Lesson video

This lesson is the first one in the topic, "Energy changes in reactions." And we're gonna talk through the different types of energy changes that take place during chemical reactions.

So let's get started on the lesson.

In today's lesson, we're going to complete a series of practicals to measure the temperature change and record appropriate results, concluding whether the reactions are exothermic or endothermic.

Here are the key words for today's lesson.

Exothermic, endothermic, solution, and calorimetry.

Now, some of those words will be familiar to you, others will be new.

So pause the video here, have a read through the different descriptions, and make any notes if you feel you need to.

Today's lesson has been divided into two separate sections.

Firstly, we're going to talk about endo and exothermic reactions and explain what they are, and then we're going to show how we can measure the energy changes that take place during chemical reactions.

So let's get started on part one of the lesson, exothermic and endothermic reactions.

An exothermic chemical reaction is a type of reaction in which energy is transferred to the surroundings from the reactants.

A good example of an exothermic reaction is combustion.

We can see here the temperature of the surroundings increases as a result of this chemical reaction.

So it shows that it's exothermic.

In this case, we can see methane plus oxygen, forms carbon dioxide plus water.

So this reaction is exothermic, energy is transferred to the surroundings.

Here is a question based on the learning so far.

"The energy in an exothermic reaction is transferred.

." So read through those statements and decide which one is correct.

Pause the video now, and I'll see you when you finished.

Welcome back.

So hopefully you've recognised that the answer to this question is B.

So the energy in an exothermic reaction is transferred to the surroundings which heat up, so the temperature of the surroundings increases.

Well done if you've got that correct.

Now, a strong acid, strong base neutralisation reaction is an example of an exothermic reaction.

So in this instance, we've got a strong acid, which is hydrochloric acid, reacting with a strong base, sodium hydroxide, forming a salt, sodium chloride, and water.

And during this reaction, the temperature of the solution, which is the surroundings, will increase.

Most neutralisation reactions are actually exothermic, but some of them are endothermic.

So if we have weak acids, for example, citric acid, and weak bases, for example, sodium hydrogen carbonate, these type of reactions might end up absorbing energy, so they're endothermic.

So most neutralisation reactions are exothermic, but some can be endothermic.

Here's a question based on the learning so far.

So which image correctly shows the measurement of the energy change occurring during neutralisation? I'll just give you a moment to think about it.

So hopefully you've recognised that it's A.

So we are measuring the temperature of the surroundings, but remember the solution is included in that.

So the solution, the reacting solution, is the surrounding.

So we're measuring the temperature of that.

Well done if you've got that correct.

Now, displacement reactions are also exothermic, and this is where a more reactive element replaces a less reactive element in the compound.

So the temperature of the surrounding solution will increase.

So this shows it's an exothermic reaction.

If we were to measure that temperature, it would increase.

Here's a good example of an exothermic displacement reaction.

So we've got silver nitrate plus copper, forming silver and copper nitrate.

Now copper is more reactive than silver, so it displaces silver from its compound to form copper nitrate.

And this reaction is exothermic.

So if you were to measure the temperature, you would see that the temperature increases.

So after an exothermic reaction has happened, the energy then dissipates into the environment from the reaction mixture.

So it goes from the surroundings into the environment, and we call that dissipation.

So the energy is dissipating.

So you can see that happening in the diagram here where the arrows are representing the energy transferring from the solution to the environment.

And this means that the solution will decrease in temperature over time.

So it cools down until it's at equilibrium with the room temperature.

So it's equal to the room temperature.

So as the environment is so large, the energy increase only has a very small impact on the temperature of the room.

So if you think about how much air there is actually in the classroom, the temperature increase there as the energy dissipates is minimal.

So here's a question based on that learning so far.

So after an exothermic reaction has taken place, it cools to room temperature due to energy transfer to the reaction solution.

So decide if that statement is true or false, and then justify your answer using either A or B.

So pause the video here and answer the question.

Welcome back.

So hopefully you've recognised that that statement is in fact false, and it's because the energy is dissipating to the environment, not to the reaction solution.

So take care when reading through the question, and make sure you've actually got the statement correct.

So well done if you got that correct.

So exothermic reactions have lots of everyday uses as well.

So we use them in real life.

So some good examples of this would be hand warmers, but also self-heating meals and self-heating cans of drink.

So we've got an example here.

So this is self-heating noodles.

And the arrows show the direction of energy transfer from the chemical reaction, which is inside that white packet there, to the surroundings.

And if you place that pack of noodles on top, that little thing of noodles there on top, then it will be heated as a result of that exothermic chemical reaction taking place in that packet there.

Now an endothermic chemical reaction is a type of reaction in which energy from the surroundings is transferred into the reaction.

