Lesson video

Hello, and welcome to today's lesson.

I'm Mrs. Adcock, and today we are going to be looking at combustion of alcohols: planning.

We are going to be planning an investigation to look at whether the type of alcohol that undergoes combustion, affects the amount of energy that is released.

Today's lesson outcome is; I can write balanced symbol equations for combustion of alcohols and investigate the energy changes of combustion reactions.

Some of the keywords that we'll come across in today's lesson include alcohol, combustion, molecular formula, calorimetry, and energy change.

You can see those keywords written here in a sentence.

It'll be a good idea to pause the video now and read through those sentences, and you might even like to make some notes so that you can refer back to them later in the lesson if needed.

Today's lesson on combustion of alcohols is split into two main parts.

The first part is looking at balancing symbol equations for the combustion of alcohols.

And then we are going to move on to the second part of our lesson, which is planning an investigation on the combustion of alcohols, and looking at the amount of energy released when we combust different alcohols.

Let's get started on the first part of our lesson.

Alcohols can fully combust and when they combust, they react with oxygen, and when they fully combust, they react in a plentiful supply of oxygen to produce two products, carbon dioxide and water.

And in the process of combustion, they release energy, and we can see an image there showing an alcohol burner.

The general equation for the complete combustion of alcohols is, alcohol reacting with oxygen to produce carbon dioxide and water as our two products.

And we've got an example reaction there where we've used a named alcohol, so we're using ethanol, and we can react to that with oxygen, and that will produce carbon dioxide and water.

And we've got the structural formula there for ethanol, which is C2H5OH We react that with oxygen, which has the molecular formula O2, and that produces carbon dioxide, which has the molecular formula CO2, and water, which we can see there has the molecular formula H2O.

At the moment, when we look at that symbol equation, we can see that there are not the same number of atoms of each element in the reactants as are in the products.

So if you have a look just at the carbon, we can see that in our reactants we've got 2 carbon atoms in ethanol, whereas in our products, we've only got 1 carbon atom that we can see there in carbon dioxide.

So the atoms of each element are not balanced, and we would say this equation is unbalanced.

To balance equations, we cannot start changing the molecular formula.

So for example, H2O and CO2, we need to leave them as they are, these are fixed formulae.

We can change the number of molecules, though, so we could change it to 2H2O, and 3CO2.

So we could change the number of molecules that we are reacting or that are produced.

We've got our example reaction there where we've got ethanol reacting with oxygen, to produce carbon dioxide and water, and we can balance this equation, and to do that, we need to adjust the number of each molecule by placing numbers called coefficients in front of the molecular formula.

If we do that now, we can see that we have 1 molecule of ethanol, reacts with 3 molecules of oxygen, and this will produce 2 molecules of carbon dioxide and 3 molecules of water.

And we can check that the atoms of each element are balanced.

If we start with carbon, we can see we've got 2 carbon atoms in our reactants and 1 carbon atoms now in our products.

We've got 6 hydrogen atoms in our reactants and 6 hydrogen atoms in our products.

And if we look at oxygen, we can see we've got 7 oxygen atoms in our reactants, and we've got 7 oxygen atoms in our products.

Therefore, all the atoms for each element are balanced, we've got the same number in our reactants as we have in our products.

So this equation is now balanced.

Time for a check for understanding, which equation shows the correct balanced equation for the complete combustion of methanol? So we're using methanol as our alcohol this time.

Have a look at A, B, and C, and decide which one is correctly balanced.

Be careful to check that the formula are written correctly, and then check they've got the correct coefficients to balance those equations.

Well done if you chose option A, A is correctly balanced, you've got the same number of atoms for each element in the reactants as there are in the products.

It's not B, because the formula are not written correctly, in B, you can see we've got no subscript numbers, and it's not C, because there have been no coefficients added, and at the moment, C is unbalanced.

Time for our first practise task of today's lesson, what you need to do is balance the following combustion equations.

So there are five combustion equations there.

You need to balance them, not changing the molecular formula, but by adding coefficients in front of the formula, to make sure you then end up with the same number of atoms of each element in the reactants as there are in the products.

Now, sometimes you might find that you end up with a half number.

So, for example, if you ended up with 4 1/2 O2.

If this is a situation, then just double up all your numbers so that you end up with whole numbers.

Pause the video now, have a go at balancing those equations, and then come back when you're ready to go over the answers.

Well done, having a go at those questions, let's see how you got on.

For A, you should have 6 O2, 4 CO2, and 5 H2O, and that equation is now balanced.

For B, you would need 2 in front of the alcohol at the beginning, then you would have 15 in front of the O2, 10 in front of the CO2, and 12 in front of the H2O.

For C, you should have 9 O2, 6 CO2, and 7 H2O.

