Lesson video

- Hi folks, my name's Mrs. Farrow.

For today's session, you are going to need a pen and a piece of paper and it's a good idea to put your devices on silent or switch them off to help avoid new distractions.

If you need a moment to sort that now, then please pause the video and press play when you're ready to resume.

So today's session is going to be about Group 7.

We're going to describe some of the trends we see in the reactivity of Group 7 elements, look at some of their physical characteristics and properties and describe how they react with each other and elements of other groups in the periodic table.

Some of the key words that you'll see as we go through today's session include volatility.

This is the tendency, the likelihood, of a substance to evaporate at room temperatures.

Sublimation, which is a process of changing state directly from solid to gas, missing out the liquid stage.

And displacement reactions.

Displacement reactions recur when more reactive elements push less reactive elements out of compounds.

And this is something that we observe within Group 7 elements.

So we've broken today's session down into four distinct learning cycles.

We've got what is Group 7? The physical properties and trends of the Group 7 elements.

And chemical properties and trends of the Group 7 elements and then displacement reactions.

So if you're ready to jump in, we'll get started with, what is Group 7? Group 7 are called the halogens and they react with other elements to make salts and this is where they get their name.

Halo meaning salt and gen, meaning former in Greek.

And so their name directly translates to the salt formers.

So when we react our halogens with other elements within the periodic table, they form what are known as salts.

The Group 7 elements, or the halogens, are found on the right-hand side of the periodic table as indicated here in the image.

The halogens include fluorine, chlorine, bromine, iodine, and astatine.

And we're gonna look at some of their chemical and physical properties today.

Properties are physical or chemical characteristics of substance that allow us to be able to describe the physical features of a substance.

Physical properties include things like the appearance.

What state they appear in at room temperature and pressure and what colour they might have.

The melting and boiling points, indicating how much we need to heat the substance in order to change it from solid to liquid or liquid to gas.

And then volatility.

The ease with which something is able to evaporate.

Some substances evaporate very readily and others less so.

And so those that evaporate easily we say are volatile and those that don't evaporate very readily are less volatile.

You can leave a cup of water on the side for a very long time before it starts to evaporate.

It has quite a low volatility.

However, if you've ever been to a petrol station, you can smell the fumes because petrol evaporates very easily, it is very volatile.

We're also gonna look at some of the chemical properties, including their reactivity.

So how they behave in chemical reactions with other elements.

The toxicity, whether it is a poisonous substance to living organisms. And the acidity.

So whether it shows acid or alkaline properties.

So let's do a quick knowledge check and see what you can recall from what we've covered so far.

Which is the first element in Group 7? Is it A, chlorine, B, astatine, C, fluorine, or D, bromine? The correct answer is C, fluorine.

Now it's time for you to have a go at a practise task.

In this practise task we're asking you to answer the question and then colour the periodic table to show the location of the halogens.

The question asks, "Why are elements in Group 7 named as the halogens?" You'll need to pause the video at this point to give this task your full attention and when you're ready to hear a model answer, press play.

Let's take a look at how you got on.

Question A, why the elements in Group 7 known as the halogens? Now this is an open-ended answer and so your response doesn't need to be word-for-word exactly the same as what I have here, but it should reflect the same themes.

We've got the elements in Group 7 are called the halogens, which means salt formers.

This is because they react with other elements to form salts.

Well done if you've got that one right.

Your next task was to colour the correct column and to show the location for halogens in the periodic table.

Love a good colouring task.

So which bit of the periodic table should you have coloured? Well, you can see here indicated in the image, you should have coloured under the number seven from fluorine through chlorine, bromine and iodine, down to astatine.

These are what we refer to as our Group 7 elements or the halogens.

Good job guys.

We've now had a look at what Group 7 is and where we find it on the periodic table.

It's time to take a look at some of their physical properties and the trends that we see as we go down the group.

What do we know about the halogens? Well, the halogens are very reactive non-metals.

Some of their physical properties include having a low melting and boiling point.

This means we don't need to get them particularly warm in order to change them from solid to liquid or liquid to gas.

As you can see here in the image, some of them are actually gases at room temperature and pressure already.

As non-metals, they do not conduct electricity and if in solid form, tend to be brittle and crumbly.

They are coloured, as we can see in the image, and they have a particularly stinky odour.

