Lesson video

Hello there, I'm Mr. Forbes, and welcome to this lesson for the energy of moving particles unit.

This lesson's called specific latent heat and it's all about the changes of state of a substance and the energy required to cause those changes.

By the end of this video, you're going to be able to describe the changes of state and the energy changes required for those changes.

You're also going to be able to calculate the amount of energy required for a change of state.

Here are the keywords that'll help you through the lesson.

The first of them is melting and that's the change of state, when a solid turns into a liquid, and that happens at a very specific temperature.

Then there's boiling the change of state from a liquid to a gas, and again, that happens at a different but fixed temperature.

Then we have specific latent heat and that's the amount of energy transferred when one kilogramme of a material changes its state from one state to another.

And finally, we've got specific heat capacity and that's the change in internal energy when the temperature of a substance changes by one degree Celsius and that's per kilogramme.

You can return to this slide at any point in the lesson.

This lesson's in two parts and in the first part we're going to look at a heating curve, a graph we generate when we provide energy to a substance that caused it to heat up and/or change state.

In the second part of the lesson, we're going to look at specific latent heat in particular and how to calculate the energy required for a change in state.

So when you're ready, let's start by looking at interpreting a heating curve.

When you heat a substance, you provide it with energy and you'd expect its temperature to increase over time as you provided it with that energy.

I've got a graph here showing the temperature of a substance where I'm heating it at a constant rate over a period of time.

And as you can see, there are several key parts to this graph.

In the first part of the graph, here we can see that the temperature is increasing with time, so it is actually getting hotter.

But then in this second part of the graph, we've got a period of time where the temperature isn't changing.

Then we have a part where the temperature changes again, so we've got a gradual increase in temperature over this period of time.

And finally, we have got another part here where the temperature isn't increasing again.

So we need to be able to explain why there's those different sections of the graph.

Let's start by looking at the first section of the graph.

And at this point, we've got a solid that we're heating.

So we're gonna heat that solid, providing it with energy until it reaches its melting point.

What's happening during this is the average speed of the particles is increasing as we're providing them with more energy.

So as we heat them up with a Bunsen or electrical heater, the particles gain energy and the average speed increases, they get faster.

The temperature is actually related to the average speed of the particles.

So because the average speed's increasing, the temperature is increasing as well.

If we continue to heat a solid, it's going to increase in temperature until it reaches its melting point, and its melting point are pure substance is a fixed value.

So I've got different substances with some different melting points here.

The one you should be familiar with is water.

If you heat up water until it reaches zero degree Celsius, it's going to start to melt.

So the ice is going to start to change into liquid water.

But other substances have different melting points.

Ethanol's got a much lower melting point, gold are much higher, over a thousand degree Celsius.

Mercury, as an example of a metal that is liquid at room temperature, it melts at -39 and so on.

Here's a question for you now.

"An engineer finds the melting point of a 0.

5 kilogramme sample of a metal and it's 600 degree Celsius.

What's the melting point of a one kilogramme sample of the very same metal?" Pause the video, make your selection and then restart, please.

Welcome back.

Hopefully, you selected 600 degree Celsius.

The melting point of a substance is the same no matter how much of it there is.

It doesn't depend on the mass of the metal sample.

If a small sample has a melting point of 600 degree Celsius, a large sample will have the same melting point.

So well done if you selected that.

Once a substance reaches its melting point, its temperature stops rising even though you're providing it with energy.

So my graph here shows that the temperature isn't rising during this next section.

So during the melting process, we're not getting a change in temperature.

Energy is still being transferred to that substance, but the speed of the particles or the average speed of the particles isn't increasing anymore because the temperature's not increasing.

The faster moving particles are instead starting to pull away from each other.

They're separating slightly, but they're being slowed down by the electrostatic forces of attraction that keep them together.

So what we're doing instead with that energy is the particles are separating without gaining any extra speed.

The temperature doesn't increase because there's no increase in speed of the particles or average speed of the particles at all.

So we're getting no temperature change because the energy is doing something else, it's causing the particles to separate rather than get faster.

If we continue to heat the substance once it's melted, that liquid's going to gradually increase in its temperature towards its boiling point.

So that's the next section of the graph as indicated here.

What's happening there is the speeds of the particles is increasing again, and because the speed of the particles is increasing, then the temperature is rising again.

So always linking that temperature to the average speed of the particles.

So during this section we get a gradual increase in temperature as we provide energy.

That increase in temperature will continue until we reach the boiling point of the substance.

And I'm gonna talk about the boiling points of just pure substances, 'cause if you mix things together then you'll get a range of boiling points, but a pure substance has a fixed boiling point.

