Hello, my name's Mrs. Niven, and today we're going to look at how we can determine a chemical equation experimentally, specifically looking at sodium hydrogen carbonate as part of our unit on Calculations involving masses.

What we do today, you will have had some experience of in your previous learning, but what we do, it will not only help us to answer that big question of "what are substances made of?", but also help us to look at linking together those three fundamental aspects of chemistry.

What we can see, observe, and measure in a laboratory, how we communicate our understanding of those reactions with other scientists, and then how we talk about the theory, the particles that are reacting in those chemical reactions that take place in the lab.

So by the end of today's lesson, you should hopefully feel more comfortable describing a safe experiment that we could use to calculate the stoichiometric or molar ratio values for the decomposition of sodium hydrogen carbonate.

Now I will be referring to some keywords throughout this lesson and they include: decomposition, ratio, stoichiometry, mole, and balanced symbol equation.

Now the definitions for these keywords are given in sentence form on the next slide, and you may wish to pause the video here so you can read through them or perhaps make a note of them so you can refer back to it later in the lesson or later on in your learning.

So today's lesson is broken into two parts.

Firstly, we'll look at how we can heat sodium hydrogen carbonate, and then we'll move on to look at how we can determine the reacting ratios for decomposition reactions.

So let's get started by looking at what happens when we heat sodium hydrogen carbonate.

So today's lesson is broken into two parts.

Firstly, we'll remind ourselves of what heating sodium hydrogen carbonate might look like, and then we'll move on to look at how we can determine the reacting ratios for decomposition reactions.

So let's get started by looking at sodium hydrogen carbonate.

Now many of you may be familiar with a food waste pile or a compost pile in which we put substances that we want to simply break down and you can create new fertiliser out of it or something like that.

But essentially we've put the materials into these piles in order to decompose, to break down, and we can use that understanding to define a decomposition reaction.

Decomposition reactions are those in which a reactant compound breaks down into two or more products.

So you can identify these decomposition reactions from their chemical equations because they tend to have one reactant and multiple products.

Now we can see this if we look at an example of limestone.

Now limestone contains the mineral calcium carbonate, and when it is heated, it breaks down into calcium oxide and carbon dioxide.

And we can represent this with a word equation and we can see clearly here that we have one reactant and multiple products indicating that this would be a decomposition reaction.

Now, decomposition reactions are usually triggered or activated by the application of energy, and that energy source then that triggers the reaction is included in the chemical equation but over the arrow.

So if we go back to that reaction of our calcium carbonate or limestone that's been heated and decomposing, we know that we heated it with a Bunsen burner, so we'd simply put heat over the arrow to indicate the energy source that activated that reaction.

Some substances will have a different energy source that activates it.

For instance, when water is mixed with sulfuric acid and electricity is applied, it breaks down into hydrogen gas and oxygen gas, and we can represent that decomposition reaction with these equations, and notice how that energy source then of electricity is listed over the arrow.

Other substances may decompose when light is present, and you usually find these substances in brown bottles.

And an example here then is hydrogen peroxide.

It decomposes into water and oxygen gas when light is present and therefore we'd put light as the energy source over the arrow.

Let's stop here for a quick check.

Which of the following is not used to activate or trigger a decomposition reaction? Well done if you said sound.

Sound is not an energy source that is used to activate a decomposition reaction.

So well done if you manage to get that correct.

What a great start to the lesson, guys.

Keep it up.

Now, if you do any baking, you'll be very familiar with sodium hydrogen carbonate already, but probably by a different name.

This compound is also known as sodium bicarbonate and it's used quite regularly in baking, and that's because any baking mixture that contains it will rise due to the decomposition reaction that takes place when the sodium hydrogen carbonate is heated, i.

e.

, when that baking mixture is put in the oven.

Now when a carbonate substance decomposes, carbon dioxide gas is usually produced.

