide and therefore the anodes need to be regularly replaced.
SUNAYANA: You can learn more about this process on the Bitesize webpages and you can listen on BBC Sounds to all our other Bitesize Chemistry podcasts.
TULELA: In the final episode of this series, there’s one more exciting electrolysis process to look at. That is the electrolysis of aqueous solutions.
SUNAYANA: Thanks for listening!
TULELA: Bye.
Listen on BBC Sounds
Question
Why is cryolite used in the process to extract aluminium from its ore?
Aluminium oxide has a very high melting point – around 2000 °C. Dissolving aluminium in cryolite lowers the melting point to around 900 °C, meaning less energy is needed to melt it.
Episode 8 - Electrolysis of aqueous solutions
In this episode, Dr Sunayana Bhargava and Tulela Pea explore electrolysis of an aqueous solution and look at the products formed at the anode and cathode.
Tulela and Sunayana explore electrolysis of an aqueous solution.
SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.
TULELA: And I’m Tulela Pea, a science communicator and podcaster.
SUNAYANA: And this is Bitesize Chemistry.
TULELA: This is the final episode of an eight-part series on chemical changes. In this episode, we’re going to look at electrolysis of aqueous solutions.
SUNAYANA: As always, it might be handy to write some notes or diagrams along the way. So hit pause when you need to. Don’t worry, we’ll wait for you to hit play again.
TULELA: And also hit rewind if you need to go over some of those key facts. And remember to head over to BBC Bitesize on the web for more useful information and diagrams.
SUNAYANA: We’ve looked at electrolysis in the previous two episodes. In episode 6, we looked at how ions in a molten ionic compound can be separated into atoms.
TULELA: Remembering that a molten ionic compound is simply one that is melted into its liquid state. And then it’s used as the electrolyte that an electric current can flow through.
SUNAYANA: And in episode 7 we looked at the extraction of metal using electrolysis.
TULELA: And we saw that this is a process used where the metal we want to extract from its ore is more reactive – or higher in the reactivity series – than carbon.
SUNAYANA: Again, we use the molten metal ore as the electrolyte.
TULELA: Have a relisten to both those episodes or visit the Bitesize website if you need a reminder.
SUNAYANA: This time we’re going to look at electrolysis not of a molten solution, but an aqueous solution. That is, one where an ionic compound has been dissolved in water.
TULELA: Does this make it more complicated, Sunayana?
SUNAYANA: Not complicated, just more exciting! So grab a pen and paper to write down the key facts and diagrams along the way and hit pause and rewind whenever you need to.
TULELA: Let’s get going with the excitement then! The difference when we do electrolysis of an aqueous solution rather than a molten solution is that, as well as the ions of the ionic compound, we also have to take into account the presence of ions in the water itself.
SUNAYANA: This happens because in the aqueous solution water molecules break down producing hydrogen H-plus ions and hydroxide OH-minus ions.
TULELA: As before when we add our electrodes…
SUNAYANA: …cathode negative, anode positive…
TULELA: …and connect them to a power supply, an electric current flows through the electrolyte.
SUNAYANA: This time, two positive ions are attracted to the negative cathode where they compete for electrons. The hydrogen ion from the water and the metal ion from the compound. To determine which substance is made at the cathode, we use the reactivity series.
TULELA: If the metal is more reactive than hydrogen, then hydrogen will be produced at the cathode. If the metal is less reactive than hydrogen, then the metal is produced at the cathode.
SUNAYANA: So if the metal is more reactive, we get hydrogen. And if the metal is less reactive, we get the metal. Similarly two negative ions are attracted to the positive anode. The hydroxide ions from the water and the negative ions from the dissolved compound.
TULELA: If the negative ion is a halide – which you’ll remember is from a group 7 element, chlorine, bromine or iodine, then the equivalent halogen gas will form at the anode. So bromide ions form bromine gas and chloride ions form chlorine gas. If the negative ion from the ionic compound isn’t a halide then oxygen will be formed.
SUNAYANA: Sounds like we need to go through a kind of decision tree or flow chart to work out what is formed at each of the electrodes. Perhaps NNICK can help us here. Hi NNICK! Can you summarise what forms at each electrode in the electrolysis of an aqueous solution?
NNICK: Nothing could be easier. Except falling off a log while mambo dancing. Believe me, I've tried it.
SONG
What's produced at the cathode?
That's easy, you see
Because the key
Is reactivity.
If the m-m-m-metal is least reactive
The m-m-m-metal is what we get.
If h-h-h-hydrogen's least reactive
Then h-h-h-hydrogen's what we get.
What's produced at the anode?
That's easy to decide
Because it can't be denied
It's all about the halide.
If a h-h-h-halide happens to be present
A h-h-h-halogen's what we get.
