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Visually understanding balancing chemical equations

A balanced chemical equation shows the same number of each type of atom on both sides of the arrow. We can relate a balanced chemical equation to the structural formulas of its reactants and products.

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Video transcript

- [Voiceover] We've now seen a couple of examples of balancing chemical equations. And we've seen that, okay, if let's say, we're trying to balance this equation right over here and we started with the carbons. I have two carbons on the reactant side. They're both sitting in this ethylene molecule. And so I would want two carbons on the product side. And right now, I only have one carbon on the product side. And so what we've done is let's just put a two out front here and so now we have two for every molecule of ethylene and we're not done balancing this chemical equation yet. We're now producing two molecules of carbon dioxide. But one thing that you might have been thinking, "Why put this big two out front "of the entire carbon dioxide, "I like the way these little subscript little twos look, "so why not put a two right over there. " And the reason why you can't do that is that's actually changing the molecule. It's no longer carbon dioxide, it's now this bizarre thing that doesn't really exist in nature. Which is a C2O2 thing. You're actually changing the reaction when you're doing that. When you're balancing chemical reactions, the reaction itself is, even before it's balanced is describing something that happens. When it's unbalanced, it just doesn't have the numbers right in terms of number of molecules. So the only thing that you can change when you're changing these is the number of molecules. You can't change the number of constituents within the molecule. And that's why we do not change these subscripts. And if you want to visualize it a little bit differently, let's draw each of these molecules So ethylene looks like this. Double bond carbons. Then each carbon is bonded to two hydrogens. Notice you have two carbons and four hydrogens. Molecular oxygen, O2. Looks like this. It's oxygen doubled bonded to oxygen. And then you have carbon dioxide. Carbon dioxide is a carbon, double bonded, to two oxygens each. And then finally, you have water. Finally, water, actually I'm going to do this in a slightly different color. Your water right over here, this is an oxygen bonded to two hydrogens. So let me write the plus signs there. So plus is right over there. So if you were to somehow write a subscript of two right over there, you would somehow be changing the structure, you would be changing what this molecule is. As opposed to that, which we don't want to do, we want to say, we're definitely producing carbon dioxide. We're definitely producing carbon dioxide, but how many carbon dioxides do we produce for each molecule of ethylene? And so that's where we say, "Okay, we have two carbons here, "we want two carbons here." We don't want to change the actual structure here, so let's write the two out front. So we're going to have two of these. Or if you want to do it, if you want to have it more visual, you could write it, okay, we're just going to have another molecule here. We're just going to have another molecule. So now we've balanced the carbons, two carbons on the reactant side. Two carbons on the product side. And then you go to the hydrogens. Say, "okay, we have four hydrogens here, "we only have two here." Well, what if we had two of these water molecules? So, let's draw another water molecule here. So O H, and actually let me write this right over here, so now we have two of these. And now we have two of these. And now we want to balance the oxygens. So let's see, on this side we only have two oxygens. On this side we have one, two, three, four, five, six oxygens. In order to balance it, we're going to have six oxygens on the reactant side. We need three of these molecules. So, another one and another one right over there. And now we're all balanced. We have two carbon atoms on both sides. Carbons, carbons. We have four hydrogen atoms on both sides. Here they're in the ethylene, here they're in the waters. And then we have six oxygen atoms on both sides. And here some of the oxygens are in the carbon dioxides, some of them are in the waters. And here, they're all in the molecular oxygens. But notice, we didn't change the actual structure of the molecules. We just changed the number of molecules we have and that's what these coefficients in front of these molecular formulas represent.