Hello, my name is Mrs. Collins, and I'm going to be taking you through the learning today.

This lesson forms part of the unit Industrial Chemistry and is called Chemical Equilibrium.

During today's lesson, you will explain the concept of chemical equilibrium and the characteristics of a system in which equilibrium is established.

Here are the key words for today's lesson.

Dynamic equilibrium, reaction rate, and closed system.

Here are the key words in sentences.

Pause the video here, read through the explanations, and make any notes you feel you need to.

Today's lesson is divided into two parts, introduction to equilibrium, and closed systems and equilibria.

Let's get started on part one, Introduction to Equilibrium.

Remember, a reversible reaction is one where reactants react to form products which can then react together to reform the reactants.

So here we've got an example showing A plus B, form C plus D, and we've got the reversible reaction symbol in the centre.

So we know it can go in either direction.

So this direction is called the forward reaction, and this direction is called the backward reaction.

Depending on the conditions, the forward and backward reaction can occur separately or occur at the same time.

So here's a question based on that learning.

What is a reversible reaction? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to that question is a reaction where reactants form products, which can then revert to reactants.

So well done if you got that correct.

Equilibrium is a state where opposing forces or processes are balanced.

So this could be mechanical equilibrium like a seesaw.

So opposing forces are equal on the seesaw.

Or thermal equilibrium where two objects at different temperatures that eventually reach the same temperature when in contact.

So an example of this would be a hot cup of coffee.

So heat will transfer from the coffee to the surroundings until they're both at the same temperature.

So these are two examples of opposing forces or processes becoming balanced.

Chemical equilibrium occurs when the forward reaction rate equals the backward reaction rate.

So we're talking about the rate of reaction here.

So at the start in this diagram, we've got the forward reaction here with a very high rate of reaction.

It's starting very high up the arrow, and we got a backward reverse reaction, which is at at zero, basically at the very start.

And then at the end, both reactions are happening at the same rate.

So that's effectively the speed of the reaction.

At equilibrium, the amount of reactants and products will remain constant.

Now that's important.

It doesn't say the same, it says constant.

So here we've got the reactants, and at the bottom we've got the products.

Now, if you look at the beginning of the reaction, we've got more reactants than products.

So in fact, we've got no products at the start of the reaction.

And then as the react reaction proceeds, the amount of reactant drops, and the amount of products increases, and then we know we're equilibrium because the amount of product remains constant, so the line is level, and the amount of reactants remains constant.

So the amount there is level.

They're parallel to each other and parallel to the x-axis, So we know that's equilibrium.

So equilibrium, there is no change in the amounts of either products or reactants, and that's important to know.

At equilibrium, the amounts of reactants and products remain constant as we just said.

So the forward and backward reactions continue to occur, and this is called dynamic equilibrium.

So even though the amount of reactant and the amount of products remain constant, the reaction is still happening.

It's still taking place.

So we say it's dynamic because it's still occurring.

At the molecular level, reactions are continuously happening.

The system is in a state of balance.

There is no overall change in the amount of reactants and products, even though both reactions are still occurring.

So here's a question based on that learning, this time true or false.

So equilibrium, the rate of the forward reaction is equal to the rate of the backward reaction.

Is that true or false? And then justify your answer using the statements below.

So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to that question is true, and this is because both reactions happen continuously with reactant making product and product making reactant.

So well done if you got that correct.

Here's the second question about that learning.

Which of these are true about chemical equilibrium? So read through the statements, decide what is true about chemical equilibrium, and I'll see you when you're finished.

Welcome back.

So the answers to that question are the forward and backward reaction rates are equal and the amounts of reactants and products remain constant.

So well done if you've got that correct.

One way to model dynamic equilibrium is with a busy road.

Cars enter and leave at the same rate, and the number of cars on the road remains constant.

So let's have a look at that here in this diagram.

So we've got two red cars and four blue cars on the road.

So we need to think about those cars entering and leaving at the same rate as each other, but the number of red cars and the number of blue cars remaining constant on that road.

So the red cars represent the amount of reactants at equilibrium, and the blue cars represent the amount of products at equilibrium.

And we can see it's dynamic because the cars are moving, but they're entering at the same rate.

So they're going at the same speed.

All of those cars are going at the same speed.

But despite the fact you've got cars entering and leaving, there's always two red cars and four blue cars on the road.

