By the end of this video hopefully you'll get a really good understanding of reversible and
irreversible reactions in open and closed systems. Now this video I'm going to run a little
bit differently. So up in your top right hand corner of your screen, I'm going to put a link,
and that link is going to take you to a free resource that is going to help you
to make some observations about some chemical reactions I'm going to show you.
So I'm going to do some demonstrations in the lab and while I'm doing those
demonstrations I want you to record down your observations of what you can see.
When enough of both reactants are present in a system, then the reaction can go to
completion. So this is where all of at least one of the reactants is completely used up
and so the reaction is no longer able to continue. It's finished. Often however,
chemical reactions don't go to completion. A reaction that doesn't go to completion is often
reversible. A reversible reaction is one in which the reaction can occur both ways. The reactants
can react to form products, we call this the forward reaction. And those products can be
reverted back into the reactants, and we call this the reverse reaction. Chemical reactions can occur
in both open and closed systems. An open system is one that is free to transfer energy and matter
with its surroundings. But a closed system is one in which no matter can enter or leave the system,
however energy, usually in the form of heat, is able to be transferred into and out of the system.
In this video we're going to explore chemical reactions in both open and closed systems
and identify whether they're reversible or irreversible.
So by now hopefully you've downloaded that free resource
and you've printed it out and have it next to you. I'm going to show you some
chemical reactions and while I'm showing them to you I'd like you to make any observations
that you can. So have a look at, you know, the way that reactants look before reacting.
Make some observations about how they look after reacting. How the products look. Make some
observations about what happens while they're reacting. And also try and identify whether
it's an open system or a closed system. Whether you think that reaction is reversible and whether
you think that reaction has gone to completion or whether it was an incomplete reaction.
Okay, here's our first reaction. And this is burning magnesium.
So in this experiment I'm going to burn magnesium in order for it to react to form magnesium oxide.
So start by looking at the properties of the magnesium before it's burnt.
So here's the magnesium here... What can you observe about it?
And now let's see what happens when it starts to react. So it's now alight.
So it's now burning. It gets a bit blinding so you might not want to watch it directly.
In real life you shouldn't look at that flame directly because it can damage your eyes.
And now that reaction has stopped. Let's look at the product that's formed. So this
is the product of magnesium oxide. What can you observe about the magnesium oxide? Are
there any similarities or differences to the magnesium before it reacted?
And have a go at ticking those boxes there as well. Is it open or closed system?
Do you think it went to completion or was it an incomplete reaction?
And do you think it's a reversible or irreversible reaction?
The next experiment we're going to look at is the reaction of cobalt chloride and hydrochloric acid.
So here, this is before we react it. This is the cobalt chloride
solution. Make some observations about the colour,
whether it's transparent or opaque. Those are the sorts of words you might like to use here.
And now I'm going to add some concentrated hydrochloric acid. So here it is after it's
now reacted. So we've got the cobalt chloride and the hydrochloric acid have reacted together
and now I'm going to add some water.
What happens when we add water? What observations can you make there?
So for part two of this experiment I'm actually going to look at the same chemicals again,
but this time I'm going to put it in a hot water bath and a cold water bath
and we're going to see what happens when we do that as well.
This is the cobalt chloride and hydrochloric acid solution in a hot water bath.
This is the cobalt chloride and hydrochloric acid solution in a cold water bath.
Experiment number three is burning of steel wool. So steel wool is made up of mostly iron
and we're going to form iron oxide when we burn in. But before it reacts let's have
a look at the properties of that steel wool. So what observations can you make?
What does it look like? Is it malleable? Is it not malleable? And this is it reacting.
what observations can you make while it's reacting?
And this is it after reacting and I've got some fresh steel
wool here just so you can see the differences between them. So the
on the left here you've got the burnt steel wool and on the right I have some fresh steel wool.
Okay the next one I'm looking at is nitrogen dioxide gas.
So what I have here are three vials of nitrogen dioxide gas. Have a look at the colour of it. And
what I'm going to do is I'm going to put one in hot water, one in room temperature water,
and one in cold water and I want you to make some observations about what happens there.
Okay so hopefully you've had a go at recording your observations for each of those experiments
and now I'm going to go through and explain what it was that you observed and hopefully
were able to communicate on your notes, and why it was that you observed those things.
So with the burning magnesium to make magnesium oxide, words you might have
used to describe the magnesium before it burnt could have included that it's shiny,
it's got a silver metallic colour, it's bendable and malleable. And these are all properties of
metals. So when you're observing a metal they're the sorts of words you would use to to describe
them. While it's reacting we can see that there's energy being released in the form of a light,
so we know it's a chemical reaction occurring, and it is an exothermic reaction. And then when
we look at the product that's formed, the magnesium oxide, we can see that it's got a
white sort of ashy colour. It's not shiny. It is brittle, it breaks apart as soon as I touch it,
and it's got this powdery sort of consistency. Now is it an open or closed system?
