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Current time:0:00Total duration:4:37
The H2O produced during combustion is actually in gas form. The subscript should read "(g)" instead of "(l)."

Video transcript

let's now see if we can balance a chemical equation with slightly more complex molecules so here we have a chemical equation describing a chemical reaction this is actually a combustion reaction you have a some ethylene right over here in the presence of oxygen and you need to get a little bit of energy to get this going but then you're going to have this reaction that's actually going to release energy as well but we're not we're not accounting for the energy at least the way we've written right over here you have some ethylene and this this little gene parenthesis says it's in the gas form or gaseous form so gaseous ethylene plus some oxy some dioxygen molecule which is the the most prevalent form of oxygen molecule that you would find in in the atmosphere and so that's also in the gas form put them together you end up with some carbon dioxide gas and some liquid water this is the classic combustion reaction but now let's think about how do we balance this thing let's make sure we have the same number of each atom on both sides and when you see something more complicated like this where you know here I have an oxygen and two different in two different molecules over here and you know a lot of these molecules have multiple elements in it it might be very daunting where do I start and this is where the art of balancing chemical chemical equation starts to come into play the general idea is try to balance the try to balance the molecules that have multiple elements in them first and leave the the the molecules that only have one element in them for last and the idea there is is that these are harder they're gonna have all sorts of implications and then at the end of the day you can just set a number here for the number of than for the number of die oxygens if you saved if you saved say the ethylene for last then every time and you're trying to balance the carbons then when you try to change the number of carbons you're gonna change the number of hydrogen's which is gonna change that you're gonna have to balance over and over you're gonna go into this really really really confusing circle so the best thing to do try to balance the complex molecules first and then save the the single element molecules for last so let's let's do that so let's start let's start with the carbons so over here I have two carbons over here I only have one carbon I only have one carbon so it seems like the best way to balance it is I should have two molecules of carbon dioxide and I haven't I haven't even thought about the oxygens yet by putting that two there that's going to change the number of oxygens I have on the right-hand side but at least it balances my carbons I now have two carbons on the left-hand side and I have two carbons on the right-hand side I'm no longer magically destroying a carbon atom all right now let's move on to the hydrogens and remember what I said is let's wait to do the oxygens last because we have a molecule that only contains oxygen right over here so we'll save oxygen for last so let's do let's do hydrogen next so hydrogen right over here we have four hydrogen's and on the right hand side we have two hydrogen's so it seems like the easiest thing to do to balance the hydrogen's is to have two of these water molecules now I have four hydrogen's here and I have four hydrogen's there now let's do the oxygen now let's do the oxygen I've balanced the carbons and the hydrogen's and the reason why oxygen is going to be interesting I can just count the amount of oxygen I now have here after throwing these after throat after changing the number of molecules I have and then I can adjust this accordingly because this is only going to affect the number of oxygens that I have on the left hand side well right now on the left hand side I have two oxygens and on the right hand side let me count this I have two oh twos really so this is going to be four oxygens here and then I have each of these water molecules has one oxygen but I have two water molecules so this is going to be two oxygens two oxygens here so on the right hand side I have four plus two oxygens so I have six oxygens on the right hand side I need six oxygens on the left hand side I need this number to be six so how do I do that well I just need three of these molecules if I have three molecules each of them have Oxygen's i'm gonna have a total of six oxygens and just like that we have balanced this combustion reaction this chemical equation