For D, you should have 18 O2, 12 CO2, and 13 H2O.

And for E, you've just got one molecule of the alcohol, plus 12 molecules of oxygen, and they react to produce 8 molecules of carbon dioxide, and 9 molecules of water.

Well done if you correctly balanced those equations.

For the second part of this task, you now need to write balanced symbol equations for the complete combustion of methanol, ethanol, propanol, and butanol.

Now, you've not been given the symbol equations here, so the first thing you need to do is work out what is the structural or molecular formula of each of those alcohols, and then you need to write out symbol equations, And then finally, you need to balance your symbol equations.

So this is a challenging task here.

Pause the video, have a go at this question, and then come back when you're ready to go over the answers.

Welcome, back.

For A, methanol, the structural formula is CH3OH, so well done, first of all, if you got that correct, that will react with oxygen, and because it's complete combustion, they will react to form carbon dioxide and water as the products.

To balance this equation, you needed to have 2 molecules of methanol, react with 3 molecules of oxygen, to produce 2 molecules of carbon dioxide, and 4 molecules of water.

Let's have a look at B, ethanol.

So the structural formula for ethanol is C2H5OH, this will react with oxygen to produce carbon dioxide and water, again, this is the products because this is complete combustion.

Balancing this symbol equation, you should have 1 molecule of ethanol, and we don't need to put the 1 if it's just one molecule, plus 3 molecules of oxygen, produces 2 molecules of carbon dioxide, and 3 molecules of water.

Well done if you got that one correct.

Propanol, the structural formula for propanol is C3H7OH, and we'll have 2 propanol molecules, react with 9 oxygen molecules, to produce 6 carbon dioxide molecules, and 8 water molecules.

And finally, butanol.

So the structural formula for butanol is C4H9OH, and 1 butanol molecule, will react with 6 oxygen molecules to produce 4 carbon dioxide molecules, and 5 water molecules.

Now in my examples here, I've used the structural formula, if you've used the molecular formula for the alcohols, then this is absolutely fine.

Well done if you are able to correctly write out those symbol equations, and then balance them as well.

It's time for us to move on to the second part of our lesson on investigating the combustion of alcohols, so let's go.

In an investigation, when we do any investigation, we need to identify the three main variables, and these are the independent variable, And this is the variable that you change or select values for.

The dependent variable, and this is the variable that you measure or observe to get your results, and the control variables.

And these are the variables that must remain the same throughout an investigation.

Time for a check for understanding.

Jacob wants to investigate whether the type of alcohol combusted, affects the energy given out.

What is the independent variable for this investigation? And remember, the independent variable is the variable that you change or select values for.

So what is the independent variable for this investigation? Is it A, the type of alcohol, B, the energy given out, or C, the time taken for the alcohol to combust? The independent variable in Jacob's investigation is the type of alcohol.

Because the type of alcohol is the variable that Jacob is changing.

It is difficult to directly measure the energy that is released during a combustion reaction, and calorimetry can be used to measure a reaction's energy change indirectly.

Calorimetry is an experiment that we use to measure the energy that is released from fuels.

Calorimetry involves measuring the mass of the alcohol that you need to combust to raise an exact volume of water by a set temperature.

And we can use calorimetry to compare the energy that's given out from the combustion of different alcohols.

So we could perform a calorimetry experiment using methanol, and ethanol, propanol, and butanol, and we could gather our data and then compare the energy that's given out, combusting these different alcohols.

The calorimetry apparatus is shown below, so we can see what the setup would look like.

We've got a thermometer, and the thermometer's obviously used to measure the temperature, and we're gonna look at the temperature change of the water.

And the water and thermometer are both held there in a calorimeter.

And quite often, we use a copper calorimeter.

We've got our alcohol burner at the bottom, so we will change the type of alcohol that we use in our alcohol burner, and then we can use this to measure the energy that's released when we combust different alcohols.

The alcohol burner is on a heatproof mat, and our calorimeter is held by a clamp, and we have a clamp stand.

Let's have a look at what our method might look like.

So first of all, we measure the initial mass of the alcohol burner.

Now it's really important that we know how much of our alcohol has combusted, and therefore, we need to measure the initial mass of the alcohol burner, and then at the end, we will measure the final mass of the alcohol burner, and we can use these values to work out the mass of alcohol that has combusted.

Secondly, we are gonna place the alcohol burner directly below the calorimeter, and it's really important that we keep that distance the same each time we repeat this experiment using a different alcohol.

And by the distance, I mean the distance between the wick and the calorimeter, so we need to keep that constant.

Number 3, place, 50 centimetres cubed of water in the calorimeter.

And again, it's important that we use the same volume of water each time, and we are going to measure the initial temperature of the water.

And you'll see in a moment, we will measure the final temperature of the water so that we can see the temperature change.

