1. Percent Yield and Limiting Reactants Advanced Chemistry Ms. Grobsky

2. Percent Yield In theory, when a teacher gives an exam to the class, every student should get a grade of 100% In theory, when a teacher gives an exam to the class, every student should get a grade of 100% Your exam grade, expressed as a percent, is a quantity that shows how well you did on the exam compared with how well you could have done if you had answered all questions correctly Your exam grade, expressed as a percent, is a quantity that shows how well you did on the exam compared with how well you could have done if you had answered all questions correctly

3. Percent Yield You might have assumed that if we use stoichiometry to calculate that our reaction will produce 5.2 g of product, that we will actually recover 5.2 g of product in the lab This assumption is as faulty as assuming that all students will score 100% on an exam

4. Percent Yield When an equation is used to calculate the amount of product that is possible during a reaction, a value representing the theoretical yield is obtained The theoretical yield is the maximum amount of product that could be formed from given amounts of reactants In contrast, the amount of product that forms when the reactionn is carried out in the lab is called the actual yield The actual yield is often less than the theoretical yield

5. A. Percent Yield calculated on paper measured in lab

6. The percent yield is the ratio of the actual yield to the theoretical yield as a percent The percent yield is the ratio of the actual yield to the theoretical yield as a percent It measures the measures the efficiency of the reaction It measures the measures the efficiency of the reaction What causes a percent yield to be less than 100%? What causes a percent yield to be less than 100%?

7. Why The Chemical World Isn’t Perfect Reactions don’t always go to completion When this occurs, less than the expected amount of product is formed Impure reactants and competing side reactions may cause unwanted products to form Actual yield can also be lower than the theoretical yield due to a loss of product during filtration or transferring between containers If a wet precipitate is recovered, it might mass heavy due to incomplete drying, etc.

8. Limiting Reactants

9. What Is a Limiting Reagent? Many cooks follow a recipe when making a new dish When a cook prepares to cook he/she needs to know that sufficient amounts of all the ingredients are available Think back to page 126 Were there stipulations to how many MacChem sandwiches you could make? Why?

10. Limiting Reactants We can’t make any more MacChem sandwiches than the number of HambChem sandwiches we have The number of HambChems limits the number of MacChems we can make If one of our ingredients gets used up during our preparation it is called the limiting reactant (LR) The LR limits the amount of product we can form; in this case, MacChems It is equally impossible for a chemist to make a certain amount of a desired compound if there isn’t enough of one of the reactants

11. Limiting Reactants As we’ve been learning, a balanced chemical reaction is a chemist’s recipe. This allows the chemist to predict the amount of product formed from the amounts of ingredients available

12. Let’s look at the reaction equation for the formation of ammonia: N 2 (g) + 3H 2 (g)  2NH 3 (g) When 1 mole of N 2 reacts with 3 moles of H 2, 2 moles of NH 3 are produced How much NH 3 could be made if 2 moles of N 2 were reacted with 3 moles of H 2 ?

13. Limiting Reactant The amount of H 2 limits the amount of NH 3 that can be made From the amount of N 2 available we can make 4 moles of NH 3 From the amount of H 2 available we can only make 2 moles of NH 3 H 2 is our limiting reactant here. It runs out before the N 2 is used up Therefore, at the end of the reaction there should be N 2 left over When there is reactant left over it is said to be in excess

14. Limiting Reactant How much N 2 will be left over after the reaction? In our reaction, it takes 1 mol of N 2 to react all of 3 mols of H 2, so there must be 1 mol of N 2 that remains unreacted We can use our new stoich calculation skills to determine 3 possible types of LR type calculations. 1. Determine which of the reactants will run out first (limiting reactant) 2. Determine amount of product 3. Determine how much excess reactant is wasted

15. Steps to Calculate Limiting Reactants

16. Limiting Reactant Steps 1 Step one o Write and balance the equation for the reaction. 2 Step two o Convert known masses to grams of product. 4 Step three o Determine limiting reactant and amount that can be made. 5 Step four o Determine the grams of excess from limiting reactant.

17. Limiting Reactants Using Moles When 4.00 mol H 2 is mixed with 2.00 mol Cl 2,how many moles of HCl can form? H 2 (g) + Cl(g)  2HCl (g) 4.00 mol 2.00 mol ??? mol Calculate the moles of product from each reactant, H 2 and Cl 2. The limiting reactant is the one that produces the smaller amount of product.

18. 18 Limiting Reactants Using Moles HCl from H 2 4.00 mol H 2 x 2 mol HCl = 8.00 mol HCl 1 mol H 2 (not possible) HCl from Cl 2 2.00 mol Cl 2 x 2 mol HCl = 4.00 mol HCl 1 mol Cl 2 (smaller number) The limiting reactant is Cl 2 because it is used up first. Thus Cl 2 produces the smaller number of moles of HCl.

19. Checking Calculations InitiallyH 2 4.00 mol Cl 2 2.00 mol 2HCl 0 mol Reacted/ Formed -2.00 mol +4.00 mol Left after reaction 2.00 mol Excess 0 mol Limiting 4.00 mol

20. Limiting Reactants 79.1 g of zinc react with 0.90 L of 2.5M HCl. Identify the limiting and excess reactants. How many liters of hydrogen are formed at STP? Zn + 2HCl  ZnCl 2 + H 2 79.1 g ? L 0.90 L 2.5M

21. Limiting Reactants 79.1 g Zn 1 mol Zn 65.39 g Zn = 27.1 L H 2 1 mol H 2 1 mol Zn 22.4 L H 2 1 mol H 2 Zn + 2HCl  ZnCl 2 + H 2 79.1 g ? L 0.90 L 2.5M

22. Limiting Reactants 22.4 L H 2 1 mol H 2 0.90 L 2.5 mol HCl 1 L = 25 L H 2 1 mol H 2 2 mol HCl Zn + 2HCl  ZnCl 2 + H 2 79.1 g ? L 0.90 L 2.5M

23. Limiting Reactants Zn: 27.1 L H 2 Limiting reactant: HCl Excess reactant: Zn Product Formed: 25 L H 2 Left over zinc HCl: 25 L H 2

24. Steps in Calculating Amount of Excess Reactant Left Over §Step One §Calculate mass of non-limiting reactant used up from ACTUAL moles of product produced §Stoichiometric calculation of: Moles A →Moles B →Mass B Step Two Subtract mass from step one from initial mass