Well because we didn't have this happening in a sealed jar it was actually out in the open, and we
were able to use the oxygen just from the air it is an open system. Now because it was in an open
system this reaction was able to go to completion and that's because the magnesium was able to react
with excess oxygen in the air. This reaction is not reversible, so how do we know that?
Burning magnesium is an exothermic reaction and that means it's actually more stable as
magnesium oxide. Now magnesium oxide does not decompose to make magnesium and oxygen.
This means that the reaction is not reversible. We say it's irreversible.
Okay onto the cobalt chloride and hydrochloric acid. So before the cobalt chloride solution,
before we react that with the hydrochloric acid, we can see that it's a really nice pink colour
and it's transparent, and that's because of the complex iron that is formed between the cobalt
and the water molecules. Hydrochloric acid before reacting is a clear liquid. Then after reacting
what happens? So when we add the hydrochloric acid you can see a colour change and this colour change
is into a blue, still transparent colour. And that is because the cobalt has now formed a complex ion
with the chloride ions from the hydrochloric acid. Now when we added more water we can see that the
colour now returns to more of a pink colour. Yes it is more dilute, so you should include that in
your observations, it's a more dilute colour. But it's still that pink. And so we can see that the
reaction has actually been reversed, that now the pink colour is back because the cobalt has more
water molecules to react with and so it's forming that complex ion again with the water molecules.
Is it an open or closed system? This was an open system. It's not
sealed and we added matter into the system. Did it go to completion? No, the reaction
didn't go to completion. The reason we were able to reverse it was because there was still
some of the reactants there in the solution. Is it a reversible or irreversible reaction?
We've already established that it was reversible. Okay so the next part of that experiment was
looking at the same reaction but we this time we were adding heat energy and removing heat energy.
Now when we added heat. Adding heat causes some of the water in the solution to evaporate and so that
means that there's less water and so the cobalt actually reacts with the hydrochloric acid again
and so that's why it turns blue. When we removed the heat, so we put it in a cold water bath, this
reverses that reaction and so it turns pink again. Is this an open or closed system? We have to say
that it's open because those test tubes are not sealed. If those test tubes were sealed we could
call it a closed system and that transference of heat can happen without the transference of matter
so we should expect to see the same result. But in this experiment, the demonstration I've shown you,
the test tubes weren't sealed therefore it has to be an open system.
Burning the steel wool. So before burning the steel wool we can see that it is shiny and it's
metallic and it's bendable and malleable. And you'll notice those sorts of words,
we use the same ones for the magnesium, and that's because they're both metals,
we expect them to have very similar properties. So you can observe the difference in colour after
the burning and that product that you see, that difference in colour that is iron oxide.
This reaction again occurred in an open system with excess oxygen and so that meant that the
reaction was able to go to completion. Is it able to be reversed? Can I "unburn" the
steel wool? The answer is no. It's not reversible. And the reasoning is the same as what we saw with
the magnesium oxide. So it is an exothermic reaction, the iron oxide is more stable than
the reactants are, and the iron oxide will not decompose to form the iron and the oxygen again.
The nitrogen dioxide gas. So before reacting the nitrogen dioxide is a nice dark brown colour.
And when we put it in the hot water we actually see that colour get darker. When we put it at room
temperature that that colour is fairly constant. Okay and then cold water we see a lighter brown.
Now the reason for that is that the nitrogen dioxide is reacting to form dinitrogen tetroxide
and the dinitrogen tetroxide is a clear or transparent gas. It's not brown. And so if
there's more of the dinitrogen tetroxide then the colour in the tube is going to become lighter
because there'll be less and less of the nitrogen dioxide. Now when we put it in hot water what
we're actually seeing there is the dinitrogen tetroxide is dissociating into nitrogen dioxide,
and that's the brown gas, and so we actually see more and more of that being created or being
produced when we put it in the hot water. Now is this an open or a closed system?
This is a closed system. We have sealed tubes. No matter is being transferred or can be transferred
into the system. Did it go to completion or was it incomplete? This was incomplete at every point
in this experiment - there were some reactants and some products - just the amounts of those
varied. And is it reversible or irreversible? Yes it's a reversible reaction. So we were
able to swap between the forward and the reverse reaction just by adding or removing heat energy.
So in conclusion for this video, some chemical reactions are irreversible while other chemical
reactions can be reversed in both open and closed systems. In a closed system if a reaction can be
reversed then it hasn't gone to completion so there's still some reactants available there.
However, a reverse reaction won't reach what we call an equilibrium unless it is in a closed
system and we will look at equilibrium reactions specifically in an upcoming video.
Let me know what you thought about doing this video a little bit differently. Let
me know what you thought about the free resource and whether it helped
you with your understanding in the comments below. Thank you!