Some of you may be familiar with chlorine, having visited swimming pools, and smelt it.

In terms of the trend in their appearance, they become darker as you go down the group.

Fluorine is a very pale yellow coloured gas.

Chlorine is more of a greeny-yellow, as we can see in the picture.

Bromine is a brown liquid, although vaporises very easily and iodine is a blue-black, crumbly solid.

In terms of their melting and boiling points, we can see that they increase as you go down the group for the halogens.

Fluorine has a melting point of minus 220 degrees C, which means it changes from a solid to a liquid in a very negative temperature.

It's boiling point is -118 degrees C and this means it would be a gas at room temperature, which is approximately 20 to 25 degrees C.

If we then move down the group, we can see these numbers getting much larger as we head towards the bottom, through chlorine and bromine and towards iodine.

Iodine has a melting point, so changes from solid to liquid, at 114 degrees C.

So this would be the temperature at which it melts.

It would need to heat it using a Bunsen burner in order for it to do this and its boiling point is much higher again, at 184 degrees C.

As we've already said, we see the halogen showing the physical property of volatility.

Now remember this is the tendency or likelihood that a substance will evaporate at room temperature.

Some things evaporating very easily and therefore often being quite smelly substances and other substances don't.

Again, think about a cup of tea left on the side, it can be left there for weeks or possibly even months, although I wouldn't recommend this, before the amount of liquid within the cup starts to decrease.

However, if you leave a small jar of essential oils or petrol open, then it will start to release vapours very readily.

It will evaporate very quickly.

Both fluorine and chlorine are already gases at room temperature and pressure and so they aren't going to be volatile because they are already in the gas state.

However, bromine and iodine have very low boiling points and therefore they are particularly volatile.

We can see here that if you put some bromine liquid into the bottom of a round-bottom flask, that's a reddy-brown vapour quickly forms above liquid iodine at just room temperature.

Can speed this up slightly by holding the bottom of the flask in our hands just to warm it and the whole of the flask turns this interesting red-brown colour.

Ionine is a particularly unusual substance in that it undergoes a process referred to a sublimation.

Sublimation is when a substance is able to change directly from a solid to a gas, so it doesn't melt and form a liquid very easily.

If we gently heat some iodine, we can see that it is producing a vapour in this image without becoming a liquid.

Let's have a quick knowledge check then and see what you can remember.

True or false? All halogens are gases at room temperature.

The correct answer to this question is false.

Some halogens are gases, namely fluorine and chlorine, but others are liquids and solids at room temperature, although they do evaporate easily and show a high volatility.

Answer the next question.

What happens to the melting and boiling points of the Group 7 elements as you go down the group? Is it A, they both increase.

B, they both decrease, or C, the boiling points increase but the melting points decrease.

The correct answer is they both increase.

Well done if you've got that right.

The next question asks, which of these halogens has the lowest melting point? Now we're not expecting you to have memorised the melting points, but you need to be able to identify the general trend and the order of the halogens within the group.

So that's the information you need to use in order to answer this question.

The correct answer is B, chlorine.

Chlorine is the halogen nearest the top of the group and therefore has the lowest melting point out of these options.

Now it's time for you to have a go at practise task.

In this task you are going to cross out the word in the brackets to make the sentence read correctly and then have a go at the two questions which follow.

Describing the trends in melting points as you go down Group 7 and then looking at the trend in their colour and appearance, predicting the colour and state of astatine at room temperature.

You'll need to pause the video at this point in order to give this task for full attention.

When you're ready to see some model answers, press resume.

How have you got on? Tricky, I know, but I'm sure you've done a good job.

Let's take a look at the answers.

In the first task you were asked to cross out the word in the brackets to make the sentence read correctly.

The halogens have low boiling points.

And so you should have deleted the word high.

Question B asked you to describe the trend in melting point as you've moved down the Group 7 elements.

Again, with an open-ended question like this, it doesn't need to be word-for-word, but you do need to have the correct general trend, which is that as you go down the Group 7 elements, the melting points increase.

And the final task set asked you to predict the colour and state astatine at room temperature.

Being that as there is a trend from colour and state and the halogens.

Astatine being at the bottom of the group would, based on the trends and the properties of iodine that comes immediately before it, likely be a black or dark coloured solid.