So if a carbon dioxide gas is produced within a baking mixture, bubbles will form within that mixture and in its attempt to try and leave that mixture, it causes that mixture to then rise.

Now we can heat sodium hydrogen carbonate in the laboratory, and we usually do this in a closed environment.

If we look closely at this diagram, we can see that we have some substance that's in a crucible and the lid is on top.

Now what this does is it prevents substances from entering or leaving that reaction environment within the crucible.

So we keep the crucible lid on for two reasons.

One, it reduces the emission of that unpleasant gas, that carbon dioxide that forms when a carbonate or hydrogen carbonate substance is heated and starts to decompose, break down.

It also prevents the loss of any product that might be forming from that decomposition reaction.

However, it does make it very difficult to see what's actually going on.

Is there some other observation that we could make to tell us that a chemical reaction is taking place? For instance, is there a colour change or not? Let's stop here for a quick check.

True or false? Sodium hydrogen carbonate is heated in an open environment.

Well done if you said false, but which of these statements best justifies that answer? Well done if you said B.

A closed environment reduces the loss of product.

So well done.

If you manage to get these correct, guys.

You're off to a cracking start.

Keep it up.

It's now time to start our first task of the lesson.

Now, there are three possible ways in which sodium hydrogen carbonate can decompose when it's heated, and those three possible ways are listed below as A, B and C.

What I'd like you to do is to write a balanced symbol equation for each of these reactions that could take place with those possible products.

So pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

Now, the main thing to balance a symbol equation is we need to keep track of the number of atoms of each element in both our reactants and our products.

And sometimes the easiest way to do that is in a table, and that's what I've created here.

So by looking at the reactant and product atoms, I can see that I have enough just as the reaction is.

And so for this particular reaction equation, it is NaOHCO3 makes NaOH + CO2.

Now for B, I have a slightly different reaction equation because I'm producing different products with this possible decomposition.

So following the same idea of creating a table to keep track of the different atoms and the number of atoms I have for those different elements, I can see that I have an uneven number and I'm going to need more atoms on my reactant side.

So I add another molecule of sodium hydrogen carbonate, change the values to count up all of those atoms, and I can see it's still unbalanced.

So I'm going to add another molecule of carbon dioxide on my product side, and I can see now by adding that, I have now evened out the atoms of both my reactants and my products.

And so my final equation will be 2NaHCO3 makes NA2O + 2CO2 + H2O.

And for C then, following those same steps, I get a final answer of 2NaHCO3 makes Na2CO3 + CO2 + H2O.

Well done if you managed to balance those correctly, guys.

Great job.

Okay, for the next part of this task, I'm gonna ask you to follow a method which I'll show you in a moment to decompose some sodium hydrogen carbonate.

Now, when you heat that sodium hydrogen carbonate, you'll produce a solid product and some gaseous products, and the equipment you'll need is shown in this scientific diagram, which includes: a heat proof mat, Bunsen burner, tripod, clay triangle, a crucible and lid, and a sample of sodium hydrogen carbonate.

Now you'll also need a balance for this, and that's because the first step that you'll need to do is to record the mass of a crucible and the lid.

Now make sure that that lid fits the crucible properly.

Into that crucible you'll add about five grammes of sodium hydrogen carbonate, and then heat the crucible with the lid on top strongly, as shown, for about 10 minutes.

After heating that you want to allow the crucible to cool and then record the final mass in a results table, which I'll show you in a moment.

Now, if you're unable to carry out this practical, you can watch a video of it in a moment.

The results table that I'd like you to use is shown here, and you can see that there are three different masses that you'll need to record the mass for and a description of what that mass includes.

Once you've finished taking your measurements and recording them in the table, I'd like you to use them to calculate the mass of the sodium hydrogen carbonate that reacted and the mass of the solid product that was formed.

So pause the video and come back when you're ready to check your work.

Now the first thing we need to do is make sure that the balance reads 0.

00 grammes.

