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Current time:0:00Total duration:4:37

Video transcript

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 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 just put a two out front here and so now we have two for every 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 well why 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 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 the c 2o 2 thing you're actually changing the reaction when you're doing that when you're balancing chemical reactions the reaction itself is is even before it's balanced it's describing something that happens it's just when it's unbalanced it just doesn't have the numbers of right it 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 you can't change the number of constituents within a 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 each of these molecules so ethylene looks like this it's a double bond with the carbons and then each carbon is bonded to two hydrogen's notice you have two carbons and four hydrogen's molecular oxygen o2 looks like this it's oxygen double 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 let me do this in a different color your water right over here this is an oxygen bonded to two hydrogen's so let me write the plus signs there so Plus this right right over there so if you were to if you were to somehow write a subscript of 2 right over there your you'd 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 okay we're definitely producing carbon dioxide we're definitely producing carbon actually well how many carbon dioxides do we produce for each 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 okay we're just gonna have another molecule here we're just going to have a another molecule so now we've balanced the carbons two carbons on the product on the reactant side two carbons on the product side and then you go to the hydrogen's you say okay we have four hydrogen's here we only have two here well what if we had two of these water molecules so let's let's draw another water molecule here so oh H and actually let me write this right over here so now we have two of these and now we have we have two of these and now we want to balance the oxygens so we see on this side we only have two oxygens on this side we have one two three four five six oxygens so what if so in order to balance so we 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 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 there with their 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 of the molecules we just changed the number of molecules we and that's what these these coefficients in front of in front of these these molecular formulas represent