Worked examples: Balancing more complex chemical equations

0:00- [Instructor] Let's practice balancing complex chemical equations in this video. And if you're new to this, well, you can watch our previous videos on middle school chemistry. But if you're ready for this, let's begin. Here's the first reaction. Carbon dioxide reacts with water to produce glucose and oxygen.

0:14This is called photosynthesis, and this is how all the plants and trees around us produce their own food. Okay, now you can pause and try and balance this yourself, but here are three samples that three students have given, and let's see which of them is balanced.

0:26We'll start with this one, all right? To see whether it's balanced or not, we just have to count the number of atoms of each element on the left and on the right. So, which elements do we have? We'll we have carbon, we have hydrogen, and we have oxygen, which is all over the place.

0:36Whenever you have elements which are present in multiple compounds, like oxygen over here, we'll save that one for the last, all right? So let's start with carbon. How many do we have on the left? Well, 6 times 1, which is 6, and on the right, we'll have 1 times 6, which is six, which is balanced.

0:49How many hydrogen do we have on the left? Well, 6 times 2 is 12, and on the right, we have 1 times 12, which is 12, so that's also balanced. Okay, what about oxygen? Well, we have 6 times 2, which is 12, plus, 'cause we have oxygen here as well, 6 times 1. So 12 plus 6 is 18 on the left, and how many do we have on the right?

1:05Well, we have oxygen here. 9 times 2 is 18. This is balanced, is it or not? It's not because we forgot the oxygen in glucose. So it's 18 plus 1 times 6, which is 6, 18 plus 6 is 24, so it's not balanced. This equation is not balanced. All right, so let's look at this one.

1:25How many carbons do we have on the left? Well, 12 times 1 is 12. On the right, 2 times 6 is 12. This is balanced. How many hydrogens do we have on the left? 12 times 2 is 24. On the right, 2 times 12 is 24. This is also balanced. What about oxygen? Well, let's see.

1:3912 times 2 is 24 plus 12 times 1, which is 36 together. And on the right side, 2 times 6 is 12, plus 12 times 2, 24. 12 plus 24 is 36. Oh, this is balanced, right? Yeah, but the last step is to check if the coefficients are in the simplest ratios, and we see they're not the lowest whole number ratios.

2:0012, 12, 2, and 12. You can divide this by 2 and get 6, 6, 1 and 6. This is the lowest ratio. So since chemical equations have to be written in the lowest, you know, whole number ratios, this is not the right one. In fact, if you look at student three, they have done the same equation but in the lowest whole number ratio.

2:18So we can know that it is balanced. But you can check it, and you can see that all atoms work out. Everything works out, and this is the balance chemical reaction. All right, onto the next challenge. So for the next one, we have magnesium chloride reacts with potassium hydroxide to give potassium chloride and magnesium hydroxide.

2:37Ooh, we see some parenthesis over here. What does that mean? Well, let's see. Let's get to that. So again, which are the elements that we're gonna deal with? Well, we have magnesium. We have potassium, chlorine, hydrogen, oxygen. Let's go through them. So for magnesium, we have 1 on the left, sorry, 1 times 1 on the left, and we have 1 on the right.

2:54So that's balanced. Potassium, we have 1 times 1, 1 on the left, and 1 times 1, 1 on the right, so that's balanced. What about chlorine? Well, we have 1 times 2,2 on the left, and 1 times 1 on the right. That's not balanced, so we need to take care of that. But let's just go on.

3:09Oxygen and hydrogen, we have 1 each on the left. How many do we have on the right? Ooh, this is the important one. Is it 1 and 2? No. You know, whenever you see a parenthesis, the subscript applies to all the elements inside the parenthesis. Okay, so here we have O and H in the parenthesis, and the subscript is 2.

3:26That means we have 2 oxygen atoms and 2 hydrogen atoms in this chemical formula. So that means we have two of each, and that means these are not balanced. So let's say a student attempts to balance this by putting a coefficient 2 over here. Is it done? Is this equation balanced now?