So we press the tare button to ensure that and then add the crucible and lid to get that first mass, which is 22.

69 grammes.

After this, you're going to start adding that sodium hydrogen carbonate until we get five grammes and we're aiming for 5.

00 grammes.

And you can do this in two ways.

You can either add it slowly using a spatula or putting some into a weigh boat and tapping it into the crucible to start.

Now we know that we started with 22.

69 grammes, and if we're adding five grammes, I should finish it at 27.

69 grammes.

And if you go past it a little bit, all you need to do is grab that spatula and take a little bit out at a time until you end up with the appropriate value, which would indicate that we have added 5.

00 grammes of that sodium hydrogen carbonate into our crucible.

Now, once you've done that, then you're going to move the crucible onto your clay triangle and pop the lid on top of that crucible.

And then what you need to do is change the Bunsen burner to a strong, blue flame and place it underneath that clay triangle and the crucible and lid.

You're gonna leave that to heat for about 10 minutes, so keep an eye on the time.

Once 10 minutes has finished then, pull the Bunsen burner out, replacing it with a safety flame and then turning the gas off and leave the crucible to cool.

Once it's cooled, again, make sure that your balance is reading 0.

00 by pressing the tare button.

And then you're going to move using the tongs, the lid, and the crucible onto the balance, so you can record that final mass, which is 24.

68 grammes.

Okay, let's see how you got on.

Now, the masses that you recorded in your table will depend on the practical that you carried out.

If you were unable to carry out this practical and you used the video, these are the results that you should have in your results table.

Mass one was 22.

69, mass two was 27.

69, and mass three was 24.

68.

You were then asked to use your results to calculate the mass of the sodium hydrogen carbonate that reacted and the mass of the solid product that was formed.

Now this is going to depend on the measurements that you recorded in your own results table.

So I'm going to show you the method that was used to calculate these masses using the results from that practical video.

So to find the mass of the sodium hydrogen carbonate that reacted, you needed to take mass two and subtract mass one from it.

So for this reaction from the video, we have 5.

00 grammes.

For the mass of the solid product that was formed, you're going to take mass three and subtract mass one.

So for the solid product that was formed in our practical video, we get 1.

99 grammes.

Now that we're feeling more comfortable talking about how sodium hydrogen carbonate might decompose when heated, let's look at how we can determine the ratios for decomposition reactions.

Now, depending on the decomposition reaction that's taken place, conservation of mass will allow a chemist to calculate an unknown mass of a substance.

So if we revisit heating limestone from earlier in this lesson, you actually start with the solid mineral of calcium carbonate, and when it's heated, it will produce solid calcium oxide and a carbon dioxide gas.

Now, what's interesting about this reaction is that we start with a solid reactant and we finish with one solid product and a gas product.

Now, the great thing about having two solid materials, one reactant and one product, is that you can measure the mass of your starting reactant and the mass of your solid product that's formed and using this conservation of mass then, we can find that unknown mass of carbon dioxide because the mass of my reactants must equal the mass of my products.

And when I carry out the calculation for this reaction, I could say that 2.

69 grammes of carbon dioxide was produced in this reaction.

Let's stop here for a quick check.

What is the missing mass in this decomposition reaction? Well done if you said B: 1.

522 grammes is the mass of oxygen that is produced in this decomposition reaction, and I've shown the working out here.

In case you didn't get that answer, you may wish to pause the video and double check your calculations, but well done if you managed to get that correct.

Now, we've said previously during a chemical reaction that all the reactant atoms are rearranging to form all of the products, and because of that, the number and type of atoms and their mass are conserved throughout that chemical reaction.

Now, the difficulty we have is that the products of a decomposition reaction could actually be in the solid, the liquid or the gas states, and that can make measuring the mass of a specific product quite difficult.

So if we look for instance, at this example of the decomposition of hydrogen peroxide, we have here two liquid materials.

We've got the hydrogen peroxide that's a liquid, and the water as a product that's also a liquid.

