1. “Limiting Reactants and Percentage Yield”
Stoichiometry
“Limiting Reactants and Percentage Yield”

2. This is the LIMITING REACTANT
Consider a Bicycle
Each bicycle needs 2 tires, one seat, and 2 handlebars.
How many complete bikes can be made from 6 tires, 7 seats, and 5 handlebars?
Which one runs out first???
This is the LIMITING REACTANT

3. Limiting Reactants
Just like the bike example, a reaction will run out of a reactant before all parts are consumed.
A limiting reactant is the substance that controls the quantity of product that can form in a chemical reaction.
The handlebars were limiting in the bike problem

4. Excess Reactants
The “other” reactant must be in excess if it is not limiting.
The excess reactant is the substance that is not used up completely in a reaction.
Some remains still.
The tires and seats were in excess in the bike example.

5. Calculating Theoretical Yield
If you are given quantities of all the reactants involved, it is a Limiting Reactant problem.
You must first determine which reactant is limiting before you can calculate the theoretical amount, or theoretical yield, of the products.

6. Calculating Theoretical Yield
Rules for making L.R. calculations:
Write out and balance the equation.
Determine if the problem is an L.R. (quantities of all reactants are given)
Convert all quantities to moles
Divide each value of moles by its own corresponding stoichiometric coefficient.
Whichever number is smallest is the L.R.
Calculate the products using that reactant.

7. Practicing With L.R.’s
Identify the limiting reactant and the theoretical yield of phosphorous acid, H3PO3, if 225 g of PCl3 is mixed with 125 g of H2O. ___PCl3 + ___H2O  ___H3PO3 + ___HCl

8. Practicing With L.R.’s
___CuO(s) + ___H2(g) ___Cu(s) + ___H2O(g) What is the limiting reactant when 19.9 g CuO react with 2.02 g H2?

9. Actual Yield and Percentage Yield
Equations tell you what should happen in a reaction, not always what does happen
The “theoretical yield” is what SHOULD be produced. What is calculated stoichiometrically.
The “actual yield” is what is ACTUALLY produced in lab by a reaction.

10. Percentage Yield
Percentage Yield is the measured amount of a product of a reaction.
In lab, we may only get 85% of the desired amount. Not everything is perfect in this world 
The percent yield is a ratio of the ACTUAL yield and the THEORETICAL yield.

11. How to Calculate
% Yield = Actual Yield x100 = % Theoretical Yield What is the percent yield if stoichiometry tells us we should get 14.7 g of product but actually only get grams?

12. Practice
Determine the limiting reactant, the theoretical yield, and the percentage yield if 14.0 g N2 are mixed with 9.0 g H2, and 16.1 g NH3 are recovered. ___N2 + ___H2  ___NH3

13. Determining Actual Yield
Even though the actual yield can only be determined experimentally, we can estimate the amount if we know the percentage yield.
Say a reaction usually yields about 80.5% of the theoretical yield. We have enough to make 200 grams (theoretically). We can assume that we will get around 80.5% of 200 g or roughly 161 grams.

14. Calculating Actual Yield
How many grams of C7H8O2 should form if 4808 g are theoretically possible and the percent yield for the reaction is 83.2%?

15. Calculating Actual Yield
The percent yield of NH3 from the following reaction is 85.0%. What actual yield is expected from the reaction of 1.00 kg N2 with 225 g H2?

16. Practice, In-Class Assignment
Due at end of block:
Page 319, Section Review
#6, 7, 8, and 11.
Separate sheet of
notebook paper,
turned in, 16 points