When scientists established that everything was made of atoms - we call this The Atomic
Theory - the Law of Conservation of Mass came to be stated as
"Atoms are neither created nor destroyed during any chemical reaction."
What this means for us is - when you are writing out a chemical reaction - a chemical equation
- you'd better make sure it has the same number of atoms on the left side of the equation
as on the right side.
You can't go creating atoms, or destroying atoms.
Make sure there's the same number of oxygen atoms on the left as on the right.
The same number of hydrogen atoms on the left and the right.
Okay?
Otherwise, you're breaking the laws of the universe.
We're going to go through 5 examples of writing balanced chemical equations.
There are a couple of popular methods for balancing chemical equations.
In this video, I'll show you the most commonly used method - we call this the inspection
method - or you might call it the "trial and error" method.
Some people prefer to use another method which uses algebra.
I'm putting that method in another video.
If you really like algebra, that's a great method, so check out that video, and see which
method works best for you.
1.
Here's our first example.
We know that hydrogen and oxygen combine to form water.
We know that hydrogen exists as a molecule of two hydrogen atoms covalently bonded together
H2 and oxygen, likewise, exists as a molecule
of two oxygen atoms covalently bonded together O2
so we have H2 + O2 → forming H2O but if we write the chemical equation like
this, we've *violated* the Law of Conservation of Mass.
We can't show atoms being created or destroyed during the course of this reaction, but that's
what we just wrote - it looks like we lost an atom of oxygen.
So we need to balance this chemical equation by including coefficients in front of the
reactants or products.
Here's the big rule we have to follow - We are only allowed to change the coefficients
in front of any single atom or molecule - but we can't change any subscripts.
If we did, we'd be changing the actual chemical.
For instance, if we wrote H2O2 on the right hand side, we might feel clever and think
we solved the problem, but we'd be so completely wrong.
Because that's not the formula for water, that's the formula for hydrogen peroxide.
What we can do, however, is write 2H2O, which means 2 molecules of water, or 3 H2O, and
so on.
Similarly, on the left, if we wrote O3, that's the formula for ozone instead of the formula
for molecular oxygen, so we're not allowed to do that.
Changing the coefficient is okay, because that just changes the amount.
1 O2 vs 2 O2 - that just means 1 molecule of oxygen vs 2 molecules of oxygen.
So let's look at our unbalanced equation.
H2 + O2 → H2O unbalanced.
If we count up the number of atoms on each side, we can see they are not correct:
we have 2 hydrogens on each side, but we have 2 oxygens on the left, and only 1 on the right.
What happens if we put a 2 in front of the H2O, so we'd have 2 oxygens on the right?
H2 + O2 → 2H2O It's still unbalanced.
Now we have 2 hydrogens on the left, and 2 oxygens on the left, but 4 hydrogens on the
right, and 2 oxygens on the right.
If we put a coefficient of 2 in front of the hydrogen molecule on the left, however...
2H2 + O2 → 2H2O now everything is balanced.
4 hydrogen atoms and 2 oxygen atoms on the left, 4 hydrogen atoms and 2 oxygen atoms
on the right.
2.
Let's try another one.
Nitrogen gas and hydrogen gas react to form ammonia.
We write that as N2 + H2 → NH3.
That's unbalanced.
We can see that if we count up the atoms on both sides.
On the left we have 2N and 2H, and on the right we have 1N and 3H.
I see I have 3 H on the right and 2H on the left.
The lowest common multiple is 6, so I'm going to try to get 6 H on both sides.
To do that, I need to multiply the H2 by 3 on the left, and the NH3 by 2 on the right.
N2 + 3H2 → 2NH3 Let's check.
On the left we have 2N and 6H, and on the right we have 2N and 6H.
That's balanced!
3.
Let's try another one with ammonia reacting with oxygen.
NH3 + O2 → NO + H2O It almost looks balanced, doesn't it?
But it isn't.
Remember to always count up the atoms on both sides.
On the left we have 1N, 3H, and 2O.
On the right we have 1N, 2H, and 2O.
There are 3H on one side and 2H on the other.
Let's give them coefficients like we did last time to get a total of 6 H and see if that
helps.