So here's a question based on that model.

What does the busy road model demonstrate about chemical equilibrium? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So you should have recognised that cars enter and leave the road at the same rate.

So well done if you've got that correct.

Here's a second question about that model.

In the busy road model, what remains constant at equilibrium? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to this question is the number of cars on the road.

So well done if you got that correct.

So let's have a look at task A.

We're going to go through this task together before pausing the video and answering it.

So reversible reaction takes place inside a glass container.

Select the diagram that best shows the colour or colours that will be observed.

So the equation for the reversible reaction between the two gases is shown there.

So we've got 2NO2 forming N2O4, And then underneath, we've got a key.

And this key is vitally important to be able to answer the question, so let's just have a look at that.

So if NO2 is present, we've got a dark brown colour.

If N2O4 is present, we've got colourless.

And if it's a mixture of the two substances, we've got light brown.

So we need to think about what's happening during that reversible reaction inside that glass container and what impact that might have on the colours.

So A, the colours change from dark brown through to colourless, B, the colour switches between dark brown and colourless, C, the colour stays like brown, or D, the colour changes straight from dark brown to colourless.

So pause the video here, think very carefully about your response, and I'll see you when you finished.

Welcome back.

So let's go through that answer then.

So there are the four potential answers.

The correct answer is C, the colour stays light brown.

And this is because as the reaction reaches equilibrium, both NO2 and N2O4 are present in the container, resulting in a light brown colour due to the mixture of both gases.

So well done if you got that correct.

So let's go on to question 2a.

Diagrams can be used to represent a chemical reaction at dynamic equilibrium, where nitrogen dioxide, NO2, can be represented by a blue circle and two red circles and dinitrogen tetroxide, or N204, can be represented by two blue circles and four red circles.

So you can see each blue circle represents a nitrogen atom and each red circle represents an oxygen atom.

So the question, some students are discussing the equilibrium 2NO2 forms N2O4, and it's a reversible reaction because we have the double-headed arrow there.

So the question A, which option most likely describes the number of reactant and product molecules at equilibrium for this reaction? So is it A where there are equal number of each type of molecule, or B, there are different numbers of each type of molecule? So read the question carefully, pause the video here, and I'll see you when you're finished.

Welcome back.

So let's have a look at the answer to that question then.

So the option that's most likely to be correct is B, and this is because whilst the concentration of reactants and products remains constant, they are very often not equal.

So constant doesn't mean equal, remember.

So B is the most likely correct answer.

It's well done if you've got that correct.

Let's have a look at 2b now.

So again, we're using these same diagrams. Some students are discussing the equilibrium.

Which option describes what is happening to reactant and product molecules at equilibrium for this reaction? So is it C where the individual molecules are not changing, or D, for each pair of smaller molecules combining, a larger molecule splits? Think very carefully about this question again.

Pause the video here and I'll see you when you're finished.

Welcome back.

So let's go through the answer to that question.

So the answer is D, because the rate of the forward and the back of reaction are equal, so for each pair of smaller molecules combining, a larger molecule must split.

So well done if you got that correct.

Now we're moving on to question three and four.

The molecules that make up nitrogen dioxide and dinitrogen tetroxide can be represented by those diagrams as we've seen before.

Question three, use molecules to explain why there are no observable changes when a chemical reaction is at equilibrium.

And four, use molecules to explain why the maximum possible quantity of product is not formed by a chemical reaction that is in equilibrium.

So you're gonna need to think very carefully about those questions.

Pause the video here, think about your response before you start answering, and I'll see you when you're finished.

Welcome back.

So let's have a look at the answers to that question then.

So for three, at chemical equilibrium, the proportion of reactant and product molecules remains the same.

So the observable properties, such as colour, will also remain the same.

So those kind of things that you are looking out for, when you're looking for a chemical reaction, they will stay the same.

And then four, product molecules may change back to reactant molecules.

Before all the reactant molecules have reacted.

There is never a full conversion of reactant molecules to product molecules.

'Cause if there was, we'd have that forward-facing arrow, that's single-headed arrow.

We wouldn't have the double-headed arrow.

So well done if you got that correct.

That was quite challenging, but I'm sure you completed it well.

Let's move on to part two of the lesson now.

Closed systems and equilibrium.

So a closed system is one where no matter can enter or leave, but energy can be transferred.

