Stoichiometry is a kind of calculation in chemistry. It’s sort of like bookkeeping.
Stoichiometry lets you figure out how much of a substance is used up or produced in a
chemical reaction. You need two things to do stoichiometry:
1. A balanced chemical equation (balanced is absolutely essential here)
and 2. Some measured values (the amount of some of the chemicals involved in the reaction).
You can use these 2 things to solve for whatever is unknown - the mass or volume of
one of the other chemicals in this balanced chemical equation. It’s really common to
measure the amount of one reactant, in order to figure out how much product you expect
to get. Or, given how much of one reactant you have, you might want to figure out how
much of a second reactant to add so you don’t have any left over.
Stoichiometry works because of the Law of Conservation of Mass. Chemical reactions
don’t create or destroy matter, so say you have a certain number of atoms of oxygen at
the start of a reaction, you will have the same number of atoms of oxygen at the end
of the reaction. You can recombine the atoms with other atoms, but you can’t destroy
them, and you can’t change the atoms into some other kind of atom.
The balanced chemical equation tells you how the atoms combine in very definite ratios.
It’s like a recipe. Let’s say you want to make grilled cheese sandwiches. You take
2 slices of bread and one slice of cheese to make one grilled cheese sandwich.
Another way you could say this is for every 2 pieces of bread, you need 1 piece of cheese
to make 1 grilled cheese sandwich. Believe it or not, this is just like a balanced
chemical equation. When you see the equation:
N2 + 3H2 → 2NH3
It’s the same as saying it takes 2 atoms of Nitrogen + 6 atoms of Hydrogen to make
2 molecules of NH3. The equation also says 1 molecule of N2 + 3 molecules of H2 produces
2 molecules of NH3. Another way to read the equation is 1 MOLE of N2 + 3 MOLES of H2 yields
2 MOLES of NH3.
Let’s look at our grilled cheese sandwich recipe again. What if I asked: If you start
with 6 pieces of bread, and assume you have excess cheese (more cheese than you’ll need),
how many grilled cheese sandwiches can you make? It’s tempting to just solve this in
your head, but let’s solve it like we would a stoichiometry problem. We can use the recipe
to find conversion factors. Remember conversion factors are ratios that equal 1.
For every 2 pieces of bread, you need 1 piece of cheese. So you can write that as
a conversion factor: 2 bread/ 1 cheese = 1.
Similarly, for every 2 pieces of bread, you get 1 grilled cheese sandwich. That conversion
factor looks like this: 2 bread/ 1 grilled cheese sandwich = 1
And for every 1 piece of cheese, you get 1 grilled cheese sandwich. The conversion factor
is: 1 cheese/ 1 grilled cheese sandwich = 1
Back to our problem: Write down what you know, and what you want to get at the end,
and leave a blank space for a conversion factor: 6 pieces of bread gives us…. some number
of grilled cheese sandwiches. We want a conversion factor that has pieces
of bread and # of grilled cheese sandwiches in it.
We’ll use the conversion factor 2 bread/ 1 grilled cheese sandwich = 1. But careful
- we want our units to cancel. So in this case, we’ll invert our conversion factor.
That’s the great thing about conversion factors - they equal 1, so you can flip them,
and they’ll still equal 1. That’s why we can use our conversion factor in either
orientation.
So we have 6 pieces of bread times (1 grilled cheese sandwich/ 2 pieces of bread)
= some number of grilled cheese sandwiches. pieces of bread cancels...
6/2 = 3 grilled cheese sandwiches.
I’m sure you could have done this problem in your head, but this is how you’ll want
to write out all your stoichiometry problems. Always write out your work. That way, you’ll
never accidentally divide instead of multiplying with a conversion factor - if you write it
out you can see when your units cancel properly. And, if you’re in a class, you might get
partial credit for showing your work. It’s just a good habit.
{{Example 1}} Now let’s try a stoichiometry problem
using a balanced chemical equation. The balanced chemical equation will be just like our recipe.
N2 + 3H2 → 2NH3 If you start with 10 molecules of N2, and
you have an excess of H2, how many molecules of NH3 will you make?
It’s a good idea to check that your equation is balanced. Count up the number
of atoms on each side. On the left, we have 2 N and 6 H. And on the right we have
2 N and 6 H. Okay, it’s balanced. If your equation isn’t balanced, you have to balance it before
you can do anything else, or stoichiometry won’t work. It would be like a bad recipe,
like trying to make grilled cheese sandwiches and starting with one piece of bread and expecting
2 pieces of bread to magically show up in your finished sandwich. You can’t create
bread and cheese out of nothing - and the same goes for your chemical reactions.
Remember we’re going to use conversion factors. Start with what you know on the left,
and what you want to wind up with on the right. 10 N2 times some conversion factor = some
number of molecules of NH3 on the right.
We get our conversion factors from our balanced chemical equation. The equation tells
us for every 1 molecule of N2 you use 3 molecules of H2 to get 2 molecules of NH3. For this
problem, we need a conversion factor with molecules of N2 and molecules of NH3. We can
write it like this: 1 molecule of N2 over 2 molecules of NH3 = 1.
Let’s put that into our problem. Remember we need to make sure the units cancel. We
want to cancel N2 and wind up with NH3, so we’re going to flip our conversion factor.
10 N2 times 2NH3 / 1 N2 = the number of molecules of NH3
N2 cancels. 10 times 2 = 20 molecules of NH3.
In this stoichiometry problem, we went from molecules to molecules. It turns out,
it’s the same kind of conversion as going from moles to moles. We could have asked - if
you start with 10 moles of N2, how many moles of NH3 will you wind up with - and you’d
get 20 moles of NH3. The Balanced chemical equation tells you the MOLE RATIOS of all
the reactants and products in the chemical reaction.
It gets a little more complicated when you start talking about mass - going from
grams to grams - but the general idea is the same. It will take 3 steps: grams to moles,
then moles to moles, and finally, moles to grams.
We’ll tackle these types of stoichiometry problems in PART II.