Then we are going to light the wick on the alcohol burner.

And when the water temperature has increased by 30 degrees Celsius, then we will extinguish the flame on the alcohol burner, and then we will record the final temperature of the water, and record the final mass of the alcohol burner.

And all of this information, the initial mass, the final mass, the initial temperature of the water, and the final temperature of the water, all need to be recorded in a results table.

Time for another check for understanding.

What is the dependent variable in this investigation? And remember, the dependent variable is the variable that you measure or you observe to get your results.

Is the dependent variable, A, the time taken, B, the temperature rise, or C, the type of alcohol? The correct answer is B, the temperature rise, so well done if you've got that one correct.

Another question here, what are the control variables for this investigation? And control variables are variables that must remain the same throughout an investigation.

So do we need to keep these things the same? Have a think as I read through them.

A, distance between the wick and calorimeter.

B, volume of water, C, the type of alcohol, D, the initial water temperature.

The correct answers are A, the distance between the wick and calorimeter, B, the volume of water, and D, the initial water temperature all need to be kept constant when you are doing this investigation.

Calorimetry data should be repeated to improve the reliability of the results.

We can see we've got our calorimetry apparatus set up there.

Heat being lost to the surroundings is a major source of error in calorimetry, and how can we prevent this? Draught screens, which can be added to the sides, and also, we can add a lid to the top of the calorimeter, and you can see these shown.

So there we've got our draught screens, which are either side of our alcohol burner, and we've added a lid to the top of the calorimeter, both of these will prevent heat being lost to the surroundings.

Let's check that we've understood that.

Why is a draught screen often used in this practical? Is it A, to prevent someone getting burnt, B, to ensure the flame does not blow out, or C, to prevent heat loss? We use draught screens to prevent heat loss.

Well done if you got that correct, you're clearly focusing well.

If an alcohol burns completely, why may the experimental temperature rise be less than the theoretical temperature rise? Why might the values that we gather when we do calorimetry be less than the theoretical temperature rise? Is it A, because there is a limited supply of oxygen, B, because some of the heat has been lost to the air, or C, the thermometer was faulty? The correct answer, is that because some heat is unfortunately lost to the air, so quite often, the experimental values that we get are less than the theoretical values.

Time for our final practise task of today's lesson, and for the first part, what you need to do is label the calorimetry apparatus.

So all the apparatus is there, you just need to add the correct keywords to each of those seven labels.

Pause the video now, have a go at that task, and then come back when you're ready to go over the answers.

The correct labels are thermometer, water, calorimeter, alcohol burner, heatproof mat, and then on the left-hand side, we've got clamp, and clamp stand.

So well done if you correctly identified all of those pieces of equipment that are used in our calorimetry experiment.

For the second part of this task, we're gonna look at a method that Aisha's written.

So Aisha has written a method to investigate the energy released from the combustion of different alcohols.

Carefully read through her method and suggest at least four improvements.

Here is Aisha's method.

"Place the alcohol burner directly below the calorimeter.

Place some water in the calorimeter.

Light the wick on the alcohol burner, and record the final temperature of the water.

Now, Aisha has missed out some key details and also has missed out some steps.

So pause the video now, and see if you can improve Aisha's method, by coming up with at least four improvements, and then we'll go over the answer when you come back.

Okay, let's see how you got on with this task.

Firstly, we need to measure the initial mass of the alcohol burner.

Now, Aisha forgot to mention that.

Place the alcohol burner directly below the calorimeter, then place a known volume of water.

Aisha said, "Place some water." Now, you might have actually mentioned a known volume, so you might have placed 50 centimetres cubed of water, for example, in the calorimeter, and measure the initial temperature of that water.

Then we could add a lid and a draught screen, now you might have thought of those key points, so well done if you did, and they are going to prevent heat being lost to the surroundings, then light the wick on the alcohol burner, and when the water temperature has increased by about 30 degrees Celsius, extinguish the flame.

So those are points that Aisha forgot to mention.

Record the final temperature of the water, and also, don't forget to record the final mass of the alcohol burner so that we can work out the mass of alcohol that has combusted.

We've reached the end of today's lesson, so well done for working so hard.

We are just gonna summarise some of the key points on combustion of alcohols.

We have looked at balancing symbol equations for the combustion of alcohols, and then we've also gone on to look at the calorimetry experiment, and how we could use this to compare the amount of energy that is released when different alcohols undergo combustion.

So balanced symbol equations can be written for the combustion of alcohols, and we looked at when it was complete combustion, we would produce carbon dioxide and water as our products.

It is difficult to measure directly the energy change of a reaction, but calorimetry can be used to measure a reaction's energy change indirectly.

It's been great learning with you today.

I hope that you've enjoyed the lesson, and I hope that you're able to join me for another lesson soon.