And a good example of this is photosynthesis.

And during photosynthesis, the temperature of the surroundings, that's the immediate surroundings, actually decreases.

So photosynthesis is a really good example of an endothermic reaction.

And you'll know from your biology lessons, the chemical equation for photosynthesis, which is carbon dioxide plus water, forming glucose plus oxygen.

And we've got the symbol equation there at the bottom too.

So here's a question based on that learning.

"The energy in an endothermic reaction is transferred.

." So read through those statements.

Decide which one is correct.

So pause the video here, and I'll see you when you've answered the question.

Welcome back.

So hopefully you recognise that the correct answer to that question is A, from the surroundings which cool down.

So the energy in an endothermic reaction is transferred from the surroundings to the reaction mixture.

Now, most decomposition reactions, where the reactants are broken down into simpler substances, are endothermic reactions.

So thermal decomposition is an example of an endothermic reaction.

So electrolysis here, in electrolysis, energy is absorbed from the surroundings via the electrical pathway to decompose or break down the reactants and form the products.

So this is another example of a decomposition reaction where energy is absorbed from the surroundings.

So heat in this instance, copper chloride, is being decomposed into copper and chlorine, and you can see the copper forming there on the electrode and the bubbles of chlorine gas.

Now in thermal decomposition, the energy is absorbed from the surroundings to break down the reactant into simpler products.

So here's a good example of thermal decomposition.

This is copper carbonate being decomposed into copper oxide and carbon dioxide.

And we can actually see that taking place during this video.

So the copper carbonate is green in colour, and copper oxide is black.

So if you watch very carefully, as the powder is heated by the blue Bunsen flame, you can see it changing colour from green to black.

So that's changing from copper carbonate to copper oxide.

And if we were to collect the gas that's being produced and test it with lime water, we'd be able to show that it's carbon dioxide.

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

So read through the statement, decide if it's true or false, and then justify it using one of the two statements, A or B below.

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

Welcome back.

So hopefully you've recognised that that statement is actually false.

And the reason it's false is because energy is transferred from the surroundings, so the temperature of the surroundings will actually decrease.

Remember, decomposition is an example of an endothermic reaction.

So the energy is being transferred from the surroundings into the reaction.

So after an endothermic reaction has taken place, energy from the environment this time, transfers into the solution, into the surroundings, and that warms it back up to room temperature again.

So as the environment is so large, the energy decrease only has a very small impact on the temperature of the room.

Remember, very much like the exothermic reaction.

And we can see that in the diagram here.

So energy from the environment is transferring to the surroundings, which is the solution.

Endothermic reactions also have everyday uses.

So we use them in self-cooling drinks and some sports injury cool pack.

So we've got an example of that here.

And the arrows show the direction of the energy transfer from the surroundings.

So that's from the injured body part where you've placed it to the chemical reaction in the cooling pack.

And this is why they feel cool.

The energy from your body is being transferred into the reaction.

So it's being used in the reaction.

Here's a question based on that learning so far.

"Which arrow is correctly pointing to the surroundings for these endothermic reactions?" So you've got three different endothermic reactions, and we need to know which one of these has got an arrow pointing to the surroundings.

So pause the video here, and I'll see you when you're ready.

So hopefully you've recognised that the answer to this question is A.

So we've got the solution here acting as the surroundings.

Now, some types of reaction might be either exothermic or endothermic, very much depending on the reactants that you actually use for the reaction.

Now, precipitation reactions involve solid products being formed from reactant solutions.

So if you have two solutions and you react to them together, the solid that's produced during that is insoluble, so it forms what we call a precipitate, and we're gonna watch that happening in a moment.

We're gonna have two colourless liquids which are going to be mixed together.

They're going to react, and they immediately formed a yellow precipitate.

So let's watch that.

See, you can see the two colourless solutions reacting together, forming that yellow precipitate.

So the yellow precipitate is lead iodide, and it's being formed by mixing lead nitrate with potassium iodide, and we can see that in a chemical reaction here.

So if you look carefully, the lead nitrate and the potassium iodide, the two reactants, are both aqueous, aq, so that means they're soluble in water, they're dissolved in water.

But if you look at the products, you can see the potassium nitrate at the end there, is aqueous.

So that's soluble in water.

But the lead iodide, PbI2, has got an s in brackets next to it, so that shows you it's solid, not dissolved in the solution, and that's the precipitate there, the yellow precipitate.

So this is an endothermic precipitation reaction.

So an example of an endothermic reaction, and it's a precipitation reaction because it's forming that insoluble solid during the reaction.

An example of an exothermic precipitation reaction is barium chloride reacting with sulfuric acid.

