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Balancing chemical equations

How to balance a chemical reaction by making sure you have the same number of atoms of each element on both sides. 

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  • leafers sapling style avatar for user Gabrielle M.
    I'm working on Chemical Reactions: Double and Single Replacement on FLVS. Now my first question for this video is, how do you have 4 aluminum atoms when it says 2Al subscript 2? Do you just add the 2 and the subscript 2? Same thing goes with the O3. I am so lost.

    My next question is...how would I write the charges out in this equation?
    Would it be like this?
    Al + O + O ----> Al + Al + O + O + O

    I don't know if I'm doing it right or not. I am completely lost. If someone can help me, I would really appreciate it. Thanks :)
    (23 votes)
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    • marcimus purple style avatar for user Samiyah
      LOL. Hi Gabrielle M.
      I do FLVS too.
      In fact, I can help you there.
      For your first question, you multiply the whole number and the subscript.
      For your second question, if I understand correctly, I would just right it as
      2Al + 3O2 = 2AlO3 because when multiplying (as I said above) you will see how you helped make the equation balanced.

      Don't worry, I was so lost and confused at the beginning too. I asked my teacher about it, watched the Khan Academy video (above) and took notes from my lesson and all started to untangle and make SO Much sense after all.
      (3 votes)
  • blobby green style avatar for user edward4810
    How does Al and O get 2 and 3 atoms?
    (1 vote)
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  • aqualine seedling style avatar for user Ulf.C.Herno
    is not 03 a molecule in the atmosphere?
    (4 votes)
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    • spunky sam blue style avatar for user Ernest Zinck
      Yes, ozone is found in the atmosphere at an overall concentration of 0.6 ppm.
      But most of it is found in the upper atmosphere.
      Ozone is highly unstable. It rapidly decomposes to oxygen in the lower atmosphere.
      In the upper atmosphere (in the ozone layer), ultraviolet rays from the sun are constantly converting oxygen molecules into ozone, and the concentration of ozone there ranges from 2 to 8 ppm.
      (21 votes)
  • blobby green style avatar for user laxeot
    At , since the equation reads 2Alsub2 Osub3, does the coefficient from 2Alsub2 apply to Osub3, or is it just assumed that we know that oxygen is doubled or something?
    (5 votes)
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  • blobby green style avatar for user Mcaela Moodley
    do you have to multiply each side by the same number?
    Sorry I'm just a bit confused.
    And also do you have to multiply each molecule?
    (7 votes)
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    • aqualine ultimate style avatar for user famousguy786
      We will have to multiply each sides by the same number if one of the molecules has a fractional value after balancing(1.5 for oxygen becomes 3 by multiplying every molecule on both sides by 2). However, when the reaction is not balanced(when both sides do not have equal number of atoms of every element) we multiply the value of the molecule/atom by a suitable number on the side where it has fewer atoms. We do this until the reaction is perfectly balanced and we are sure that the Law of Conservation of Mass is followed. I hope this answers your question.
      (7 votes)
  • winston default style avatar for user HarleyQuinn21345
    So no matter how many times I watch this video and how many times my Chemistry teacher explains it I cat seem to understand how to balance equations. If I have
    N2+ H2---> NH3 what would I get and how would would I get that answer? I have a work sheet due on this this Wednesday so I need help fast.
    (9 votes)
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    • mr pants purple style avatar for user Ryan W
      Write it out on a piece of paper and underneath each side of the equation write out how many of each atom there are.

      N2 + H2 -> NH3

      On the left there is 2 N and 2 H
      On the right there is 1 N and 3 H

      If we tried to balance starting with H you'd need to use a fraction or decimal and would get messy, so let's start with N.
      There's 2 on the left and 1 on the right, so we need to change the coefficient of NH3 to 2

      Now we have
      N2 + H2 -> 2NH3

      Total the atoms up again:
      On the left there is 2 N and 2 H still
      On the right there is 2 N and 6 H now

      So now all we need to do is make the left side have 6 H in total. So all we need to do is make the coefficient of H2 3

      N2 + 3H2 -> 2NH3
      (2 votes)
  • female robot grace style avatar for user yamaha
    is the equation technically balanced already at ? say we don't care about molecules being half molecules. Just wanna make sure i understood
    (5 votes)
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  • purple pi purple style avatar for user Michael
    Can someone explain why in water (H20) the subscript isn't written after the O, because there is 2 Hydrogen and 2 Oxygen in the compound.? H2O2 isn't correct, however, it makes more sense to me.
    (3 votes)
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    • mr pants purple style avatar for user Ryan W
      The subscript number tells you how many of the atom before the subscript number there is. H2O, so there are 2 hydrogen atoms and 1 oxygen atom (not 2 oxygen atoms)

      H2O2 is hydrogen peroxide which is a different molecule, 2 hydrogen atoms, 2 oxygen atoms, the oxygen atoms are bonded together.
      (4 votes)
  • duskpin seedling style avatar for user Elizabeth
    how do you balance equation with just whole number?
    (3 votes)
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    • duskpin ultimate style avatar for user Cowboy
      The coefficient in a balanced equation is an idea; the concept of telling the chemist that if the atoms, molecules and compounds are balanced, there are balanced amounts of the atoms, molecules and compounds on the other side in the product. It gives us a way to measure a reaction and use stoichiometry to gain the exact amounts desired of a specific product. You can read my explanation below to get an idea for this, but basically, the coefficient is just telling the chemist how much of a specific atom, molecule or compound it takes to gain the desired product.