The problem we have here is that hydrogen peroxide is actually missable or mixes in with the water, and it'd be really difficult for us to be able to determine if the mass of the water that we are trying to measure at the end of a reaction contained any un-decomposed hydrogen peroxide in it.

So it's quite a tricky one in order to find a mass to then go on to calculate another unknown mass.

But if a substance decomposes and forms only one solid product, then chemists can determine the mass of that solid product, and that's particularly useful if the substance decomposes to a constant mass and using this information then, we could determine the molar ratios for a decomposition reaction if we understand what those possible decomposition products are, the stoichiometry or the molar ratios of the reactions that form those possible products, and then we need to use that mathematical relationship of the massing grammes is equal to the number of moles times the relative mass of these substances.

Let's look at an example.

If I have 6.

97 grammes of potassium hydrogen carbonate and heat it to form a solid product, and that's measured to be 3.

98 grammes, there are actually three possible decomposition reactions that could have taken place, and these are listed here.

If I take a closer look at these three possible decomposition reactions, I can see that there's one solid product that was formed and multiple gas products.

What I need to do then is to determine which of these possible reactions actually occurred.

Now, the first step in being able to answer that question is to create a calculation grid for each solid substance from those reaction equations with space to include the mass and grammes, the relative mass and the number of moles.

And when I look at those equations a little bit more carefully, I can see it's the same solid reactant with three different possible solid products, and I want to keep track for all of them.

Then what I'm going to do is populate the grid with the mass of the reactant, which I got from my equation is 6.

97, so I'm just gonna copy that into my grid.

The next thing I need to do then is to calculate the relative mass for the decomposing substance and each solid product.

So I'm going to use my periodic table for that, and when I calculate the relative masses, I'm going to then populate my grid with those values.

I then need to calculate the number of moles of reactant that has actually decomposed, and I'm gonna use that equation of moles as equal to the massing grammes divided by the relative mass.

So going down that column, I'm going to be dividing and I have now a value of the number of particles of my reactant that have decomposed.

Now the next step is a little more complicated.

What I need to do is I need to use the calculated number of moles of my reactant and the molar ratios from the balance symbol equation to determine the number of moles of each solid product that would've been produced if that reactant had fully decomposed.

So I take my calculated value of my moles that I just found out in my previous step, I'm going to divide it by the molar ratio of the reactant and multiply it by the molar ratio of my product.

So what does that look like? I'm going to have to look at each of my reaction equations separately.

So here I have the reaction equation that is producing the product K2O.

So I start with the number of moles of my reactant, I'm going to divide it by the molar ratio for the reactant and then multiply it by the molar ratio of that specific product.

And that value then is going to be added into my calculation grid under that specific product.

Now, it's really important that you are careful in your calculations and the placement of your values within this calculation grid, so just take care.

I'm then going to use that same process for my next product of the potassium carbonate.

Again, start with that reactant moles, divide it by the molar ratio for the reactant and multiply it by that molar ratio for the product to give you a value that you're going to put into your calculation grid.

Now, sometimes you do come across a reaction equation where there's a little bit of a shortcut.

If we look at the molar ratios for this particular reaction, we can see that the molar ratio is one to one, so it's the exact same coefficient, and that can be read then that the same amount of potassium hydroxide is produced when potassium hydrogen carbonate decomposes.

So I don't need to do any maths here.

I can simply copy the value of the number of moles of my reactant into the product square.

Now, this only happens if those coefficients are exactly the same, but it is a little bit of a shortcut if you want to save yourself some brain space.

Now that we know the number of moles for each product that should have formed if that reactant had fully decomposed, we need to know what their mass is because that's what we would've measured in the laboratory.

We can't count those different particles.

So to calculate the masses for each of these possible products, I need to multiply the number of moles for each by their relative masses.

So as we go up each column, we're going to be multiplying as we go up to get those different masses that would've been produced if that decomposition reaction had occurred.