So for example, the boiling point of water, I'm sure you're aware, is 100 degree Celsius.

And I've got the boiling points of some other substances here.

Ethanol's got a lower boiling point, that will boil much more easily.

You can boil gold if you actually raise its temperature to almost 3000 degree Celsius.

So it's very difficult to do, but it can be done.

And something like nitrogen, well, its boiling point is very, very low, and so it's a gas at room temperature.

It's already all boiled and turned into a gas.

Another check for you here.

I've got a type of wax and it's got a melting point of 55 degree Celsius and a thermometer is placed into a mixture of solid and molten wax.

What temperature will that thermometer show when I've got solid and molten wax together? So pause the video, make a decision and restart, please.

Welcome back.

Hopefully you picked 55 degree Celsius.

The melting point is 55 and that's the only temperature out which you'll have solid and liquid wax at the same time.

So well done if you selected that.

When that liquid reaches its boiling point, its temperature stops rising again.

So we've got another section of the graph here where there's no increase in temperature.

What's happening there is energy is still being provided to the substance.

So it's still gaining energy 'cause we're still heating it, but the average speed of the particles isn't increasing again.

Remember, average speed is linked to temperature.

So what's going to happen is that the faster particles in that liquid are gonna start to escape from the liquid.

And those faster particles are taking away excess energy and that means that the particles left behind, the average energy and the average speed, isn't changing.

So that remains constant and so the temperature remains constant 'cause we're getting no changes in the average speed of the particles.

That temperature cannot increase anymore until all of the particles in that liquid have gained enough energy to escape the electrostatic forces of attraction between them and the whole of the substance has changed into a gas.

So we get no change in temperature at the boiling point.

Okay, another check for you now.

The melting point of ice is zero degree Celsius.

Ice is kept inside a freezer and the temperature of that freezer is set to -10 degree Celsius.

As soon as the ice is taken out the freezer, it's placed in a beaker and its temperature is measured each minute until all the ice has melted.

Which of those three statements is correct, please? So pause the video, make your decision and restart.

Welcome back.

Hopefully you selected the last of those.

So the temperature of any water in the beaker during the melting process is going to be zero degree Celsius until all the ice has melted in it.

Well done if you got that.

Okay, now it's time for the first task of the lesson, and what I'd like you to do is to have a look at this graph.

It shows the cooling of a pure substance over a period of time.

So I've got this pure substance and put it somewhere very cold until it changes state and it's all turned into a solid.

What I'd like you to do is to label the boiling point and the melting point of the substance on the graph, and describe what happens to the temperature and the behaviour of the particles in the substance during the cooling processes X and Y there.

So there's just two phases in the graph.

So pause the video, work out your answers to those and restart, please.

Welcome back.

Hopefully, your answers look something like this.

I've marked the boiling point and the melting point there.

The boiling point is the higher of those temperatures where there's no change in temperatures.

So the flat lines here towards the top is the boiling point and the flat line lower down is the melting point.

And the descriptions of what's happening to the temperatures and the particle behaviour, during phase X, the temperature is decreasing.

You can see that the temperature goes down.

So that must mean the average speed of the particles is decreasing.

Some of the energy's being dissipated to the surroundings, that's where the energy goes.

During phase Y, the energy, the particles, is still decreasing.

I'm still cooling it there, but the temperature's not decreasing.

And what's happening there is the energy is being dissipated as the particles move closer together and the forces of attraction, the electrostatic forces between them, get stronger.

Well done if you got answers like that.

Now it's time to move on to the second part of the lesson.

And then we're going to be looking at specific latent heat, which has to do with the energy requirement for a change of state, and we'll be doing some calculations.

So let's do that.

You'll need different amounts of energy to change the state of different substances.

It's easier to change the state of some than it is to change the state of others, and that's because there's different strengths of bonds between the particles in that substance.

The amount of energy required to change the state is called the latent heat.

So for a particular substance we'll have a latent heat, and that's a measure of how much energy it would require to change its state.

And for a fair measure of how difficult it is to melt or boil a substance, we'll use the specific latent heat.

Now that's the amount of energy per kilogramme for a particular substance.

So specific latent heat is how much energy is required to change the state of one kilogramme of a substance.

As we've seen already, there are two different changes of states, we can have melting and we can have boiling, or conversely freezing and condensing.

So there's two different changes of state there.

So there's two different values for the specific latent heat for each substance and those can be very different.

So we've got these changes of state.

I've got water changing from a solid to a liquid, and that's melting, and then changing from the liquid to a gas, and that's evaporation, and the reverse of those processes which is condensation and freezing, and the amount of energy per kilogramme for those changes of state for water is shown here.