So if we see in that first diagram there, we we're heating up that substance inside that round bottom flask, and matter is able to leave from the open round bottom flask, so we can have gas escaping there through the neck.

In the other example, this system's considered closed as matter cannot leave, but obviously energy can transfer through the round bottom flask.

So here's a question based on that learning.

What is a closed system? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

So the answer to that question is B, a system where no matter can enter or leave, but energy can be transferred.

So it's not that nothing can enter or leave, matter cannot enter or leave.

So well done If you got that correct.

Dynamic equilibrium can only be achieved in a closed system.

If reactants and products can enter or leave the system, then the equilibrium cannot be maintained.

So continuous addition or removal of substances disrupts the balance between the forward and the backward reaction rates, and it prevents the system from reaching a state where reaction rates are equal.

Here is a question based on that learning, this time true or false.

Dynamic equilibrium can be achieved in an open system.

Is that true or false? And explain your answer.

Pause the video here and I'll see you when you're finished.

Welcome back.

So the answer to the question is false.

In an open system, matter can enter or leave, disrupting the balance needed for equilibrium.

So well done if you got that correct.

We're now moving on to task B.

So to understand chemical equilibrium, we can use a simple visual model involving two containers of water with food colouring.

So this you may carry out as an individual practical or is a demonstration, but we fill two containers with equal amounts of water, add a few drops of food colouring to one container and stir it until it's evenly mixed, and then pipette water from the first container to the second container.

And at the same time, transfer the same amount of colourless water from the second container to the first.

And keep repeating it several times, recording your observations.

We need to make sure that the rate remains the same, so we're doing them simultaneously.

So here is your potential observations for that reaction.

So the initial state, how it looks at the start is the first container will contain coloured water, and the second container will contain colourless water.

After the first transfer, you'll see a slightly less intense coloured water in one and very slightly coloured water in the other.

And over time, the colour intensity in both containers will become very similar.

And in equilibrium, the colour intensity remains the same in both containers with each transfer.

So it doesn't matter how much you transfer backwards and forwards, that colour will remain the same, and that's demonstrating equilibrium, dynamic equilibrium.

It's dynamic because you are still moving liquid from one container to another, and it's a equilibrium because the colour intensity is remaining the same.

So moving on to question two, evaluate the model you've just created.

Is it helpful when explaining equilibrium? Is it a good model? Does it demonstrate a closed system? That's the transfer of water.

Discuss that one.

And then three, repeat the process and evaluate the model you saw earlier involving cars entering and leaving a road.

Remember, this is where the red cars represent the amount of reactants at equilibrium, and the blue cars represent the amount of products at equilibrium.

So evaluate those two models, pause the video here, and I'll see you when you finished.

Welcome back.

So let's answer those questions.

So your answer to question two might be it demonstrates equilibrium, the colour, the amount of reactants and products of the mixture remains constant, and that indicates no observable change, which is something we mentioned earlier, remember? And it effectively mimics a closed system, which is essential for achieving equilibrium.

The model seems to produce two solutions, the reactant and product of equal concentration, and that's not typical, remember? That's a problem with the system.

Remember, we said earlier that it's unlikely that those concentrations are going to be the same for the reactants and the products.

And the model does not explain the submicroscopic level or the dynamic aspect of equilibrium where individual molecules are continuously changing.

So it doesn't talk about the dynamic part of the molecules.

So well done if you've got something similar to that.

For question three, it demonstrates equilibrium because the number of cars, the reactants and products on the road remains constant, and cars enter and leave at the same rate.

And this indicates that the rate of the forward reaction is equal to the rate of the backward reaction.

So that's some advantages of the model.

So problems with the model.

Ensuring no cars enter or leave from or to the external sources would be a better mimic in a closed system.

So it looks like an open system because the cars are disappearing off the end of the road and appearing on the end of the road, and that's essential for a true dynamic equilibrium for this closed system.

So it doesn't demonstrate a closed system very well, and the model does not explain, again, the submicroscopic level, the dynamic aspect of the equilibrium.

So it's not looking at those individual molecules and what's happening to them and how they're changing.

So that's an issue with that model.

So well done if you've got something similar to that.

Here is a summary of today's lesson.

At equilibrium, both reactants and products are formed at the same rate.

Amounts of reactants and products remain constant at equilibrium.

Equilibrium can only occur in a closed system.

So thank you for joining me for today's lesson.