Well done if you got those right, it's fantastic work.

Predicting is very difficult and you should be really chuffed with yourself if you've managed to spot those patterns and make a prediction.

So now we know what physical properties the halogens have, it's time to move on to look at some of their chemical properties and reactions.

The halogens are very reactive and this in fact actually makes them poisonous.

They used chlorine gases during some of the World Wars.

Halogens will readily react with metals and other metals to make what are called halides.

These are a type of salt.

So as we go down the group, we see that the reactive actually decreases.

So fluorine at the top of the group is by far the most reactive and dangerous, followed them by chlorine, bromine and iodine.

Iodine being poisonous or toxic to some living organisms, such as microbes, and used as an antiseptic disinfectant during surgery.

So how do the halogens react? Well, let's first of all take a look at their reactions to metals.

The halogens react with all metals to produce metal salts.

And the general equation for this is a metal plus a halogen gives a metal halide, which we know is salt.

All reactions of metals follow this same general reaction.

So we're always going to get the same products.

Metal halide.

If we take an example, for example, iron and react it with a halide, let's take chlorine, then it's going to form the metal halide iron chloride.

This can also be shown as a symbol equation, where we're reacting two iron atoms with six chlorine atoms to form two lots of iron chloride, 2FECL3.

Now you don't need to be able to write or read these equations at this point using the symbols, but it's a good idea to start becoming more familiar with them, ready in preparation for further chemistry studies.

Some of the most vigorous reactions are seen between the halogens and the very reactive group one metals.

So let's see how you get on having a go at some word equations.

I'm gonna model a few for you and then you are going to use this as a guide to complete the task.

We know that we react to metal with a halogen.

This is going to form a metal halide.

So if we take calcium for example, and react it with fluorine please being on the left-hand side of our equation are our reactants.

These are going to react to form our product, calcium fluoride.

Notice how the end of the name of the halogen is changing once it's become pass of a compound with the metal atom.

What we'd like you to do now is to have a go at writing the word equations for the reactions of lithium with chlorine, bromine and iodine.

So in this instance, lithium will be the metal for each of your reactions, but you'll be writing out to show what's formed when it reacts with chlorine, what's formed when lithium reacts with bromine and what's formed when lithium reacts with iodine.

You going to need to take a little bit of time to have a go at this task.

So we suggest you pause the video now and then press resume when you're ready to see the correct answers.

Good luck and have a go.

How'd you get on? I'm sure you've done great.

Let's take a look at what your reaction should look like then.

So initially we've got lithium reacting with chlorine.

And this forms lithium chloride.

Then we ask you to react lithium with bromine.

So lithium plus bromine reacts to form lithium bromide.

And then we asked you to react lithium with iodine and when this reaction takes place it forms lithium iodide.

Well done if you've got all of those right, it's a great effort.

So now we're going to take a look at the reactions with non-metals because we set the halogens are incredibly reactive, reacting with both metal and non-metal elements.

One of the more notable reactions of the halogens is of that with hydrogen to create something called a hydrogen halide.

Notice it's still a halide.

Hydrogen halides are gases, but they dissolve easily into water to become very strong acid.

The general equation's reaction of hydrogen with halogen is hydrogen plus the halogen gives a hydrogen halide.

If we take a look at an example of hydrogen reacting with chlorine, it forms hydrogen chloride.

Notice it's very similar pattern to that which we saw with the metals.

Only this time we have hydrogen in the place of the metal.

Hydrogen chloride dissolves in water, to make an acid I'm sure you'll have come across in this lab before, hydrochloric acid.

So again, let's take a look at some practise equations and get you having a go at writing some for yourself.

So we know that we're going to take the non-metal hydrogen and we're going to react it with the halogen.

When that reaction takes place, it forms a hydrogen halide.

Let's take a look at one more example for you.

So if our reactants are hydrogen and the halogen fluorine, for example, then it's going to react to produce a product, calcium fluoride.

In this instance we'd like you to write the word equation to the reactions of hydrogen with bromine, iodine and astatine.

So it would be hydrogen plus bromine, hydrogen plus iodine and hydrogen plus astatine.

And then you are going to tell us the products.

Again, this will take you a little time to complete.

So we recommend pausing the video now and pressing play when you're ready to take a look at the correct answers.

