1. Percentage Yield
Chemistry 11
Ms. McGrath

2. Percentage Yield
In this last unit, we will earn how chemists calculate a percentage that will determine how successful a reaction will be, ie how much product will be produced.

3. Percentage Yield
The amount of product that is predicted by stoichiometry is called the theoretical yield. This predicted product is not always the same as the amount of product that is actually obtained during the reaction. The amount of product obtained in an experiment is called the actual yield.

4. Percentage Yield
A reduced yield may be caused by a competing reaction – a reaction that occurs at the same time as the principal reaction. For example, phosphorus reacts with chlorine to produce phosphorus trichloride. Phosphorus trichloride then may react with chlorine to form phosphorus pentachloride.

5. Percentage Yield
There are certain reasons why the percentage yield may be off. What do you think these may be?

6. Percentage Yield Some causes are: competing reactions;
poor experimental technique;
impure reactants;
faulty measuring devices.

7. Percentage Yield
The percentage yield of a chemical reaction compares the mass of a product obtained by the experiment (the actual yield) with the mass of the product determined by stoichiometry calculations (the theoretical yield).

8. Calculating Percentage Yield
Ammonia can be prepared by reacting nitrogen gas, taken from the atmosphere, with hydrogen gas. When 7.5 g of nitrogen reacts with sufficient hydrogen, the theoretical yield of ammonia is 9.10 g. If 1.72 g of ammonia is obtained by experiment, what is the percent yield of the reaction?

9. Calculating Percentage Yield
Actual yield = 1.72 g Theoretical yield = 9.10 g Percentage yield: = 1.72 g x 100 % 9.10 g = 18.9 %

10. Calculating Percentage Yield
Try page 139, #31, 32, 33

11. Predicting actual yield based on percentage yield
Calcium carbonate can be thermally decomposed to calcium oxide and carbon dioxide. CaCO3(s) -> CaO(s) + CO2(g) Under certain conditions, this reaction proceeds with a 92.4 % yield of calcium oxide. How many grams of calcium oxide can the chemist expect to obtain if 12.4 g of calcium carbonate is heated?

12. Predicting actual yield based on percentage yield
Let’s plan our process:
Balanced chemical equation
g of CaCO3 to mol CaCO3
mol ratio of CaCO3 to CaO
mol of CaO to g CaO
Actual yield = g CaO x 92.4
100

13. Predicting actual yield based on percentage yield
Try page 141, #34, 35, 37