0:33

Likewise if we have two heads, two middles and

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

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

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

Â 1:03

We're limited to making three snowmen,

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

Â Once we run out of one part or

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

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

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

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

Â are limited by the number of heads that we have.

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

Â Our reaction can not proceed any further.

Â 1:37

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

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

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

Â product that we can form.

Â The excess reagent is the species that is

Â left after all of the limiting reagent is consumed.

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

Â 2:09

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

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

Â the bottom were our excess reagents, or excess reactants,

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

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

Â 2:35

Now let's look at another example.

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

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

Â where we needed one of each part.

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

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

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

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

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

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

Â based on what we know from our equation here,

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

Â 3:19

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

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

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

Â Now I know that I have 6 slices of bread.

Â That was the amount that was given in the problem.

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

Â the number of sandwiches I can make, and

Â I get that from the coefficients in my balanced equation.

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

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

Â where we looked at the relationship,

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

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

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

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

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

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

Â So now, I assume bread is limiting.

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

Â therefore that the bread is the excess reagent.

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

Â I have five [SOUND] pieces of turkey.

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

Â To determine the ratio between turkey and sandwiches, and

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

Â Again, I see that turkey cancels with turkey.

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

Â 5:19

So what I'm looking for

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

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

Â That lets me know that this is the maximum amount

Â of product I can produce [SOUND] and,

Â that the bread is, in fact the limiting reagent.

Â [SOUND] Because I

Â run out of bread,.

Â Before I run out of turkey,

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

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

Â see that if bread is limiting,

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

Â And so the lesser amount will always determine which reagent is

Â the limiting reactant.

Â 6:21

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

Â we're not told that it is?

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

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

Â iron oxide.

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

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

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

Â And I can't simply compare the amounts and

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

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

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

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

Â I can't compare grams to grams to directly.

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

Â the limiting reagent and which one is the excess reagent or

Â if I have stoichiometric amounts of those two reactants.

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

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

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

Â 7:44

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

Â product formed for each reactant given.

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

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

Â used that for a second calculation.

Â 7:59

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

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

Â So it always goes back to the reactant, but

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

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

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

Â we have to look at the mole to mole ratio.

Â 8:26

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

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

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

Â 8:41

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

Â how much we have of each substance.

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

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

Â And what we have here are three flasks.

Â And in the balloons, we have samples of magnesium.

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

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

Â the hydrochloric acid to produce the hydrogen gas.

Â And we can see which one is the limiting reagent and

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

Â So I'm going to lift the balloon here.

Â This is the .1 magnesium.

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

Â 9:32

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

Â And we see a much more vigorous reaction.

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

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

Â 10:03

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

Â I notice a couple of thing.

Â One.

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

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

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

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

Â So when I look at the second and

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

Â We produced about the same amount of hydrogen gas.

Â However, if I look at the third flask,

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

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

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

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

Â our reactant.

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

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

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

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

Â So the magnesium is the excess reagent, and

Â the hydrochloric acid is the limiting reagent.

Â