O-o-o-o-o-o-o-o-o-otherwise
o-o-o-oxygen's what we get.
SUNAYANA: Thanks NNICK! Nice singing. That helps, but perhaps an example or two might help us more.
TULELA: OK, let’s take our good friend sodium chloride NaCl. We dissolve it in water – the aqueous solution – add our electrodes and connect to a power supply. What happens next?
SUNAYANA: It can be handy to write out all the ions present in the solution. In this case the positive ions are hydrogen H-plus from the water, and sodium Na-plus from the compound.
TULELA: And the negative ions are hydroxide OH-minus from the water, and chloride Cl-minus from the compound.
SUNAYANA: The positive H and Na ions are attracted to the cathode and compete for electrons. We see that in this case sodium is a more reactive element than hydrogen – so it is hydrogen that gains those electrons and become neutral hydrogen gas. And indeed we see bubbles of hydrogen at the cathode. So what about the anode?
TULELA: The hydroxide OH-minus from the water and the chloride Cl-minus from the compound are attracted to the anode. Is there a halide present? Yes – the chloride ions. So in this case, at the anode the chloride loses electrons to become chlorine gas. And again this is what we see.
SUNAYANA: Another example, Tulela?
TULELA: How about we give our dear podcast listening friends a chance to do this before we work through the answer?
SUNAYANA: Good shout.
TULELA: Let’s go with an aqueous solution of copper sulfate, CuSO4. Hit pause, write out the ions present and work out what we see at each electrode.
SUNAYANA: And in the meantime we’ll look at picture of cats on social media…. Ahhhh!
TULELA: Ahhh!
SUNAYANA: Enough cats! How did you get on with that aqueous solution of copper sulfate?
TULELA: The positive ions present are the hydrogen H-plus from the water and Copper Cu 2-plus from the compound. And the negative ions are the hydroxide OH-minus from the water and the sulfate SO4 2-minus from the compound.
SUNAYANA: At the cathode the hydrogen and copper ions compete for electrons. We compare their reactivity in the reactivity series and see that copper is less reactive than hydrogen. So it will be copper that forms at the cathode. And indeed we’ll see a coating of copper on the cathode.
TULELA: At the anode, the hydroxide and sulfate ions are present. There is no halide ion present. So it will be oxygen that forms at the anode which we’ll see as bubbles of oxygen.
SUNAYANA: Correct! Hope you got that. There are more of these examples on the Bitesize webpages, the more you practise the easier it is and you’ll be doing them in your sleep.
TULELA: So why do we need to know about this anyway, Sunayana? Let’s look at some real-world examples of electrolysis of an aqueous solution.
SUNAYANA: Well, as with everything in this series, the chemistry knowledge that we’ve revised isn’t there just to pass exams. It’s the foundation of really useful and important processes that help in our daily life. So, for example, electrolysis of aqueous solutions is a process used to produce hydrogen gas which can be used instead of fossil fuels.
TULELA: And that helps us to develop green and renewable energy sources.
SUNAYANA: Also, maybe some of the jewellery that you’re wearing has been electroplated to protect it and make it look shiny. You may have also copper-plated a key in the class by using your key as the cathode with a copper sulfate electrolyte.
TULELA: Also done by electrolysis in an aqueous solution?
SUNAYANA: Correct! And it’s also an important process in the treatment of wastewater, production of chlorine and refining of metals.
TULELA: Chemistry saves the day yet again.
SUNAYANA: Final summary of the series, Tulela? You start.
TULELA: In the electrolysis of an aqueous solution, we need to take account of the hydrogen and hydroxide ions as well as the ions from the ionic compound.
SUNAYANA: If the metal in the compound is more reactive than hydrogen, then hydrogen will form at the cathode.
TULELA: And if the metal is less reactive than hydrogen, then the metal will form at the cathode.
SUNAYANA: If there are halide ions in the compound then the equivalent halogen gas will form at the anode.
TULELA: If there are no halide ions in the compound, then oxygen will form at the anode.
SUNAYANA: Remember there’s loads more hints and tips and revision aids on the Bitesize webpages.
TULELA: And you can listen on BBC Sounds for loads of other Bitesize series in this subject and many others.
SUNAYANA: Thanks so much for listening to us and good luck with your chemistry exams.
TULELA: We should thank NNICK also. Thanks NNICK!
NNICK: Please, don’t mention it! Oh, too late, you already have. Never mind. Bye!
SUNAYANA: Bye from me, Sunayana…
TULELA: …and me, Tulela.
TOGETHER: Bye!
Listen on BBC Sounds
Question
In the electrolysis of potassium iodide in an aqueous solution, what is produced at the anode?
When a halide is present, a halogen is produced at the anode. Iodide is the halide here, so iodine is produced at the anode.
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