It’s time to learn about . . .It’s time to learn about . . .

Stoichiometry: Limiting Stoichiometry: Limiting ReagentsReagentsAt the conclusion of our time At the conclusion of our time together, you should be able to:together, you should be able to:

1. Determine the limiting reagent2. Use the limiting reagent to determine the

amount of product produced in a reaction3. Determine the excess amount(s) in the

reaction

How many burgers can we How many burgers can we make?make?

Limiting ReactantLimiting Reactant

Most of the time in chemistry we have more of one reactant than we need to completely use up other reactant.

That reactant is said to be in excessexcess (there is too much).

The other reactant limits how much product we get. Once it runs out, the reaction ’s. This is called the limiting reactantlimiting reactant.

Limiting ReactantLimiting ReactantTo find the correct amount of product

produced, we have to try allall of the reactants. We have to calculate how much of aa product we can get from each each of the reactants to determine which reactant is the limiting one.

The The lowerlower amount of amount of aa product is the product is the correct answer. correct answer.

The reactant that makes the least amount of product is the limiting reactantlimiting reactant. Once you determine the limiting reactant, you should ALWAYS start with it!

Be sure to pick Be sure to pick aa product! You can’t product! You can’t compare to see which is greater and which compare to see which is greater and which is lower unless the product is the same!is lower unless the product is the same!

Limiting Reactant: Page 18Limiting Reactant: Page 18Combustion of propane:

Finding the Amount of Excess – Finding the Amount of Excess – Page 18Page 18

By calculating the amount of the excess reactant needed to completely react with the limiting reactant, we can subtract that amount from the given amount to find the amount of excess.

Limiting Reactant: RecapLimiting Reactant: Recap1. You can recognize a limiting reactant

problem because there is MORE THAN ONE GIVEN AMOUNT.

2. Convert ALL of the reactants to the SAME product (pick any product you choose.)

3. The lowest answer is the correct answer.4. The reactant that gave you the lowest

answer is the LIMITING REACTANT.5. The other reactant(s) are in EXCESS.6. To find the amount of excess, subtract

the amount used from the given amount.7. If you have to find more than one

product, be sure to start with the limiting reactant. You don’t have to determine which is the LR over and over again!

Stoichiometry: Limiting Stoichiometry: Limiting ReagentsReagentsLet’s see if you can:Let’s see if you can:

1. Determine the limiting reagent2. Use the limiting reagent to determine the

amount of product produced in a reaction3. Determine the excess amount(s) in the

reaction

In the lab where you combined iron (II) with sulfur, if 5.00 g of iron is combined with 5.00 g of sulfur, what is the limiting reagent and how much excess reagent is left?

5.00 g Fe x 1 mol Fe

55.85 g Fe

= 0.0895 mol Fe

Fe + S FeS

5.00 g S x 1 mol S

32.07 g S

= 0.156 mol S

0.0895 mol Fe + 0.156 mol S FeS

The ratio must be 1:1, therefore Fe is limiting and will combine with 0.0895 mol S to produce 0.0895 mol of FeS

0.0895 mol S x 32.07 g S

1 mol S

= 2.87 g S used

5.00 g S – 2.87 g S used =

2.13 g excess S