So to melt one kilogramme of water I need 334 kilojoules for every kilogramme.

To freeze it then would have a change of energy that's exactly the same.

It would give out 334 kilojoules per kilogramme of water on a gen state.

And that's very different from changing the state from liquid to a gas.

It requires much, much more energy to do that.

2,260 kilojoules per kilogramme to boil or evaporate one kilogramme of water.

And obviously, the reverse process, that energy is dissipated back into the surroundings in exact same amount.

The table here shows you some of those specific latent heats for some different substances.

So you can see the values for water put in the.

They are 334 kilojoules per kilogramme to melt or freeze and 2,260 to evaporate or condense.

So you can see quite a lot of energy required to change the state of water.

Ethanol, it's easier to change the state.

Again, it's more difficult to change it from a liquid to a gas than it is from a solid to a liquid, alright? But it's easier to affect in water.

Mercury's very easy to change the state and see the values much lower, and nitrogen even lower still.

So in nitrogen it's very easy to change the state off.

We need very large amounts of energy for those changes of state.

So that's why we're measuring them in kilojoules per kilogramme.

And changes of state from a solid to liquid or liquid to a solid involve those specific latent heats of fusion.

So the changes of state involving a solid, it's called a specific latent heat of fusion.

And the changes of state from a gas, or back into from a gas, is the latent heat of vaporisation.

As usual, we have an equation that links the variables that we've been discussing, energy mass and specific latent heat.

And that equation is thermal energy for a change in state is mass times specific latent heat.

In symbols that can be written as E = ml.

or E is the energy change in joules.

M is the mass in kilogrammes and L is the specific latent heat in joules per kilogramme.

Let's have a look at an example of using that equation.

So I've got a question here.

The specific latent heat of fusion for ice when it's melting is 334 kilojoules per kilogramme.

Calculate the energy required to melt 1.

5 kilogrammes of ice.

So what I do is I'll write out the equation E = ml.

I substitute the values for the mass and the latent heat of fusion from the question.

So that's 1.

5 kilogrammes for the mass, 334 kilojoules per kilogramme for the latent heat of fusion, and I just multiply those together to get 501 kilojoules.

So I need 501 kilojoules to melt 1.

5 kilogrammes of ice.

Now it's your turn.

What I'd like you to do is calculate the energy required to evaporate 1.

5 kilogrammes of water.

So pause the video, work out your answer to that and restart when you're done, please.

Welcome back.

Hopefully you did this.

Write out the equation, substitute the values from the question, 1.

5 kilogrammes, 2,260 kilojoules per kilogramme, and that gives an answer of 3,390 kilojoules.

Well done if you've got that.

We can also use the equation to find other things such as the mass.

So we've got another example here.

The specific latent heat of vaporisation for ethanol is 108 kilojoules per kilogramme.

Calculate the mass of boiling ethanol, which will evaporate when I provide it with 50 kilojoules of energy by heating it.

So to do that I'll write out the equation and I substitute the values that I know.

Now I know the energy that I'm providing, it's 50,000 joules, and I know the latent heat of vaporisation, it's 108,000 joules per kilogramme.

I don't know the math, so I'll leave that as a symbol M.

And then I can rearrange that equation in terms of M and write out M = 50,000 joules divided by 108,000 joules per kilogramme, and that gives me a mass of 0.

46 kilogrammes.

So 0.

46 kilogrammes of that ethanol would evaporate when I provide it with that energy.

Now it's your turn.

What I'd like you to do is to calculate the mass of mercury, which is at its melting point that melts when 550 joules of energy is transferred to it.

And I've given you the specific latent heat of fusion for mercury there.

So pause the video, work out your answer and restart, please.

Welcome back.

Hopefully you did this.

Write out the equation, substitute the known values, rearrange it, and then that gives you a final value of mass of 0.

05 kilogrammes.

So 0.

05 kilogrammes of mercury melted when I provide it with that energy.

Well done if you got that.

The equation for specific latent heat can easily be confused with the equation for specific heat capacity because they've got very similar terms in them and they're both talking about heat.

So the first of those equations are specific latent heat is what we've been studying today, and that's the energy required to change the state of one kilogramme of the material.

And you see the equation, the energy is mass times specific latent heat, that's the symbol L.

And it doesn't involve anything to do with temperature, and that's the key thing to know.

The specific heat capacity of a material is the energy required to increase the temperature of one kilogramme of that material by one degree Celsius.

And so we've got an equation here, delta E equals mc delta theta that does involve a temperature change.

