- Today on Real Chemistry,
we're gonna be talking about stoichiometry.
Stoichiometry is a big word that basically just means
thinking about chemical reactions as being recipes.
And so we're gonna use a lot of analogies in this video
about food to get you thinking about chemical reactions
as recipes for the products they're producing.
So, in this first slide, we have a simple recipe.
You combine two slices of bread,
one slice of cheese and you get out a cheese sandwich.
Straightforward enough, right?
And we're gonna compare this in the next slide
to a chemical reaction.
And we're gonna use, thinking about the ingredients
that go into something,
to calculate how much product could come out.
For example, if I put in four slices of bread,
how many sandwiches come out?
Well, with this recipe,
since there's two slices of bread needed for each sandwich,
we could make two sandwiches with four slices of bread.
What does it look like when you have a chemical equation?
Well, here's a chemical reaction that's combining
chlorine and titanium to make titanium chloride.
And notice that it has the exact same stoichiometry.
By that, I mean you put together two chlorines
with one titanium and you get out one titanium chloride.
And the same way, if I put together two slices of bread
with one slice of cheese, I get out one sandwich.
And so, we can use these recipes.
Stuff goes in, stuff comes out,
to figure out how much of different types of product we
can make whether its cheese sandwiches or titanium chloride,
just by looking at the recipe and thinking about it.
So let's take a look.
All right, in the first case, when we're making sandwiches,
our question asks how many sandwiches can you make with
10 slices of bread, and excess cheese?
The excess cheese part is important.
What that means is that when I go into the kitchen
I got plenty of cheese, I don't need to worry
about how much cheese I have, all I need to worry about
is how much bread I have.
And this is the way that stoichiometry
problems work at first, we just consider one ingredient.
Later on when they get a little more complicated,
we'll consider both ingredients, and then we have to think
about which one runs out first.
In this case, we know that cheese is not gonna run out,
and so all we're thinking about is how many
sandwiches can I make with a different amount of bread.
And here, I'm just gonna say okay, well if I have
10 slices of bread,
and I'll use B to stand for bread,
well then all I need to do is divide that
by the number of slices of bread in my recipe.
So if I multiply that by one over two,
that gives me five sandwiches.
So, 10 slices of bread can make five sandwiches because
it takes two slices of bread in each case.
Now, that problem's really easy and everybody looks
at that and says okay, I don't even know why you spent
your time telling me that, I could figure that out.
But notice, that's exactly the same as it is
in this chemical equation below.
This one says how many moles of titanium chloride
can you make with 10 moles of chlorine?
And well, if I start out with 10 moles of chlorine,
then I'm really gonna do the same thing.
I look at my recipe and I see okay,
I need two moles of chlorine
for every one mole of titanium chloride I get out,
so all I have to do is again divide it by two,
because I need to put in two of those chlorines
for every titanium chloride.
And I'll see that oh, hey,
I can make five moles of titanium chloride.
So these are basically stoichiometry problems.
Now there's a lot of variations that we can do here
on stoichiometry problems,
so there's a fair amount more to learn,
but this is the basic idea.
You think about how much of something you can make,
or how many ingredients you need
for a certain quantity of product.
So, let's take a look
at a little bit more complicated example.
Now let's make brownies.
So we put in a brownie mix, and we combine that with two
cups of waters, and we get out 12 brownies.
Why is this more complicated?
Well, now we have to take into account that each time
we run the recipe, we don't just get one sandwich,
we get 12 brownies, and that makes our math
a little more complicated.
And you'll notice that the chemical reaction below
from making ammonia, which is NH3, is the same.
We combine one nitrogen in that case,
with three hydrogens, and we get out two ammonias.
And so, when we have problems like this
we have to take into account not just the number
in front of the ingredient we're looking at,
like in this case water, but also the number
in front of the product, how many brownies we get out.
So if I wanna do this problem,
I start with four cups of water,
so four cups of water,
and I wanna get to my brownies.
So what I need to do then is take into account
how many cups of water it takes for each batch,
two, so I can make two batches,
and the way I can calculate that is
again just dividing by 2.
And then for each batch I get out 12 brownies,
so that means I need to multiply by 12.