3:42Well, let's see. By putting a 2 over here, the oxygen and hydrogen on the left has become 2, but the potassium has also become 2. So hydrogen and oxygen is balanced, but now the potassium is no longer balanced, and even chlorine is still not balanced. So what can we do?

3:58Hey, hey, hey. We have 2 potassium and 2 chlorine on the left, so we just need to double this. We can put a 2 in front of this. That doubles the potassium and chlorine. We get two each, and that balances the whole thing. Oh, man, isn't it super fun to balance chemical equations?

4:14All right, final challenge. We have aluminum reacts with copper sulfate to produce copper and aluminum sulfate. Again, feel free to pause and balance this yourself, but otherwis.e here are the three attempts given by three students. So let's start with this one.

4:28So how many elements do we have? We have aluminum, we have copper, we have sulfur, and we have oxygen. But wait a second. Do you notice something? We see that sulfur and oxygen comes as a combination, SO4, called the sulfate. And that combination, we can see on the left side, and we see that same combination appearing on the right side.

4:44We don't see sulfur or oxygen anywhere else, only in this combination. Since it always appears as a sulfate, instead of counting them separately, why not just count them as a sulfate group? That will just reduce the amount of counting we have to do, isn't it? That's a trick that we can apply.

4:59And this applies whenever we see groups like this. For example, let's just take a couple of examples. Look at this equation. We see a NO3, which is called nitrate group, over here, and we see the same thing on the right hand as well, the same combination. And we don't see nitrogen or oxygen in any other combinations anywhere else.

5:14And so we could have done the same thing over here. We could have counted just the nitrate groups instead of considering nitrogen and oxygen individually. But what about this equation? We might consider CO3 as a single group and just count that because we see it on the left, we see it on the right.

5:29But wait, we also see CO2, which is a different combination, and we also see O over here, right? So over here, we wouldn't be able to do it because they don't appear just as this combination. We also see them in different combos. We also see them combined with other elements.

5:42So this is only applicable when we don't see them in other combinations. We only see them as one particular combination, as you can see here and here. Okay, so let's go over here and count how many aluminum we have on the left. Well, we have 1 on the left, and 1 times 2, 2 on the right.

5:57So this is already not balanced, but let's go see, you know, what happens to copper and sulfate. Well, we have 1 times 1, 1 on the left of copper and 1 on the right of copper. So copper is balanced. What about sulfate? Well, we have 1 times 1 because this is a whole group now.

6:13So there's only 1 on the left and on the right, we have 1 times 3. Remember this subscript applies to all the, you know, elements in the parenthesis. So 3 sulfates altogether. So 1 times 3, so we have 3 on the right, so this is not balanced. So how could we go about balancing this?

6:29Well, the first thing we could do is just put a 2 over here so that aluminum gets balanced, and that's exactly what the other two students have done. So that's done. How do we balance the sulfate? Well, we need three on the left somehow. Look at what student three has done.

6:41He's put a 3 over here. So oh, no, that can't be done because you can't change the subscripts. You know, you can only change this coefficient. Subscripts cannot be changed. You can't change chemical formulas. So this is instantly wrong. Look at student two. She has done it right by putting a 3 as a coefficient.

6:57We have 3 sulfate on the left side. So let's take stock. We have 2 aluminum on both sides, so that's good. We now have 3 sulfates on both sides. That's great, but look, as a result of this 3, we now have 3 copper on the left side, but she has also put 3 on the right, making 3 coppers on the right side.

7:15So this is the correct chemical equation. So putting it all together. To obey the law of conservation of matter, a balanced chemical equation must have the same number of each type of atoms on both sides of the arrow mark. And to balance, we change the coefficients in front of the chemical formula.

7:29We are not allowed to change the subscripts. And finally, after balancing everything, we need to make sure that the coefficients are simplified, and they represent the lowest whole number ratio.