The next step is a simple comparison.

You want to compare the calculated mass of the possible solid products to the measured mass of the solid product that was actually formed in this reaction.

And what you're looking for then is the mass that is closest is going to be the likely product that was formed.

So in this case, it looks like the closest mass would be that for the potassium hydroxide.

Let's stop here for a quick check.

Why is it so important to use molar ratios when calculating the mass of solid products that form in a decomposition reaction? You may wish to pause the video here so you can discuss your ideas with the people nearest you and then come back when you're ready to check your answer.

Well done if you said C.

Molar ratios indicate the ratio of particles that might react and what might form in a chemical reaction.

They don't indicate the mass of the products and they definitely don't give you a specific number of particles.

They simply tell you the ratio of what you start and what you end with.

Time for the final task in today's lesson.

For this first part, I'd like you to use your results from the practical from Task A, part two to determine the correct chemical equation for the decomposition of the sodium hydrogen carbonate that took place earlier.

So pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

Now remember, everybody's results are going to be slightly different based on the individual practical results that you collected from Task A.

I'm going to go through the answers based on the results from the practical video that was included and using that information my completed calculation grid looks like this, and you may remember that we calculated that we formed 1.

99 grammes of a solid product in this reaction, and as a result of it, that matches most closely with the sodium oxide that's formed, and as a result, the final reaction equation I have for this decomposition reaction was 2NaHCO3 makes Na2O + 2CO2 + H2O.

Now, you may have a different answer based on your decomposition, and that will result in a variety of answers.

But the main thing here is have you completed your calculation grid correctly? If you weren't sure, you may wish to pause the video and go back through those steps from the earlier part of this learning cycle.

But overall, really impressed with the work you guys are doing.

Great job.

For the last part of this task, I want you to consider the method that was followed to decompose that sodium hydrogen carbonate and the measurements recorded during Task A, part two, and I want you to reflect and think about how might the measurements have been improved.

So as a reminder, this was the equipment that you used and the method you were asked to follow.

So pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

Now, if you struggled with this particular part of the task, don't worry.

That's normal.

Asking someone to reflect and consider how to improve something is actually quite a tricky skill, and it's one that does come with practise and hopefully going through this will give you some ideas of how you might consider to improve future practicals.

So for this particular method, I thought there were two main ways that we could have improved it.

One of them is actually looking at what we were recording.

We were recording the mass of our materials, and one of the things I could do is look at what I'm actually using to record that mass.

I could potentially use a higher resolution balance to reduce the uncertainty of each of the measurements that I was taking.

So I'm looking for a balance that has potentially more decimal places on it.

Another way that I could improve this method was how it was heated.

They indicated in the method that it should be heated for around about 10 minutes, but to improve this and ensure that as much of that sodium hydrogen carbonate had decomposed as possible, I could have heated this until it reached a constant mass, indicating that as much had decomposed as possible.

Hopefully, that's given you some ideas of ways that you could consider improving methods in the future.

But don't worry if you didn't get these.

It is a very tricky, tricky task, but well done if you got one of those ideas and fantastic work if you suggested both of those improvements.

Well done, guys.

Let's take a moment to summarise what we've managed to learn in today's lesson.

Well, we know that heating sodium hydrogen carbonate in a crucible will lead to its decomposition, and the risk that's involved with heating that compound is actually quite low.

And because a gas is not a reactant in this reaction, the crucible lid can actually remain closed throughout that entire heating process.

Unlike if you are heating, say, magnesium, you'd need to open the lid to let that oxygen in.

You don't need to do that with the sodium hydrogen carbonate decomposition.

And we've also learned that the stoichiometry or those molar ratios for decomposition reaction can be calculated given experimental data, and you can use that then to determine the correct decomposition reaction that's taken place during the reaction.

I hope you've had a good time learning with me today.

I've had a good time learning with you, and I hope to see you again soon.

Bye for now.