>> Welcome to a screencast on an introduction to reaction stoichiometry.
Well, first of all, what is stoichiometry?
It's the part or aspect of chemistry that deals with quantitative relationships
between the substances involved in a chemical reaction.
So in other words it is the part of chemistry that deals with how much
of this reactant is needed to react with how much of that reactant.
How much of a certain product can be produced and so on.
An example here is the space shuttle or rocket often uses something like hydrogen as its fuel,
reacts with oxygen and we have representations of the molecules involved in the reaction.
What you start with and what you make.
And the reaction itself and the stoichiometric relationships, we'd say, are that two molecules
of hydrogen gas react with one molecule of oxygen gas to make two molecules of water.
And then, since we typically don't count molecules, we would have to deal with quantities
in terms of masses or some other quantity that we could measure very well.
So that's what stoichiometry is -- dealing with these quantitative relationships.
So let's start with an example.
Several parts to this one.
We have the reaction of iron 3 oxide with carbon monoxide to make iron and carbon dioxide.
So this is the reduction of iron 3 oxides to iron
and carbon monoxide is what allows this to happen.
And we have three parts to this question.
How much carbon monoxide is needed to completely react with a given amount of iron oxide?
How much iron could we produce by the complete reaction of a different amount of iron oxide?
And then, if we actually produce a specific amount of iron from that original iron oxide,
what's our percent yield of iron.
So let's deal with these one at a time, starting with how much CO is required
to completely react with 3.067 moles of Fe2O3?
Well, in order to do this, we need to have a chemical reaction and it needs to be balanced.
So notice, this reaction isn't balanced.
Shouldn't be too hard to do.
That's something you should probably know how to do at this point.
And there is the balanced chemical equation which we now need.
And note, what it tells us is that one Fe2O3 unit reacts with three CO units
to make two Fe units and three CO2 units.
So that -- those are the stoichiometric relationships within this reaction.
The Fe2O3 are formula units of an iron oxide ionic compound.
CO, the units are molecules.
Fe, the units are atoms.
And CO2, the units are molecules.
Now note, of course, the relationship is one to three to two to three on the basis of units.
And if we scale it up to a unit of moles, then the relationship still holds.
And that's the key to chemical stoichiometry.
This reaction tells us that one mole of Fe2O3 reacts with three moles of CO,
produces two moles of Fe, and produces three moles of CO2.
In all chemical reactions, the stoichiometric relationships are in terms of moles.
That's typically what we're going to use.
So this is going to essentially just be a conversion problem.
We have 3.067 moles of Fe2O3.
We need to convert it into or determine how much CO is required.
And what we know is that for every one mole of Fe2O3 that reacts,
we know that three moles of CO are needed.
And so if we set up a conversion factor like we've done before but with moles of Fe2O3
on the bottom, which we want to cancel out, and moles of CO on the top,
which we want to get, then we simply do the math.
3.067 times 3 is 9.201 moles of CO and that is a perfectly acceptable 'how much.'
We can have amounts of terms of moles.
Now very often you will want amounts in terms of grams.
So if we wanted to continue, we could easily convert to grams,
but this is a perfectly good answer to the question -- 9.021 moles of monoxide are needed.
If we want to convert from moles to grams,
that's something you should know how to do at this point.
Use the periodic table.
Determine the mass of -- molar mass of carbon monoxide, 28.01 grams per mole.
And then convert from moles of carbon monoxide to grams of carbon monoxide
and in this case the answer is 257.7 grams of CO.
Okay. Now what we just did is at the heart of stoichiometry calculations in general.
Let's do a somewhat more challenging one next.
How much iron could be produced by the complete reaction of 247.36 grams of Fe2O3?
And here, let's get the 'how much iron' in terms of an actual mass.
So we're going to really be looking for how many grams of iron could be produced.
Okay. Well, couple things we're going to need to know.
An iron -- a mole of Fe2O3 is 159.688 grams.
Use the periodic table and determine that.
And the reason we want that is because we need to get the iron 3 oxide converted from grams
to moles because the reaction happens on the basis of moles.
This doesn't tell us how many grams of this react with how many grams of that.
It tells us moles.
So 247.36 grams of Fe2O3 convert from grams to moles.
This is something that should be familiar to you at this point.
Set up your conversion factor.
And that is 1.5490 moles of Fe2O3 -- that's equivalent to the grams of Fe2O3 that was given.
Now next, we note that for the Fe2O3, one mole of Fe2O3 requires three moles of CO,
produces two moles of Fe and three moles of CO2.
So if what we want is moles of Fe, now our conversion factor is two moles of Fe produced
for every one mole of Fe2O3 that reacts.
We set up our conversion to cancel out the moles of Fe2O3 that we started with to get us moles
of Fe -- Fe which is what we want.
And that turns out to be 3.0980 moles of Fe.
And if the question truly was just how much iron could be produced, we could stop here
because this is a perfectly good amount of iron.
But I did say that we're going to want the amount of iron in terms of grams
so we'll continue and do the last step -- 3.0980 moles of Fe.
One mole of Fe is 55.845 grams and when we do that calculation, we get 173.01 grams of Fe
that could be produced by the complete reaction of 247.36 grams of Fe2O3.
Okay. Now note what we did in this problem is going to be the same as we do in general.
We have a known amount of some substance.
In this case, it was Fe2O3.
And we could know it in grams or in moles.
We want to get to a known amount or an equivalent amount of a different substance
which we could know in moles or grams.
And we have to go through the equation when we convert from moles
of one thing to moles of another.
And then we use molar mass to go from grams of one thing to moles, or from moles to grams.
So we're going to have several conversions, potentially.
Always goes through the equation -- possibly involves molar masses as well.
Now the last part of this problem is --
what if we actually produce 146.82 grams of iron, what's our percent yield?
And notice, this amount of iron that was actually produced is less
than what we calculated in the previous part for the amount
that could potentially have been produced.
That part, the amount that could potentially be produced,
is what's called our theoretical yield.
And that's the amount that you would get if all of the substances react as fully as possible.
In general, that's not what happens and so we normally end up having a percent yield
which is our actual yield divided by our theoretical yield,
or what we potentially get if everything was perfect yield.
And then we multiply by 100% to get percent.
So in our case, our values are 146.82 grams for the actual yield,
173.01 grams for the theoretical yield.
Divide those and multiply by 100 and we get 84.862%
for our actual yield or our percent yield.
And that's it for an introduction to reaction stoichiometry.