So I can run this recipe twice, and each time I run it
I get 12 brownies, and that means I'm gonna
get out a total of 24 brownies.
So whenever you look at a chemical reaction,
don't look at it as just numbers and symbols on a page,
realize that what a chemical reaction is telling you is a
recipe for how to make the product.
And if we go down to the example down here,
we're making ammonia.
And it says if we use three moles of hydrogen
and excess nitrogen, again, we don't need to worry
about nitrogen, we're just worrying about the hydrogen,
how much ammonia can I make?
So again, what we need to do is we start out
with our hydrogen, three moles hydrogen,
and each time we run the recipe we need three moles
of hydrogen, that's what that three is telling us.
So, this time we're gonna multiply it by one over three,
and every single time we run the recipe,
we get out two ammonias,
so that means I'm gonna multiply by two.
So then, we basically are gonna be able to run this recipe
exactly once, and each time we run the recipe up
we get two ammonias.
So this is gonna give us two moles of ammonia.
Now, so far we've just gone thinking about this in sort of
conceptual terms, we haven't really drawn
careful conversion factors.
But you might be able to see already that we can
do these problems with conversion factors, and we'll
get the right answer over time, and so that's what we're
gonna develop on the next slide.
We're gonna talk about how you do these exact type
of problems, if you use conversion factors.
So, in this case, again,
we're gonna be dealing with brownies,
and we're gonna write this problem out very explicitly now
as a conversion factor.
It's good to be able to both think through the problem,
like we've been doing in the past few slides,
and also to solve it in a sort of systematic
mathematical way, which is what we're about to do.
And so, if we wanna do these stoichiometry problems with
conversion factors, I've broken the process down
into three steps.
We just write the starting quantity,
and in this case it says how many brownies can we make
with six cups of water, and excess brownie mix?
So we got plenty of brownie mix,
and we're limited only on water.
So that means we're only gonna think about water.
And we write down what we're starting with,
which is six cups of H2O.
And now, I'm gonna write in my second step
the desired quantity that I wanna get out.
And I wanna know how many brownies I can make, right?
This says, "How many brownies?"
And so my desired quantity over here is brownies.
And now what I'm gonna do is I'm gonna use a
conversion factor, and the numbers in that conversion
factor are gonna come from my recipe.
And, what I'm gonna do here is I'm gonna say
okay I wanna get rid of water, so I'm gonna write
H2O down here.
In particular it's cups of H2O.
And I wanna get brownies, so I'm gonna write
brownies up here.
And now what I do is I go up to my recipe, and I see hey,
there's a two next to my water and a 12 next to my brownies.
And so if I put a two next to my water down here
and a 12 next to my brownies, that's always gonna use
my recipe up there to convert between cups of water and
brownies, and so I have six cups of water,
which lets me run that recipe three times,
I get 12 brownieout three times, so if I just multiply
all that through, I'll get out 36 brownies.
So this is kind of the conversion factor method of doing
stoichiometry problems, and that's how you typically want
to approach them.
We'll do a problem now with a chemical reaction.
In this case, it says, "How many moles of ammonia can you
"make with six moles of hydrogen and excess nitrogen?"
So now our starting quantity is moles of hydrogen.
So we have six moles H2,
and we don't need to worry about how much nitrogen we have,
and the quantity we want to get to is moles of ammonia.
So we wanna get to moles of ammonia,
and we're starting out with moles of hydrogen,
and so we just write our conversion factor.
We wanna get rid of the H2, the moles of H2,
and we want to get moles of ammonia.
And again, I just go up to my chemical recipe, and I say
oh, okay, I need to put in three moles of hydrogen.
And for every three moles of hydrogen I put in,
I'll get out two moles of ammonia.
So that two goes next to my ammonia,
and the three goes next to my hydrogen.
And, what that means is I'm gonna take six, multiply by two,
and divide by three, and that's gonna give me
four moles of ammonia.
Now, we've been doing relatively straightforward
stoichiometry problems where we've been dealing directly
with the units that the recipe gives us.
So when we made brownies, we talked about cups of water
'cause the recipe was in cups of water.
And here our chemical reactions are in moles,
and we're talking about moles.
One other wrinkle that happens in stoichiometry problems
is we're often given not exactly the same units
for our quantities of reactant
that our recipe's in terms of.
