0:33

Likewise if we have two heads, two middles and

two bottoms we can form two snowman, again with nothing left over.

However, in real reactions, we frequently have more of one substance than another.

And so then we have to determine how much product can we actually make?

1:03

We're limited to making three snowmen,

because we are limited by the number of snowmen heads we have.

Once we run out of one part or

one reactant, we can not continue the reaction any more.

And so it is going to control the amount of product that we make.

So even if we had 50, or 30, or 100 middles and bottoms for

our snowmen, we would be left with only three snowmen because we

are limited by the number of heads that we have.

And once we'd run out of that reactant, we're done.

Our reaction can not proceed any further.

1:37

So for this, we can redefine two new terms, limiting and excess reagents.

So the limiting reagent is the reactant that is used up first in the reaction.

We frequently determine this by looking to see which one will limit the amount of

product that we can form.

The excess reagent is the species that is

left after all of the limiting reagent is consumed.

Depending on the number of reactants, we could have two or more limiting reactants.

2:09

In this case, our head was our limiting reagent, so

this limited the amount of snowmen we could build, and both the middle and

the bottom were our excess reagents, or excess reactants,

because we had some of those left over after the reaction was complete,

because the reaction stopped as soon as we ran out of one of our reagents.

2:35

Now let's look at another example.

Let's look if we have, we're making sandwiches.

Now it's not a one to one to one ratio, like we saw with our snowman,

where we needed one of each part.

Now, to make a sandwich, we need two pieces of bread and one piece of turkey.

Therefore, we have to figure out how many sandwiches can we

make if we have six slices of bread and five pieces of turkey.

Now, I probably don't have to show you the math for you to realize that the bread is

going to run out first, that we're only going to be able to make three sandwiches.

But what we want to look at is how we're going to set that up

based on what we know from our equation here,

which happens to show two bread, one turkey equals one sandwich.

3:19

Let's start by assuming that the bread is limiting.

[SOUND] So if the bread is limiting the agent,

that means we're assuming that the turkey is the accessory agent.

Now I know that I have 6 slices of bread.

That was the amount that was given in the problem.

And I also know the relationship between the number of pieces of bread and

the number of sandwiches I can make, and

I get that from the coefficients in my balanced equation.

So, I know that for every two slices of bread, [SOUND] I can have one sandwich.

Now this is the same way we looked at in the previous unit,

where we looked at the relationship,

the mole-to-mole ratio between two substances in a chemical equation.

The difference is now that we're not going to be able to do just one calculation to

find the amount of product, we're going to have to do two calculations.

So here, if we assume that the bread is limiting, we know that six slices of

bread, our slices of bread cancel out with one another, and

what we're left with, is that we can form three sandwiches [SOUND].

So now, I assume bread is limiting.

Now I want to assume [SOUND] that the turkey is the limiting reagent and

therefore that the bread is the excess reagent.

So I do the same thing I did with the bread.

I have five [SOUND] pieces of turkey.

Again, I go to my balanced chemical equation or my balanced equation here.

To determine the ratio between turkey and sandwiches, and

see that I need 1 piece of turkey [SOUND] for every 1 sandwich.

Again, I see that turkey cancels with turkey.

When I do the math, I see that I get 5 sandwiches.

5:19

So what I'm looking for

here is looking to see which one is produced the least amount of products.

Because I produce only three sandwiches when I assume bread is limiting.

That lets me know that this is the maximum amount

of product I can produce [SOUND] and,

that the bread is, in fact the limiting reagent.

[SOUND] Because I

run out of bread,.

Before I run out of turkey,

it is going to limit the amount of product that I can form.

And that's easy to see when I look at my two assumptions, my two calculations, and

see that if bread is limiting,

I can make three sandwiches, if the turkey is limiting, I can make five.

And so the lesser amount will always determine which reagent is

the limiting reactant.

6:21

Now, how do we recognize that something is a limiting reagent problem if

we're not told that it is?

What we want to look for is to see if we have amounts of two or more reactants.

In this case we're given a balanced equation of iron plus oxygen yields

iron oxide.

And I see that I have 25 grams of iron and 25 grams of oxygen.

Because I'm given the amounts of both of these substances,

I don't know which one is going to be limiting.

And I can't simply compare the amounts and

say that we have stoichiometric amounts, because they both have the same mass.

Because, remember, when I'm looking at stoichiometric amounts,

[SOUND] okay, [SOUND] everything has to be in units of moles.

I can only compare moles of one substance to moles of another.

I can't compare grams to grams to directly.

So without knowing the moles of each of these, I can't determine which one is

the limiting reagent and which one is the excess reagent or

if I have stoichiometric amounts of those two reactants.

So for this calculation, I'm actually going to have to do similar to

what I did with my sandwich problem, where I have to do

two calculations to determine which one will produce the least amount of product.

7:44

The first thing we're going to do is we're going to calculate the amount of

product formed for each reactant given.

So, looking back at our sandwich example, we assumed bread was limiting and

used that for a calculation, then we assumed turkey was limiting and

used that for a second calculation.

7:59

Then we looked at the amount of products produced, and we see the reactant which

produces the least amount of product, is the limiting reagent.

So it always goes back to the reactant, but

it's about the one that produces the least amount of product.

It is very possible to have the less mass of a reactant be our excess reagent,

because we have to look at it in terms of moles and

we have to look at the mole to mole ratio.

8:26

Then we're going to use the information provided, to us in the problem as well as

the identity of the limiting reagent to finish our problem, to determine what

the actual answer needs to be or what units we need our answer to be in.

8:41

So when we look at stoichiometric reactions we have to worry about

how much we have of each substance.

In this demonstration we going to look at the reaction between magnesium which is

a metal, it's a solid and hydrochloric acid.

And what we have here are three flasks.

And in the balloons, we have samples of magnesium.

We have .1 grams of magnesium, one gram of magnesium and two grams of magnesium.

And what we going to see happening is the magnesium reacting with

the hydrochloric acid to produce the hydrogen gas.

And we can see which one is the limiting reagent and

which one is the excess reagent depending on how the reaction proceeds.

So I'm going to lift the balloon here.

This is the .1 magnesium.

And I'm going to lower the magnesium into the container.

9:32

So now we can drop the one gram of magnesium into our hydrochloric acid.

And we see a much more vigorous reaction.

We see more hydrogen being produced because our balloon is getting larger.

And now we can go to the two grams of magnesium.

10:03

So now that the reaction has finished on all three of them,

I notice a couple of thing.

One.

The balloon in the first one is very small indicating that one of our reactants is

completely consumed because we have the same amount of hydrochloric acid in each.

So our magnesium ran out here in the first one and so

we weren't able to produce much of our H2 gas product.

So when I look at the second and

third flasks, I notice the balloons are about the same size.

We produced about the same amount of hydrogen gas.

However, if I look at the third flask,

what I see is that there's still magnesium in the hydrochloric acid.

So they started with the same amount of hydrochloric acid, but

I have left over magnesium, indicating that, that is the excess free agent.

And so, in the middle when we actually have stoichiometric amounts of

our reactant.

So for every one mole of magnesium we have two moles of hydrochloric acid.

So when I look at the third flask, what I see is that I have excess magnesium.

So all of the HCl has been consumed, so it's my limiting reagent, and

is limiting the amount of hydrogen gas that can be formed.

So the magnesium is the excess reagent, and

the hydrochloric acid is the limiting reagent.