2NH3 + O2 → 2NO + 3H2O On the left we have 2N, 6H, and 2O.
On the right we have 2N, 6H, and 5O.
We've balanced the Hs, but now O is wonky.
Don't give up.
Keep adding coefficients.
We have 2O on the left, and 5 on the right.
The lowest common multiple is 10, so let's try for 10 total oxygen by putting a coefficient
of 5 on the left.
We'll have to multiply the right by 2 to get up to 10 oxygen on that side.
2NH3 + 5O2 → 4NO + 6H2O On the left we have 2N, 6H, and 10O.
On the right we have 4N, 12H, and 10O.
If we multiply that NH3 by 2 on the left, we'll be square.
So we'll put a coefficient of 2 on to NH3.
Let's check.. 4NH3 + 5O2 → 4NO + 6H2O
On the left we have 4N, 12H, and 10O.
And on the right we have 4N, 12H, and 10O.
Balanced!
That one was hard.
But we got it!
Just don't give up.
4.
How about how carbon and oxygen form carbon dioxide?
Remember carbon exists as a single atom, but oxygen is a diatomic molecule.
C + O2 → CO2 Count up the number of atoms on both sides.
On the left we have 1C, and 2O.
On the right, we have 1C, and 2O.
It's already balanced!
Sometimes, you get a little gift.
What if you saw something like this: 4C + 4O2 → 4CO2?
There are the same number of each kind of atom on both sides, but by convention, we
always simplify down to the smallest whole number.
So we'd simplify this by dividing through by 4 to get
1C + 1O2 → 1CO2, and since we don't write the ones in front of an atom or molecule - they're
understood - we just write C + O2 → CO2 Another thing to remember is
that chemists generally don't write coefficients as fractions.
Every now and then you'll see that, but it's somewhat frowned upon.
Stick to the smallest whole number.
4b.
I'm going to give you another real example # 4, because that one was already balanced,
it doesn't really count.
How about the decomposition of hydrogen peroxide.
You know like when you put hydrogen peroxide on a cut and it's bubbling, this is what's
happening.
hydrogen peroxide breaks down into water and oxygen gas.
H2O2 → H2O + O2 Count up the number of atoms on both sides.
On the left we have 2H and 2O.
On the right we have 2H and 3O.
There's an even number of oxygens on the left hand side, and an odd number of oxygens on
the right hand side.
See that one oxygen sitting on its own on the right?
If I multiply that by 2, now there will be an even number of oxygens on the right hand
side as well.
Let's try that.
H2O2 → 2H2O + O2 So now on the left we have 2H and 2O.
On the right we have 4H and 4O.
Now there is twice as much hydrogen and oxygen on the right as on the left.
If we put a coefficient of 2 on the left, we should even everything out.
2H2O2 → 2H2O + O2 On the left there's 4H and 4O.
And on the right there's 4H and 4O.
Balanced.
5.
Let's do our last example.
Let's pick a big important reaction - the combustion of glucose.
This is an essential reaction for getting energy from food.
You can think of it like this: you eat glucose, and you breathe in oxygen, and you breathe
out carbon dioxide and water vapor.
You are a combustion engine!
You'll learn more about it when you study biochemistry.
For now, let's balance this equation.
C6H12O6 (that's glucose) + O2 → CO2 + H2O On the left we have 6C,12H, and 8O.
And on the right there's 1C, 2H, and 3O.
That's unbalanced.
Start by putting a coefficient of 6 in front of the CO2, to balance the number of carbons,
and recount.
C6H12O6 + O2 → 6CO2 + H2O On the left we have, still, 6C,12H, and 8O.
On the right we have 6C, 2H, and 13O.
Let's put a 6 in front of the H2O to balance the number of hydrogens and recount.
C6H12O6 + O2 → 6CO2 + 6H2O On the left we still have 6C,12H, and 8O.
On the right we have 6C, 12H, and 18O.
Let's put a 6 in front of the O2 and recount.
C6H12O6 + 6O2 → 6CO2 + 6H2O So
on the left there's 6C, 12H, and 18O.
On the right, there's 6C, 12H, and 18O.
BALANCED.