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Balancing more complex chemical equations

A balanced chemical equation shows the same number of each type of atom on both sides of the arrow. In this example, we balance the combustion reaction of ethylene, C₂H₄, and provide tips on how to balance more complex chemical equations.

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  • old spice man green style avatar for user Glen R.
    When referring to C2H4, Sal kept saying that it was ethylene. Would the name of that molecule not just be "Ethene". We are in the middle of studying Organic Chemistry in school and I am under the impression that is how it is pronounced. Any help with this?
    (49 votes)
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  • duskpin ultimate style avatar for user keeyan000
    Why must C2H4+O2 makes CO2 and H2O, why not H2O3 and C?
    (10 votes)
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    • piceratops ultimate style avatar for user Just Keith
      That is because trioxidane (H₂O₃) is exceedingly unstable and reactive, especially in the presence of water. Even if trioxidane were formed, it would decompose into water and O almost immediately. Then, because single atoms of O are one of the most reactive known substances, the O would react with C or O₂ to form either ozone, CO or CO₂.

      However, if there is not enough oxygen available, the combustion of ethane will also produce CO (g) and C(s) instead of just CO₂ and water.
      (29 votes)
  • piceratops seed style avatar for user KAYSY FRESNARES
    to balance the equation don't you just ad a 2 at h2o and a 2 at the 02
    (14 votes)
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  • male robot hal style avatar for user Ted Molkentin
    When you multiplied 2 times the hydrogen atoms, why would you in turn multiply 2 times the oxygen atoms? Are you multiplying two times the entire subset? Or do you not have to multiply the whole equation by two? Also, is this equation equal to start? Or is this equation technically not equal. Because if it is equal, shouldn't we be doing the reverse, and doing it to both sides? Or does that not work in chemistry?
    (11 votes)
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    • piceratops tree style avatar for user henry231580
      When balancing chemical equations, it is not necassary to perform operation on the whole equation like you would do in certain math fields e.g. Matrices. Therefore, it would be a sheer concidence that you would be balancing by 2. The equal of atoms in the equation is not equal and that would be the reason you would have to balance it. Hope that helps.
      (4 votes)
  • primosaur seed style avatar for user Anculus
    I'm a little confused one of my Chemistry problems. Any help would be greatly appreciated.

    Give a balanced chemical equation for the reaction between phosphoric acid (H₃PO₄) and CsOH.

    The balanced chemical equation:
    H₃PO₄ + 3CsOH → Cs₃PO₄ + 3H₂O

    I understand it for the most part, but why does Cs have a subscript of 3 on the product's side? If someone could explain this to me, it would be awesome. Thank you!
    (6 votes)
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    • duskpin sapling style avatar for user Iris Nogueroles Langa
      It's helpful to understand why H₃PO₄ has this formula. This is because it is made up of H⁺ and the phosphate polyatomic ion PO₄³⁻. In order to balance out these charges, you need three hydrogens. This will result in the charges being 3+ and 3- which when added give 0 (no charge overall).

      In this reaction, the caesium displaces the hydrogen as you can see in the balanced equation H₃PO₄ + 3CsOH → Cs₃PO₄ + 3H₂O. Caesium is in group 1 as well so the same principle applies as above. You need three Cs atoms (total charge 3+) bonding with the phosphate ion (charge 3-) so that the charges balance out to zero.

      Hope that helps!
      (8 votes)
  • duskpin sapling style avatar for user slayer
    It might seem like a weird question but the valency of carbon is 4 and that of hydrogen is 1

    So..... why is the formula of ethylene C2H4? Is it because of it's isotope?

    It's my first experiencing class 10 and it's the first time I ever heard of ethylene thus question arises. Please explain in simple language
    (2 votes)
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    • leaf red style avatar for user Richard
      So isotopes are atoms of the same element with different numbers of neutrons. Isotopes have the same chemical properties among themselves because they are still the same element with the same number of protons which does not change. So if we were dealing with carbon-12 (carbon atoms with 6 protons and 6 neutrons) compared to carbon-13 (carbon atoms with 6 protons and 7 neutrons) there would be no difference and they would both still be tetravalent while still being isotopes.

      Ethylene (or more properly ethene) is what is known as an alkene which means that the two carbons have a double bond between themselves joining them together. So each carbon in ethene has two bonds to the other carbon and two single bonds to two of the hydrogens giving each carbon atom a total of four bonds. This becomes more clear when you begin doing Lewis structures for organic molecules.

      Hope that helps.
      (7 votes)
  • blobby green style avatar for user denyall7
    I understand why it is necessary to balance chemical reaction equations as it needs to represent reality by being consistent with the Conversion of Matter. But why would anyone ever right an unbalanced chemical reaction equation in the first place? Where do these unbalanced equations come from? I would guess it comes from real world chemistry applications maybe? I'm not sure, its not that obvious to me where these unbalanced equations come from.
    (4 votes)
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    • mr pants purple style avatar for user Ryan W
      When you react hydrogen gas (H2) and oxygen gas (O2) together you produce water, H2O

      We can turn this into an equation:
      H2 + O2 -> H2O

      But this equation isn’t balanced, yet it is correctly describing the chemical reaction that takes place.
      (2 votes)
  • leaf blue style avatar for user Aaditya Barai
    Like in , it's written "Sal said dioxygen..." in the bottom-right corner
    how can we also correct mistakes in videos
    BTW I know stupid question
    (3 votes)
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  • piceratops seed style avatar for user Varun Menon
    At the video says there is a correction from ethylene to ethane, but isnt C2H4 ethylene ? I mean Sal was correct right ?
    (4 votes)
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  • blobby green style avatar for user valhernandez2
    At , how would I make a chemical equation balanced if the number of atoms is an odd number?
    (3 votes)
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Video transcript

- [Voiceover] 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 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 accounting for the energy, at least the way we've written it. Right over here, you have some ethylene, and this little g in parentheses, says it's in the gas form or gaseous form, so gaseous ethylene plus some dioxygen molecule, which is the most prevalent form of oxygen molecule that you would find 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 molecules over here, and 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 equations 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... molecules that only have one element in them for last. And the idea there is, is that these are harder. They're going to have all sorts of implications, and then, at the end of the day, you can just set a number here for the number of dioxygens. If you saved, say the ethylene for last, then every time, and you're trying to balance the carbons, you try to change the number of carbons, you're going to change the number of hydrogens, which is going to change the... You're going to have to balance over and over, and you're going to 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 single element molecules for last. So let's do that. So, 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 even thought about the oxygens yet. By putting that two there, that's going to change the number of oxygens I have on the righthand side. But at least it balances my carbons. I now have two carbons on the lefthand side, and I have two carbons on the righthand 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 hydrogen next. So, hydrogen, right over here, we have four hydrogens. And on the righthand side, we have two hydrogens. So, it seems like the easiest thing to do to balance the hydrogens is to have two of these water molecules. Now I have four hydrogens here, and I have four hydrogens there. Now, let's do the oxygen. Now, let's do the oxygen. I've balanced the carbons and the hydrogens. And the reason why oxygen's going to be interesting, I can just count the amount of oxygen I now have here, after changing the amount 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 lefthand side. Right now, on the lefthand side, I have two oxygens, and on the righthand side, let me count this, I have two O two's, 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 righthand side, I have four plus two oxygens. So, I have six oxygens on the righthand side. I need six oxygens on the lefthand 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 two oxygens, I'm going to have a total of six oxygens. And just like that, we have balanced this combustion reaction, this chemical equation.