Have you found it? Hopefully you start a spot pattern of emerging.

So let's take a look.

We've got hydrogen reacting with bromine to form hydrogen bromide.

Good job.

We've then got hydrogen plus iodine, reacting to make hydrogen iodide.

And hydrogen plus astatine will react to form hydrogen astatide.

Well done if you've got all of those correct.

Writing equations can be quite a daunting task, but you guys are showing that you are becoming experts already.

Let's do a quick knowledge check then and see how much you can recall.

True or false, the reactivity of the halogens increases down the group.

The answer to this question would be false.

Halogens become less reactive as we go down the group, with fluorine at the top of the group being the most reactive and astatine at the bottom being the least.

The first three elements in Group 7 are fluorine, chlorine and bromine.

But which one is the most reactive? Is it A, fluorine, B, chlorine, or C, bromine? The correct answer is A, fluorine.

Now it's over to you for a practise task.

You'll need to pause video in order to complete this task and give it your full attention.

Part A asks you to describe the trends in reactivities to halogens, and then task B asks you why the halogens are so poisonous.

When you're ready to hear a model answer, press play to resume.

Ready? Let's take a look at how you did.

So we've been asked to describe the trend in reactivity of the halogens, as you go down the group.

We know that they become less reactive, so we should have something that says, "The reactivity of the halogens decreases as you go down the group." But why are the halogens poisonous? Well the halogens are poisonous to living things because they are so reactive and the chemical reactions that occur within the living organism with the halogens often results in them being incredibly poisonous and toxic.

The last part of this practise task, task C, asks you to complete the word equations.

Again, it's a good idea here to pause the video and give this your full concentration.

When you're ready to see how you've done, press play to resume.

Let's take a look at how you've done.

Hopefully you are starting to feel more and more confident with these equations.

So the first one is a non-metal hydrogen reacting with the halogen fluorine and this is going to form hydrogen fluoride.

We've then got the metal sodium reacting with bromine to form sodium bromide.

The next equation asks, what are we reacting aluminium with in order to form aluminium iodide? It must be iodine.

If the product is iron chloride, what did we react with chlorine? The answer is iron.

If the product is hydrogen iodide, what did we react with iodine? The answer is, hydrogen.

And then finally, copper reacts with what to form copper fluoride? And the correct answer is fluorine.

Well done if you've got these right.

This is not easy and I think you're doing really well for trying so hard.

So let's go back to our lesson outline.

We've looked at what Group 7 is.

We've talked about their physical properties and their chemical properties.

Now it's time to look at something called displacement reactions.

Displacement reactions occur when the halogens react with compounds of other halogens.

A more reactive halogen can displace a less reactive halogen from its salt if it's in solution.

So for example, if we take lead bromide and we react it with chlorine, which is more reactive because it's higher up the group, when a chemical reaction will take place, chlorine will push the bromine out of its compound and form lead bromide and leave the bromine all on its own.

It's a little bit like a bullying situation and quite unpleasant.

The lead bromide is, you could think of it as a child with a lolly, and then a bigger child, chlorine, comes along and steals that lead lollipop off of the child.

So we've got a much stronger halogen displacing, which means to push out, the less reactive halogen.

So chlorine is higher up in Group 7.

It's more reactive than the bromine.

Because the chlorine is more reactive, it pushes the bromine out of the compound and we'll know that a reaction has taken place because this will show a colour change.

As we said, all of the halogens have a slightly different colour and so do their compounds.

So when the colour change takes place, we know that there has been a chemical reaction.

Displacement reactions only occur when the halogen added is more reactive.

If halogen that's added is lower in the group or less reactive, then no reaction occurs.

So for example, if we have a metal halide, like sodium fluoride, and we react with iodine lower in the groups than fluorine and so less reactive, sodium chloride remains and the iodine still stays alone.

If you like, the person trying to steal the lollipop is too weak and the sodium keeps hold of it.

Iodine being further down the group is less reactive than the chlorine.

Iodine is therefore unable to push the chlorine out of the compound.

This means no reactions occurred.

The products and reactions are the same and we will not observe a colour change in this situation.

Let's do a quick knowledge check then.

Which two of the halogens would be able to displace bromine from calcium bromide? Is it A, astatine, B, iodine, C, fluorine, or.