      So, balancing an equation is just like balancing the x in an algebraic equation... you need to make sure (because of the law of conservation of matter—matter cannot be destroyed or created) that if you have a definite amount of something on one side, you have an equal amount of it on the other side...
      The equation (Mg) + (HCl) ⟶ (MgCl2) + (H2) is clearly unbalanced because on one side, there is only 1 hydrogen atom, but on the other side, there are 2 (also unbalanced Chlorine, but they both come from the same compound so this becomes really simple to balance).
      This equation is easily balanced by placing the coefficient "2" in front of molecule (HCl) to form the balanced equation (Mg) + 2(HCl) ⟶ (MgCl2) + (H2).
      you can think about it this way; 1 atom (Mg) + 2 compounds (HCl) combines in a reaction to form the products of 1 compound (MgCl2) + 1 Molecule (H2).
      (3 votes)
  • old spice man green style avatar for user Jacob
    Does the number in front of the element mean the same thing as the subscript? (like 2O & Ov2 for example) Wouldn't that be redundant to put a number in front of an element that has a subscript?
    (2 votes)
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Video transcript

- Balancing chemical equations is one of those concepts in chemistry that often confuses people. But I think we'll see that if we work through this carefully and methodically, and we also appreciate the art of balancing chemical equations, that it's actually not too bad. So first of all, what is a chemical equation? Well this is a chemical equation right over here. It's describing a reaction. So if I take an atom of aluminum and I add it to a dioxygen molecule, so a molecule that has two oxygens with it, under the appropriate conditions they will react to form aluminum oxide. And the aluminum oxide molecule has two aluminum atoms and three oxygen atoms. And so you might say, "Okay, well what's "the balancing business all about? "I have a chemical reaction. "What do I have to balance?" Well if you look carefully, you might notice that you don't have the same number of each atom on both sides. For example, right over here on the left-hand side, how many aluminums do we have? Well on the left-hand side, we have one aluminum. How many do we have on the right-hand side? Well on the right-hand side, we have two aluminums. And so aluminum just can't appear out of thin air by virtue of some magical reaction. You have to have the same amount of aluminums on both sides, and the same thing is true for the oxygens. Over here on the left-hand side, we have two oxygens. They form one dioxygen molecule that has two oxygen atoms. And then over here in the aluminum oxide molecule, we have three. We have three oxygen atoms. So once again, we can't just have miraculously an oxygen atom appear out of nowhere. So we have to balance the number of aluminums on both sides, this number and this number should be the same, and we have to balance the number of oxygens, this number and that number should be the same. So how do we do that? Well one thing might be to say, "Okay, if I've got "two aluminums here and I have one aluminum here, "well why don't I just double the number "of aluminums right over here?" I could just write a two in front of it, so now this has two aluminums, so I no longer have one aluminum here. I now have two aluminums, and so it looks like the aluminums are balanced, and they are indeed balanced. But still we have an issue with the oxygens. Over here I have two oxygens. Over here I have three oxygens. So one thing that you might say is, "Okay, well how do I go from two to three? "I could multiply by 1.5." So I could multiply by 1.5, and if I multiply 1.5 times two, that's going to be three. So now I have three oxygen atoms on this side and three oxygen atoms on this side. But the convention is that we don't like writing "1.5 molecules." We don't like having this notion of a half molecule, which is kind of this bizarre notion. We want whole number molecules. So what can we do? Well, you can imagine that this makes it very similar to what you did in algebra, an algebraic equation. We just can multiply both sides by the same number that gets rid of having this fraction or this decimal here. So if we multiplied both sides by two, we're going to do that. This is going to be a four, this is going to be a three, this is going to be a two right over here. So let me do that. Let me multiply both sides by two. So instead of two aluminum atoms, let me have four aluminum... Actually, let me just write the chemical equation first in the form that it was before. So I had aluminum plus dioxygen, a molecule of two oxygens, yielding in the reaction -- these are the reactants, this is the product -- aluminum, aluminum, aluminum oxide. So what I'm saying here is to get rid of this 1.5, to turn it into a whole number, let's multiply all of these, all of the number of molecules by two. And here, there's implicitly a one... Let me do this in a different color. There is implicitly a one right over here. So let's multiply all of these by two. So two times two is... Let me do that same color. Two times two is four... That's not the same color. Two times two is four. 1.5 times two is three. And then one times two, one times two is two. And now you can verify how many aluminums do we have on each side? Well I have four aluminum atoms on the left-hand side, and how many do I have on the right-hand side? I have four aluminum atoms. How many oxygens do I have on the left-hand side? I have three molecules of dioxygen. Each molecule has two oxygen atoms, so I have six oxygens on the left, and I have two times three oxygens on the right, or I have six oxygens. So my chemical equation is now balanced.