And here the temperature of the surroundings, that is the solution, will increase.

So if you put a thermometer in there, the temperature would increase.

And that whiteness in the solution is the white precipitate formed, which is barium sulphate.

And you can see that in the equation here.

So barium sulphate is a solid, so it forms a precipitate in the reaction.

Now, dissolving salt in a solvent to make a solution can be exothermic or endothermic again, and that's depends on the salt and the solvent being used.

So if you were to dissolve sodium ethanoate in water, that would be endothermic.

So if you had a thermometer in there, the temperature would decrease.

And here's the equation for that reaction.

So the temperature of the surroundings, that's the solution itself, will decrease.

But if we dissolved potassium chloride in water, that would be exothermic, and there's the reaction for that, and the temperature of the surroundings this time would increase, because it's an exothermic reaction.

So now we're going to have it go at Task A, which is based on that first part of the lesson.

So you need to draw a Venn diagram like the one below, and then decide whether those reactions or processes inside the table are exothermic, endothermic, or could be either.

So they could be exothermic or endothermic, depending on the reactants that are used.

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

Welcome back.

So let's go through the answers to that question then.

So here's the Venn diagram.

We've got exothermic, endothermic, or an either, and then we're going to fill in the different reactions.

So neutralisation could be either exothermic, combustion, displacement, and respiration, and then endothermic, electrolysis, thermal decomposition, and photosynthesis.

And then we've got dissolving, salt and water, and precipitation can be either as well.

So well done if you've got all of those correct.

So let's move on to question 2.

"Complete the following for each type of reaction." So for each reaction, exothermic and endothermic, you need to answer A, B, and C.

So A, B, and C for exothermic, and then A, B, and C for endothermic.

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

So let's go through those answers then.

We'll talk about exothermic to start off with.

So, exothermic reactions are a type of reaction in which energy is transferred from the reaction to the surroundings, and the temperature of the surroundings increases.

So you really need to talk about energy and temperature in your answer.

So do check that.

Give an example.

So there's lots of different examples you could have given.

So either combustion, neutralisation, displacement, or respiration.

And then an example of everyday use could be a hand warmer, or self-heating meals or self-heating cans.

Now, for endothermic reactions, this is a type of reaction in which energy from the surroundings is transferred to the reaction, and the temperature of the surroundings decreases.

So again, talk about energy and talk about temperature.

Lots of different examples again that you may have given.

So photosynthesis, electrolysis, thermal decomposition, and an example of an everyday use could be self-cooling drinks or sports injury cool packs.

So well done if you've got those answers correct.

So we're now going to move on to the second part of the lesson, the measurement of energy changes.

So we're going to talk through the experimental technique you can use to measure energy changes happening during chemical reactions.

So the energy changes taking place during exo and endothermic reactions can be measured using temperature, and this process is called calorimetry.

So the initial and the highest or the lowest temperature are measured, and then these are used to calculate the temperature change.

So let's have a look at an example.

So we've got the initial temperature there and the final temperature.

And we can see there's a change in temperature, there's an increase in temperature.

So we could put some numbers to this to give an example.

So we've got the final temperature minus the initial temperature, giving us the temperature change.

And it's important to get that way round and you'll see why later.

So an example could be 39 degrees Celsius minus 22 degrees Celsius equals 17 degrees celsius.

So the temperature change is plus 17 degrees Celsius, the temperature's gone up.

So here's a question based on that learning.

I want you to select the correct statements from the list.

So pause the video here, read through the statements, and I'll see you when you finished.

Welcome back.

So let's go through the statements and see which ones are correct.

So you should have recognised the A is correct.

So we measure the initial and the lowest temperature reached for endothermic reactions.

And then C is also correct.

We measure the initial and the highest temperature for exothermic reactions.

So well done if you got that correct.

So here is the equipment needed for calorimetry experiments.

So we've got a beaker, and that's stabilising the reaction to prevent it from being knocked over.

We've got insulated polystyrene cup with lid.

We've got a thermometer or a temperature probe, a stirring rod, and then obviously we've got the reactants themselves.

Now, there are several points to consider when we're measuring energy change using temperature.

So the reason we've got that insulated cup with a lid is to help minimise that heat loss or gain to and from the surroundings.

We need to make sure that the reactants are thoroughly mixed, so that's why we have the stirring rod.

And to reliably measure the temperature, we need to think about repeating the experiment more than once and taking a mean.

And we need to ensure that the reaction is left for long enough to make sure we've actually found the maximum temperature change.

So that's the maximum high or maximum low temperature.

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

So it says, "A polystyrene cup is used to provide insulation and reduce any heat loss only." So I want you to read that statement, decide if it's true or false, and then justify your answer using the two statements below.