That's the delta theta part.

That's a change in temperature.

M is the mass, so that's the same.

And C there is the specific heat capacity.

So make sure you don't confuse those two equations which prop up sometimes in the same question.

Okay, what we can do now is use both of those equations together to calculate total energy required to heat something and to melt it at the same time.

So I'm gonna calculate the total energy required to melt 4.

00 kilogrammes of aluminium starting at a temperature of 20 degree Celsius.

And I've got the data I need for aluminium there that will always be provided to you.

So step one, what I'm going to do is to find the energy needed to heat the aluminium from its starting temperature to its melting point.

And to do that, I'll write up the equation that involves heating, changes in temperature, so that's involving specific heat capacity.

So we've got energy is mass times specific heat capacity times in temperature.

I can substitute all those values in and I've got a mass of four kilogrammes.

I've got a specific heat capacity of 900 joules per kilogramme per degree Celsius.

And then I've got the temperature change.

And if you notice, I've heated the metal from 20 degrees Celsius up to 660 degree Celsius.

So that means the temperature change was 640 degree Celsius.

And I can calculate all that and that gives me a total energy required to heat the aluminium of 9.

22 megajoules.

The next step is I've got to melt the aluminium.

It's reached its melting point but it's not melted yet, so we need to provide more energy.

So I'm gonna calculate that.

And that time I'm gonna use the specific.

Sorry, I'm gonna use the specific latent heat of fusion involved there and the mass.

So I can write up that equation.

I can substitute the mass in and the specific latent heat and that gives me an energy change required to melt the aluminium of 1.

5 megajoules.

Finally, all I need to do is to add those values together, the energy needed to heat it to its melting point and the energy needed to melt it.

So I'll do that.

And that gives me a total of 10.

8 megajoules.

If I ran that to three significant figures, 10.

8 megajoules there.

So that's the answer.

Okay, I'd like you to try and repeat that process for yourself now.

This time you're going to melt in gold.

You've got 1.

25 kilogrammes of gold and it's got a starting temperature of 20 degree Celsius.

And I've got the data for gold there.

So pause the video and work out the energy required to completely melt that gold, please.

Welcome back.

Hopefully you answered something like this.

Step one, energy needed to heat the gold to its melting point, the equation substitute in the values and that gives us a value of 168.

3 kilojoules.

Then the energy required to melt the gold, energy = ml.

Find the values from the question there, 1.

25 kilogrammes, 63,000 joules per kilogramme, and that gives an answer of 78.

8 kilojoules, and add those two together, and that gives the total energy needed.

And that is 247.

1 kilojoules around that to three significant figures, 247 kilojoules.

So well done if you've got that.

Okay, now it's time for the final task of the lesson, and what I'll need you to do is to answer these three questions.

So read them carefully.

You're going to be using the equations for specific latent heat and the specific heat capacity for the last question as well.

So pause the video, work through your solutions and restart when you're done, please.

Welcome back and here's the solutions to those.

So for the first question, that's just a simple use of the specific latent heat of vaporisation equation energy = ml.

That gives us 221 kilojoules.

For the second one, that involves a bit more complexity because first of all, we need to calculate the energy transferred, 30,000 joules.

And then we need to use the specific latent heat equation again, but this time rearrange it in terms of M, think of a mass of 13.

3 grammes.

Well done if you've got that.

And here's the answer to the second part of that.

And this involved multiple stages.

We've got, first of all, we need to calculate the energy needed to heat the gold to its melting point.

Then we need to calculate the energy needed to melt that gold, and finally add those two together.

And that gives a final answer of 39.

4 kilojoules.

So well done if you've got that.

That was a quite a tough question.

And we've reached the end of the lesson now.

So here's a quick summary of everything we've covered.

During changes of state, the temperature remains constant.

So when you're boiling something, the temperature reaches a boiling point and stays at that until all of the liquid is turned into a gas.

And similarly, for melting in their melting point.

When solids melt the energy transferred increases the spacing between the particles rather than the kinetic energy in the temperature.

Then we looked at specific latent heat of a substance, and that's the energy required to change one kilogramme from one state to another.

And we've got an equation for that.

Thermal energy for a change in state is mass times specific latent heat or E = ml.

What is the energy change in joules? M is the mass in kilogrammes and L is the specific latent heat in joules per kilogramme.

And we have two values for specific latent heat for each material, we've got the specific latent heat of fusion, and that's for changes between a solid and liquid.

And we have the specific latent heat in vaporisation and that's for changes between a liquid and a gas.

Well done for reaching the end of the lesson.

I'll see you in the next one.