C, chlorine, sorry, or D fluorine? So we are looking for two elements within Group 7, which appear higher on the table than bromine.

The answer to this question then must be C, chlorine and D fluorine.

True or false, displacement reactions occur when less reactive halogens push more reactive halogens out of compounds.

The answer to this question is false.

There's one word within that sentence which makes it incorrect and it's less.

What actually occurs is displacement reactions occur when more reactive halogens displace or push less reactive halogens from their compounds.

Well done if you've got that right.

Now it's over to you for a practise task.

In this task we'd like you to tick the box as to whether you think a reaction will take place or not.

So we are looking to see whether you are reacting the halide with a more or less reactive halogen.

So for each reaction, place a tick in the box whether a displacement reaction happens or no reaction happens.

Pause the video now to take your time and I advise looking at a list of elements within the periodic table to help you.

And press play when you are ready to see the answers.

Right, let's take a look at how you've got on.

So the first one says we've got potassium iodide and we are reacting it with chlorine.

Chlorine's higher than iodine, and so a displacement reaction will take place.

We've then got magnesium bromide with fluorine.

Fluorine's right at the very top of the group and so more reactive than the bromine and a displacement reaction will take place.

The next equation, the next reaction, sorry.

We have lead chloride plus iodine.

Iodine is lower than chlorine in the periodic table and so no reaction's going to occur here.

You then have sodium fluoride with bromine.

Fluorine is at the top of the table and bromine lower down and so no reaction will occur here.

Zinc astatide plus chlorine, astatide, astatine, sorry, is the halogen at the bottom of the table and chlorine quite close to the top.

So chlorine will displace the astatine and a chemical reaction will occur.

We've then got rubidium bromide plus iodine.

This is tricky 'cause these two are close together on the table.

You may need to refer to a table for this one to have a look.

In this case, iodine is just below bromine on the periodic table and so no reaction will occur as the iodine is not reactive enough to push the bromine out of the compound.

It's a fantastic effort, guys.

Well done if you manage to get all of those right.

Onto the next part of this practise task.

Part B, we're going to ask you for each of the displacement reactions that we identified in the table above.

Can you write the full equation for the reaction with the expected products? And then task C, describe an experiment you could do and the reaction you would observe to show that chlorine is more reactive than bromine.

You're going to want to take a little bit of time to have a go at these tasks.

So we suggest you pause the video now and press play when you're ready to hear an answer.

Great effort guys.

Let's take a look at how you've got on.

As always, when it comes to open-ended questions, they needn't necessarily be word-for-word the same.

For each of the displacement reactions that we identified in the table, we had to write the equation for the reaction, with the expected products.

So we said that potassium iodide will react with chlorine.

The products of this reaction would be potassium chloride plus iodine.

The next equation we identified was magnesium bromide reacting with fluorine.

The fluorine pushes the bromine out of the compound and forms magnesium fluoride and bromine.

And then lastly, zinc astatide and chlorine reacts to form zinc chloride and astatine.

Well done if you got all of those right.

Now you were asked to describe an experiment that you could do, that would show that chlorine is more reactive than bromine.

Hopefully you've got something similar to the following.

You would need a solution of any metal bromide, sodium bromide, for example, or lithium.

It doesn't matter.

You would then add chlorine.

A chemical reaction would take place because the chlorine would displace the bromine from the solution.

We would know that the chlorine had displaced the bromine when a colour change took place as this would indicate that a reaction had occurred.

A fantastic effort guys.

Really well done.

So we've reached the end of our lesson for today.

Let's take a look at our key learning points.

We have, "The Group 7 elements are non-metals called halogens.

The physical properties include being coloured, smelly, 'cause they're very volatile, they're poor conductors as they're non-metals and have very low melting and boiling points.

The halogens are very reactive, but show decreasing reactivity as you go down the group.

Their reactions form halides, which are salts.

And lastly, the halogens undergo displacement reactions in which more reactive halogens will push less reactive halogens out of the compound.

I hope you've enjoyed today's lesson and found it useful.

If you found any of the sections of this lesson tricky, then feel free to go back and watch any of the learning cycles again.

I'd like to thank you for all your hard work and for choosing Oak National Academy today.

I hope to see you again soon, bye for now.