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 Introduction to reversible chemical reactions.

So let's get started on the learning.

During today's lesson, you will describe reversible chemical reactions, represent them using chemical equations, and give examples.

Here are the keywords for today's lesson: reversible reaction, hydrated, anhydrous, double-headed arrow, and single-headed arrow.

Here are those keywords used in sentences.

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

Today's lesson will be divided into two parts: reaction arrows and reversibility, and investigating reversible reactions.

So let's start on part one, reaction arrows and reversibility.

An irreversible reaction is one where reactants form products and the reaction goes to completion.

So we have an example here of carbon reacting with oxygen to form carbon dioxide.

Now this reaction will only stop when one of those reactants runs out, and we call that reactant the limiting reactant.

The other reactant we say is in excess.

A single-headed arrow is used to show how the reactants turn into products.

These chemical reactions do not readily reverse.

So we can turn carbon dioxide back into carbon and oxygen, but it's difficult to do.

Physical changes are easily reversible by changing the conditions.

So water can change state from ice to liquid when heated, and on further heating turn into steam.

And we can represent this using this equation.

So you can see here we've got solid water, or ice, turning into liquid water and then into gaseous water, which we would call steam.

Steam can then be cooled to condense back into liquid water, and frozen again to become ice.

And we can represent this using an equation, this time with the gaseous water at the start and the solid water at the end.

We can simplify these two equations down into this equation here.

We can see we've got a double-headed arrow in between each of the substances there.

In a reversible reaction, both the forward and backward reactions can occur under the right conditions.

So when we're talking about conditions, we might be talking about temperature or pressure, for example.

So here's an equation for the reaction between nitrogen and hydrogen to form ammonia.

Reversible reactions like this are symbolised using this double-headed arrow, and these reactions can proceed in both directions given the correct conditions.

So we could have nitrogen and hydrogen forming ammonia, for example, but ammonia can break down into hydrogen and nitrogen as well, and it's relatively easy for that reaction to go in either direction.

So this direction we call the forward reaction, and this direction we call the backward reaction.

So here's a question based on that learning.

In a reversible reaction, the forward reaction is the one where.

Pause the video here, choose the right answer, and I'll see you when you're finished.

Welcome back.

So hopefully you've realised that the correct answer to that question is reactants form products.

So in a reversible reaction, the forward reaction is the one where reactants form products.

So well done if you got that correct.

Here's a second question based on that learning, this time true or false.

So reversible reactions are symbolised using a double-headed arrow.

So is that true or false? And then justify your answer using one of the two statements below.

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

Welcome back.

So the correct answer is true, and this is because the reaction can proceed in both the forward and backward directions.

So well done if you got that correct.

So why are some reactions reversible? While many reactions are irreversible, some are reversible, and this is because the reactants and products may have comparable stability.

So in other words, they may both be stable.

The conditions might be easily met.

So it might be easy to get to a particular temperature or pressure to enable that reaction to go in either direction.

So here's a good example.

Again, we're making ammonia using nitrogen and hydrogen.

So at room temperature, both the forward and the backward reactions occur at a very slow rate.

However, at higher temperatures and pressures, the rates of both reactions increase significantly.

In reversible reactions, the forward and reverse reactions have different energy changes.

So again, we are looking at that production of ammonia from nitrogen and hydrogen.

One direction of a reversible reaction is always exothermic and the other endothermic, but which one will depend on the reaction.

So some will have a forward reaction that's exothermic, and some will have a forward reaction that's endothermic.

So the same amount of energy is transferred in each case.

It doesn't matter which direction you're going in, it'll be the same amount of energy.

So in this instance, the forward reaction is exothermic and the reverse reaction is endothermic.

And that will become important later on when we're talking about the conditions that affect reversible reactions.

The direction of a reversible reaction can be influenced by changing directions, such as temperature.

So here we've got the exothermic and the endothermic reaction.

By supplying lots of energy, in other words, by heating, the endothermic reaction can be favoured.

By cooling a reaction down, we can favour the exothermic reaction.

Here's a question based on that learning.

Which of the following statements about reversible reactions is true? So pause the video here, answer the question, and I'll see you when you're finished.

Welcome back.

Which of the following statements about reversible reactions is true then? Hopefully you've recognised that reversible reactions can proceed in both directions under the right conditions, and that changing the temperature can influence the direction of a reversible reaction.

Well done if you got that correct.

So moving on to task A then.

