Chapter 12.3 Pages

Cake example: Recipe: 2 cups flour1½ TBSP baking powder 2 eggs1 cup water 1 cup sugar1/3 cup oil Suppose in your kitchen you have 14 cups flour, 4 eggs, 9 cups sugar, 15 TBSP baking powder, 10 cups of water, and 3 1/3 cups oil. How many cakes can be baked?

Flour... need 2 cups, have 14 cups = Eggs…need 2 eggs, have 4 eggs = Sugar… need 1 cup, have 9 cups = Baking powder…need 1½ TBSP, have 15 TBSP = Water…need 1 cup, have 10 cups = Oil…need 1/3 cup, have 3 1/3 = 7 cakes 2 cakes 9 cakes 10 cakes

Back to the question, how many cakes can be baked? Answer is 2 Only 4 eggs, so we have extra of all the other ingredients Eggs, in this case, is the limiting reactant

Now, assume you have 26 eggs and the quantities of the other ingredients stay the same. Which ingredients is now the limiting reactant? Flour... need 2 cups, have 14 cups = How many cakes can be made with this new limiting reactant?

Limiting Reactant The reactant you run out of (or use up) first Makes the least amount of product Excess Reactant The reactant you have left over (waste). The limiting reactant determines how much product you can make

Limiting Reactant You can recognize limiting reagent problems because they will give you 2 amounts of chemical Do two stoichiometry problems one for each reagent you are given

If 10.6 g of copper reacts with 3.83 g sulfur, how many grams of the product (copper (I) sulfide) will be formed? 2Cu + S  Cu 2 S 10.6 g Cu 63.6g Cu 1 mol Cu 2 mol Cu 1 mol Cu 2 S g Cu 2 S = 13.3 g Cu 2 S = 19.0 g Cu 2 S 3.83 g S 32.06g S 1 mol S 1 mol Cu 2 S g Cu 2 S

If 10.3 g of aluminum are reacted with 51.7 g of CuSO 4 how much copper (grams) will be produced? 2Al + 3CuSO 4 → 3Cu + Al 2 (SO 4 ) 3 The CuSO 4 is limited, so Cu = 20.6 g How much excess reagent will remain? 4.47 grams

Percent yield is the amount of product made in a chemical reaction. Three Types: Actual Yield – What you actually get in the lab Theoretical Yield – What the balanced equation tells should be made Percent Yield – Actual x 100 = Theoretical

Percent yield tells us how “efficient” a reaction is. Percent yield CANNOT be bigger than 100%. Theoretical yield will always be larger than actual yield. Why? Impure reactants, competing side reactions, loss of product in filtering or transferring between containers

Practice 6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate. What is the actual yield? 6.78 g Cu What is the theoretical yield? 13.8 g Cu What is the percent yield? 49.1%