In this problem I want to calculate the mass of
the product that forms
And we're given the thermite reaction
but we havent balanced it yet
And this is used it has a very high heat of reaction
and so it's used for welding metals
and we're gonna start out with 25g of aluminum
metal. 85g of the iron oxide and the question is
How much iron metal, how many grams of
iron metal are produced?
And then which of the two reacts is the excess reactant?
So any time I have a chemical reaction
the first thing I want to do is balance the reaction
before I do anything else
We have to work with a balanced reaction
so if I write the unbalanced reaction
So now I can choose how I'm going to do this
Let's say that I have two irons on the left
That means I would have two metalic
iron atoms on the right
So that balances the iron, and now
I need to also balance the oxygen
And aluminum. So is I put a two here
two aluminums on the left, two aluminums on the right
and three oxygen atoms on both sides
So it turns out to be relatively simple to balance
We always want to write down what we know
We start with 25g of this
85g of iron oxide, and we wanna know
how many grams of this, and then which of these
are in excess. So first we have to convert to moles
So first we look up the atomic wieght
So I've used the periodic table to look up
these atomic wieghts for the elements
So that means I can calculate atomic weight of Fe2O3
And that atomic wieght is gonna be two irons
plus three oxygens
Multiply those out so I can use that to determine
the number of moles of Fe2O3, and that's the
number of grams multiplied by, in this case divide by
the grams per mole. So this is gonna give me the
the number of moles of Fe2O3
So let's calculate now the number of moles
of aluminum. We have 25g and if we look up in
the periodic table then the molecular mass
26.96g for 1 mole of aluminum
So we calculate that and get 0.927mol of aluminum
and we look back at the reaction
We needed 2 moles of aluminum for 1 mole of the iron oxide
So we need this much aluminum for a stoichiometric
amount. We have less aluminum so therefore
this is the limiting reactant
And so that means that the iron oxide is in excess
Which is one of the questions the problem was asking
The next question is how much iron is produced?
So we can see from the reaction that 2 moles
of aluminum reacting makes 2 moles of iron
So if all the aluminum reacts, since it's the limiting reactant
that's possible, then we would end up with
0.927mol of metalic iron
So now we go back to the periodic table again we
have 55.84g of iron for 1 mole
If we do this multiplication, units, we have grams of iron
51.77 or 51.8g of iron