Some students are discussing reversible reactions.

Identify who is correct and update any incorrect statements.

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

Welcome back.

So let's go through those answers then.

So in this instance, Aisha and Izzy are both correct.

So reversible reactions can be influenced by changing conditions such as temperature, and in a reversible reaction, the reactants can form products, and the products can react to form the reactants again.

So Alex is incorrect.

Alex says that combustion reactions can easily be reversed, and actually combustion reactions are generally considered to be irreversible.

So they would have an arrow going in one direction.

And then Jacob, water changing from ice to liquid and then to steam is a reversible chemical reaction.

Remember, this is a physical change, not a chemical reaction.

So do take care.

Even though it's reversible, it's a physical change, not a chemical reaction.

So well done if you got that correct.

So moving on to part two of the lesson then, investigating reversible reactions.

Copper II sulphate is often seen as blue crystals, and you may well have used these in lessons.

However, it's actually only one of two forms of copper II sulphate, and we can see that in this equation here.

So we can see the equation is reversible because it's got the two-headed arrow in the centre.

And on one side we've got hydrated copper sulphate, which is in combination with water.

And on the other side we've got anhydrous copper sulphate, which means it's not in combination with water.

And if we look at the equation below that, we can see on one side it being combined with water.

So we've got.

5H2O, and on the other side, the 5H2O is separate from the copper sulphate.

So you can see the difference between the two there.

And we've got a forward reaction and a backward reaction.

So this reaction is reversible under the right conditions.

So we have an endothermic forward reaction and an exothermic reverse reaction.

Ammonium chloride will decompose on heating to form ammonia and hydrogen chloride.

And again, we can see this in the reaction.

So we've got ammonium chloride forming ammonia and hydrogen chloride in the forward reaction.

And then we've got the backward reaction in the reverse, and we can see it's a reversible reaction again because it's got the double-headed arrow.

So this reaction is reversible under the right conditions.

So if you look this time, the forward reaction is endothermic, and the reverse reaction is exothermic.

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

So in a reversible reaction, the forward reaction is always endothermic.

Is that true or false? And justify your answer.

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

Welcome back.

So the answer to that question is false, and this is because the forward reaction can be either exothermic or endothermic depending on the specific reaction.

So well done if you got that correct.

Here is task B.

So you may well carry out this task.

It's a couple of practicals.

So the first one, you've got five grammes of powdered hydrated copper sulphate in a test tube, and you're going to heat it until the colour no longer changes.

And you are forming anhydrous copper sulphate because you are removing the water.

And this is a reversible reaction.

So it is asking you to record any observations made during the heating process and when the water was poured back into the anhydrous copper sulphate.

So pause the video here if you're carrying out the experiment, and follow the method.

Here is experiment number two.

This time we're putting five grammes of ammonium chloride in a test tube, plugging the end of the test tube, and gently heating, and recording any observations made during the heating process.

We need to be careful because both ammonia and hydrogen chloride are harmful gases.

So it may well be that you see this as a demonstration rather than a class practical.

If you carried out experiment number one, your results table might look something like this.

So at the start, the hydrated copper sulphate will look like blue powder or blue crystals, and upon heating, the solid will change colour from blue to white and you might see steam as well.

And then the cooling anhydrous copper sulphate will remain as a white powder.

And then when you add water to the anhydrous copper sulphate, it will turn blue and heat will be released.

So it will be an exothermic reaction.

The reaction will feel warm.

So hopefully your observations agree with that.

And if you carried out or observed experiment number two, your results table might look like this.

So at the start, we've got a white powder or white crystals.

Upon heating, the white solid will slowly disappear as it's decomposing, and it will produce a colourless gas or some colourless gases.

So that will be the ammonia and the hydrogen chloride.

And then observations during the cooling process: white deposits form in the cooler regions of the apparatus as ammonium chloride reforms from these two gases.

Here is a summary of today's lesson.

Different types of arrows are used in chemical equations to indicate the nature of the reaction.

So we can have a single arrow or a double-headed arrow.

Reversible reactions are symbolised using the double-headed arrow in chemical equations.

In a reversible reaction, both the forward and reverse reactions can occur under the right conditions.

And this is exemplified by hydrated copper sulphate forming anhydrous copper sulphate and water, with a colour change from blue to white, or ammonium chloride decomposing into ammonia and hydrogen chloride.

And both of those reactions are reversible, so they can go in either direction.