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

Now, the word only in that question is actually really important, because the statement is false.

And it's false because it's provided to minimise heat loss for exothermic reactions and endothermic reactions.

And remember, in endothermic reactions, we're talking about heat gain rather than heat loss.

So well done if you've got the answer to that question correct.

Now, temperature can be measured using a thermometer or a temperature probe.

So the advantage of using a thermometer is it's cheap, it's very easy to use, but unfortunately it's fragile, and it's also easy to misread as well.

A temperature probe, however, is easy to read and it's got greater resolution because the decimal places you can see there on the screen.

But it is expensive and can be difficult to set up and use as well.

So the two different methods have got advantages and disadvantages.

Now, here's a method to perform calorimetry, a suggested method.

And in later lessons, we're going to go through this in a bit more detail, but we are just touching on it at the moment.

So we'd set up the equipment as shown in the diagram.

We would add one reactant to the polystyrene cup, and then we would measure and record the initial temperature.

This is before we add the other reactant.

Then we'd add the second reactant, and we'd stir it thoroughly to make sure that the reaction has taken place correctly.

And then measure and record the highest or the lowest temperature reached, that's the final temperature.

So let's have a go at Task B.

So we're gonna carry out the following reaction using the previous method.

We're gonna record your results in a table like the one below.

So we've got four different sets of reactants there, and we're gonna have a go at reacting those together, and we're gonna record the initial and the final temperature, we're gonna calculate the temperature change, and then work out if they're exothermic or endothermic.

So if you've carried out that practical, your results may well look like this.

So there is reactant 1 and reactant 2, and we can just fill in some temperatures.

So here we've got an initial temperature of 25 degrees celsius, a final temperature of 42, so the temperature change is 17.

And because that's an increase in temperature, we know that's an exothermic reaction, but it's also a neutralisation reaction as well because we've got an acid and an alkali there.

For the second one, we've got the start temperature or initial temperature of 25, the final temperature of 20.

So this time the temperature has a decreased by 5, so we put -5, and this is an endothermic reaction.

And again, this is a neutralisation reaction.

So we've got a weak acid this time and a weak alkali.

For copper sulphate and magnesium powder, the start temperature, again, 25, and the final temperature 47, so this is an increase in temperature of 22 degrees Celsius.

So again, this is an exothermic reaction, and this time it's a displacement reaction.

So the magnesium has displaced the copper from the copper sulphate.

And then lastly, we've got barium chloride with sulfuric acid.

The start temperature of 25, the final temperature of 28, so we've got a slight increase in temperature of 3.

This is an exothermic reaction.

And this time, if you remember from earlier, this is a precipitation reaction which will form a white precipitate.

Now going to answer question 2 about how to improve this experiment.

So A, energy loss/gain to and from the surroundings is the main problem when conducting these experiments.

How could you improve the measurement of the energy change? So think about how you might improve that measurement to prevent that energy transfer between the reaction itself and the surroundings.

B, explain why taking only one measurement for the initial temperature and the final temperature could be a problem.

And then C, what are the advantages and disadvantages of using a temperature probe rather than a thermometer? So pause the video here, and I'll see you when you finished.

So let's start with question 2A.

So how can we minimise this transfer of energy? So we need to ensure that the container is insulated and it has a lid.

We need to thoroughly mix those reactants to ensure the reaction fully takes place.

And we need to allow it time to fully take place as well to ensure we've got that range of temperature, from the initial temperature to the absolute final temperature to get that proper temperature range.

So for 2B, there are several different answers that you could have for this one.

So there might be an anomalous result.

And if you've only carried out the experiment once, you wouldn't know necessarily if that was anomalous or not.

So the thermometer might have been misread, for example, or not enough time allowed for the maximum temperature change to be reached.

So what we could do to overcome this, is the initial temperature reading could be taken multiple times over a longer period of time, and then a mean value taken.

The experiment itself could be repeated multiple times, and then the mean initial and final temperature taken.

And also, the temperature could be monitored for a longer period of time to make sure we've actually measured that final temperature.

And then 2C, the advantages and disadvantages of using temperature probe rather than thermometer.

So you may have some of the following in your answers.

So the advantages, it's more sensitive, and also the digital readout is easier to read and easier to record.

And then disadvantages, it's expensive and it might be difficult to set up and use.

So well done if you've got the answers to those questions correct.

Here's the summary of today's lesson.

So changes in energy accompany neutralisation, displacement, and precipitation reactions.

Changes in energy accompany salts dissolving in water.

When reactions take place in solution, temperature changes can be measured to reflect those energy changes.

And exothermic and endothermic reactions have everyday uses.

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