Main content

### Course: High school chemistry > Unit 3

Lesson 2: Balancing chemical equations# Balancing chemical equations

A balanced chemical equation shows the same number of each type of atom on both sides of the arrow.

## Want to join the conversation?

- 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 :)(28 votes)- Al2 is written with a subscript because it is bonded to O3. The leading 2 shows that there are two Al2O3's- giving four aluminum and six oxygen. This is found by multiplying the leading number by the subscript.(34 votes)

- How does Al and O get 2 and 3 atoms?(14 votes)
- Aluminium has a charge of +3 and Oxygen has a charge of -2. So by using the criss-cross method we get Al2O3. Hope you understand. Thank you(6 votes)

- 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)- 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(28 votes)

- is not 03 a molecule in the atmosphere?(4 votes)
- 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)

- At4:46, 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)
- The coefficient 2 in 2Al₂O₃means 2×(Al₂O₃).

Everything inside the parentheses gets doubled, so there are 4 Al atoms and 6 O atoms.(14 votes)

- 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)- 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.(8 votes)

- I have a question, for the Al oxide how did he get one?(5 votes)
- I’ll assume you’re referring to4:00. The top equation is balanced. If an equation is balanced and if a chemical doesn’t have a coefficient, then we assume it has a coefficient of 1. It’s implicit as Sal mentioned. Similar to how variables are assumed to have coefficients of 1 if they do not have explicit coefficients (e.i. x = 1x).

Hope that helps.(5 votes)

- I was trying to do this, but I failed, is there an easier way to try this?(5 votes)
- well...not really...

Just practice(4 votes)

- is the equation technically balanced already at2:55? say we don't care about molecules being half molecules. Just wanna make sure i understood(5 votes)
- Yes, the equation is technically balanced at2:55.(2 votes)

- how do you balance equation with just whole number?(3 votes)
- 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).(5 votes)

## 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.