Lemme show you what I mean.
Let's say now that I have the same brownie problem,
but instead of giving me how many cups of water I have
it gives me how many ounces of water I have.
So it says,"How many brownies can you make
"with eight ounces of water and excess brownie mix?"
Well, now there's one extra step to this problem right?
And I've highlighted that in my steps below where you can
see it says you must use the same units as the recipe.
So I can't just run this problem forward with eight ounces,
'cause that's not what my recipe is telling me,
it's telling me about cups.
So the very first step,
if I'm now using slightly different units,
is to do a unit conversion.
So I write out what I'm starting with, once again,
which is eight ounces of H2O.
And now my first step instead of using my recipe as a
conversion factor is just to convert from ounces to cups.
So you always need to be in the same
units that your recipe's in.
And in this recipe, we need to be in cups.
For our chemical reactions, our recipe is always in moles,
so we always have to go to moles.
And I've written the conversion factor
between cups and ounces here, so eight ounces is one cup,
so we're just gonna put eight ounces down below,
and that gets rid of that, and we're gonna put
one cup up top.
Okay, now our cups are gonna cancel,
or our ounces are gonna cancel, rather,
and we'll get out cups.
So we take eight, we multiply it by one,
and divide it by eight, and that's gonna give us one cup.
And now we do our stoichiometry part of the problem,
where we use our chemical recipe.
So we have cups of water and now we wanna know how many
brownies we can make, so we still wanna cancel out water,
which I put on the bottom,
and we wanna get brownies, which I put up top.
So we wanna get brownies, and we'll notice that 12 is
in front of my brownies in my recipe,
and two is in front of my cups of water,
and so now I'm just gonna multiply through.
And what this will do, is it first takes me from ounces
to cups, which is the same terms as my chemical recipe,
and then it'll take me from cups of water to brownies,
and I'll get out six brownies.
So that's how many brownies I can get out
from this chemical recipe.
So now what we're gonna do, is we're gonna take a look
at a problem that's like this, where we're not given
the correct units, but for a chemical reaction.
So here it says, "How many moles of ammonia can we make
"from 3.5 grams of hydrogen?"
And notice, that since our chemical recipe is always
in terms of moles, the very first thing we're gonna
have to do is take those grams of hydrogen to moles.
That's the very first thing we're gonna have to do.
And that's very common.
We always need to get to moles
before we do stoichiometry problems.
There's all sort of variations on stoichiometry
problems, and almost always the very first step
is get whatever our current units are into moles.
So we have 3.5 grams of hydrogen,
that's what we're starting with,
and we need to go to moles.
So we go to moles with molar mass,
we go from grams to moles with molar mass,
and the molar mass of H2, that is molecular hydrogen,
is 2.02 grams.
So our 2.02 goes on the bottom,
and our one mole goes up top.
So that step there is just to get our units
in the same terms as our chemical reaction.
Our chemical reaction is dealing with moles, an amount of
hydrogen molecules, and so we first take it from a mass
of hydrogen molecules to an amount of hydrogen molecules,
so that we can use our recipe, so that we can do
the stoichiometry problem.
So that gets rid of our grams, and what we're left with now
is moles of hydrogen.
And what we wanna figure out
is how much ammonia we can make.
So the last step here, our last conversion factor
we're gonna have to write, should take us
from moles of hydrogen to moles of ammonia,
so we put H2 down here to cancel that out,
and we wanna get out NH3.
And again now we go look at our chemical recipe
and we see that there's a two next to our ammonia,
so we put a two there, and there's a three next to our
hydrogen, so we put a three next to our hydrogen.
So we're set.
We've gotten our amount of hydrogens
and amount of our ingredient on the same terms,
and now we're just gonna multiply it by our conversion
factor for our stoichiometry problem, and we'll get out
1.2 moles NH3.
Alright, so that's a basic introduction to stoichiometry.
We've talked about what stoichiometry basically is,
and we've built up the tools we need
to do some basic problems.
I suggest you go ahead and watch stoichiometry practice
problems so that you can do some more examples
of these with more variations on the initial units you're
given for and what final number you're actually asked for.
Thanks for watching.
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