Worked examples: Balancing chemical equations

0:00- [Instructor] Chemical equations are the most efficient way to communicate chemical reactions. For example, this equation communicates that hydrogen reacts with oxygen to produce water. But the important thing about a chemical equation is they should obey the law of conservation of matter, which says matter can either be created nor destroyed.

0:16Meaning in chemical equations or chemical reactions, atoms can either be created nor destroyed. So for every chemical equation, the number of atoms of each element on the left side should equal that on the right side, and when that happens we say the chemical equation is balanced.

0:30So the goal of this video is to practice identifying whether an given equation is balanced or not. So let's begin. Let's start with the chemical equation where hydrogen reacts with nitrogen to form ammonia. Ammonia is the key ingredient of fertilizers, which is super important to produce enough food to feed the world, okay?

0:48Now, how do we balance chemical equations? Well, remember these subscripts that you see over here, those are the things that we cannot change, all right? But what we can change is the coefficients that are in front of the chemical formula. Now, let's say that there are three students who attempt to balance this chemical equation, and here's what they get.

1:06Our goal is to look at them and see if the equations are balanced or not. How do we do that? Well, we just have to check whether the number of atoms of each elements is the same on the left and on the right side of the arrow mark, and the best way to do that is to write it down and keep track of it, so let's start with this one, okay?

1:20So we have two elements, nitrogen and hydrogen. Let's see how many of each one we have, how many atoms of each one we have on the left and on the right. So how many atoms of nitrogen do we have on the left? Well, how do we calculate that? Just multiply the coefficient with this subscript.

1:33Okay, so one times two is two. How many do we have on the right? Well, again, multiply coefficient with the subscript. If you don't have any subscript, it just means one, okay, so two times one is two. So the number of nitrogen atoms works out. It's the same on the left and right, so that's good.

1:47And by the way, remember, if you don't have anything as a coefficient, it means it's one, so this one was not really necessary, okay, but now, let's look at hydrogen. How many do we have on the left? Well, two times two is four. On the right side, we have two times three, which is six.

2:03So the number of hydrogen atoms is not the same on the left and right, and therefore, this equation is not balanced. Okay, what about this one? Feel free to pause the video and try it on your own if you want, but let's check. For nitrogen, well, we have one times two, and on the right side, we have two times one, so that works out.

2:21What about hydrogen? Well, we have three times two on the left, six, and two times three on the right, six, so hydrogen also works out. This chemical equation is balanced. Okay, what about this one? Well, again, let's see, hydrogen, sorry, nitrogen. We have two times two, which is four, on the left and four times one, which is four, on the right.

2:42Okay, that's good, and we have six times two, that is 12 on the left, and we have four times three, that is 12 on the right, so that works out. So is this equation also balanced? Well, technically, the law of conservation of matter holds, but the problem with this equation is we like to write chemical equations with the smallest number of molecules that will show their relative proportions of the extremely large quantities that we are dealing with over here, okay?

3:10In other words, we want the coefficients to be simplified. But look here, the coefficients are just doubled. And so yes, the law of conservation of matter holds, but that would be true if you had triple or if you had quadruple as well, right? But because these are not in the lowest whole number ratios, this is not the correct way to write the chemical equation.

3:28This is the correct way to write the chemical equation. All right, in the next one we have aluminum reacts with diatomic oxygen to produce aluminum oxide. Feel free to pause and try and balance yourself. Otherwise, let's look at the three samples and see which one of them is balanced.

3:42Let's start with this one. Again, we have aluminum and we have oxygen, so let's keep count of their atoms. So if you start with aluminum on the left side, how many do we have? Well, two times one is two, and on the right, side we have one, times two is two, works out.

3:54What about oxygen? Well, on the left we have three times two, which is six, and on the right side, we have one times three, which is three, it doesn't work out. This equation is not balanced. Let's try over here. How many aluminum on the left side? Four times one is four.

4:08And over here, two times two is four, so aluminum works out. And what about oxygen? Well, on the left, you have six times two, which is 12. On the right, we have two times three, which is six. No, so close. Oh, even this does not work out. This equation is also not balanced. So what about this one?

4:25This could be it. Let's see. Aluminum, four on the left, two into two, four on the right. This works out. Three times two for oxygen, six on the left and two times three, six on the right. This works out. This is the one. Yay, this equation is balanced. All right, let's move on to the next one. Onto the last one.

4:45We have methane reacts with oxygen to produce water and carbon dioxide. Ooh, this one's a doozy. So again, we have three samples and we have to check which of them is balanced. All right, and here's a quick tip for balancing. Whenever we notice that there are certain elements which are on multiple compounds on the same side, like oxygen that you can see over here, we wanna save that one for the last, right?

5:07So let's look at how many of each we have on the left and right. So let's start with carbon. How many do we have on the left? Well, we have one times one, which is one, and on the right, we have one times one, which is one. So carbon works out. What about hydrogen?

5:21Well, on the left, we have one times four, which is four, and on the right, we have two times two, which is four. Hey, hydrogen works out as well. Could this be the one? Let's see, oxygen. On the left, we have three times two, which is six. On the right, we have two times one, which is two, but there's more.

5:41We also have one times two, which is another two. So two plus two is four, we get four, and that does not work out, this equation is not balanced. Okay, what about this one? Start with carbon again. We see one on the left and we also see one on the right. This is good. What about hydrogen?

5:57We see four on the left and we see two times two, four, on the right. So this, again, good. What about oxygen? We see two times two, four, on the left and we see, on the right, two times one is two and one times two is two, two plus two is four. This is the one! Yes, got it right.

6:15So this equation is indeed balanced. Okay, what about last one? Well, let's see again, carbon. One on the left, one on the right. This is good. Hydrogen, we have one times four on the left, two times two, four, on the right, which is also good. What about oxygen?

6:30Well, we have one times four on the left and we have two times one is two and one times two is two, two plus two, four, on the right. This is also, wait, what? Is this also correct? So this is also balanced. Ah, did you see? Can you spot something? What's wrong over here?

6:49Look, this student change the subscript altogether. That's not allowed. Oxygen is supposed to be diatomic. You can't just change the subscript, you can only change the coefficient. So that's why this is wrong. This is a wrong chemical equation altogether. So this is the right one.

7:04So long story short, chemical equations are efficient ways of communicating chemical reactions or chemical changes, but importantly, in a chemical reaction, atoms are not created or destroyed. Matter cannot be created nor destroyed. Law of conservation of matter, okay?

7:17So atoms only rearrange themselves, therefore, the number of atoms of each element on the left should always equal that on the right, and when that happens, we say the chemical equation is balanced, but remember, the coefficients must be simplified and they must represent the lowest whole number ratio.

7:33And finally, remember, you cannot change the subscripts. That would change the chemical formula altogether. That's not allowed. The only thing you're allowed to do while balancing is to change the coefficients.