Chapter 9 Stoichiometry

Pancake Recipe 1 cup flour 2 eggs ½ cup water ¼ tsp baking powder Makes 5 pancakes We can write this recipe as an equation.

1 cup flour + 2 eggs + ½ cup water + ¼ tsp. baking powder → 5 pancakes If I have a 3 cups of flour how many pancakes can I make?

1 cup flour + 2 eggs + ½ cup water + ¼ tsp. baking powder → 5 pancakes How many cups of water will I need if I have 1 dozen eggs?

1 cup flour + 2 eggs + ½ cup water + ¼ tsp. baking powder → 5 pancakes How many tsp. of baking powder will I need to make 13 pancakes?

Chemical Equations are Recipes for Chemicals

IT TAKES MOLES TO MOVE 5C + 2SO2 → CS2 + 4CO How do we interpret this equation? 5 mol C + 2 mol SO2 → 1 mol CS2 + 4 mol CO IT TAKES MOLES TO MOVE

IT TAKES MOLES TO MOVE 5C + 2SO2 → CS2 + 4CO 3.0 mol CS2 require how many mol C? IT TAKES MOLES TO MOVE

IT TAKES MOLES TO MOVE 5C + 2SO2 → CS2 + 4CO 7.42 mol C will produce how many mol CO? IT TAKES MOLES TO MOVE

IT TAKES MOLES TO MOVE C6H12O6 + 6O2 → 6CO2 + 6H2O 3.60 mol C6H12O6 produce how many mol CO2? IT TAKES MOLES TO MOVE

IT TAKES MOLES TO MOVE 2KClO3 → 2KCl + 3O2 5.42 mol KClO3 will produce how many mol O2? IT TAKES MOLES TO MOVE

Mass – Mass Calculations Chapter 9 Mass – Mass Calculations

2Al + 6HCl → 2AlCl3 + 3H2 How many mol Al will react with 6.37 mol HCl? 2.12 mol Al

C6H12O6 + 6O2 → 6CO2 + 6H2O How many grams of O2 are needed to react with 12.5 g of C6H12O6? How is this problem different than the previous problem? This is an example of a mass – mass calculation.

General Plan for Mass – Mass Calculations Mass of Different Substance Mass of substance Stoichiometric factor Moles of substance Moles of Different Substance

C6H12O6 + 6O2 → 6CO2 + 6H2O How many grams of O2 are needed to react with 12.5 g of C6H12O6? 13.3g O2

2KClO3 → 2KCl + 3O2 If 80.5 g O2 are produced how many grams of KCl will be produced? 125g KCl

Zn + 2HCl → ZnCl2 + H2 Does this reaction look familiar?

Zn + 2HCl → ZnCl2 + H2 9.23g of Zn will produce how many grams of H2? 0.282g H2

Percent Yield

Zn + 2HCl → ZnCl2 + H2 9.23g of Zn should produce 0.282 grams of H2. But will it?

Theoretical Yield vs. Actual Yield Theoretical yield is the amount of product we should produce from the complete conversion of a given amount of reactant to product must be calculated or given. Actual yield is the amount of product that is actually produced during a reaction. it is always less than theoretical yield because of incomplete reactions, impure reactants, side reactions, etc must be given within the problem or measured during an experiment. Theoretical yield is calculated from the amount of limiting reactant.

Percent Yield

9.23g of Zn should produce 0.282 grams of H2. But will it? Zn + 2HCl → ZnCl2 + H2 9.23g of Zn should produce 0.282 grams of H2. But will it? The metal contained some impurities. Some hydrogen may have escaped. What would the percent yield be if 0.196 g were produced? 69.5%

A reaction was supposed to make 2. 50 grams of Zn. You actually made 2 A reaction was supposed to make 2.50 grams of Zn. You actually made 2.32 grams. What is the percent yield? 92.8%

2Al + 3CuCl2 → 3Cu + 2AlCl3 A chemistry student is trying to prepare copper metal by the reaction of aluminum with copper (II) chloride. →

2Al + 3CuCl2 → 3Cu + 2AlCl3 If the student is able to prepare 3.67g of copper metal by the reaction of 1.27g of aluminum with excess copper (II) chloride. How do I know that the aluminum gets used up and the copper (II) chloride does not? This is the concept of the “limiting reactant”

2Al + 3CuCl2 → 3Cu + 2AlCl3 If the student is able to prepare 3.67g of copper metal by the reaction of 1.27g of aluminum with excess copper (II) chloride. What is the percent yield? 81.9%

Homework “Mass-Mass Problems” Worksheet (due Thursday).

NaOH + Cr(OH)3  NaCr(OH)4 Chromium (III) hydroxide will dissolve in concentrated sodium hydroxide according to the following equation: NaOH + Cr(OH)3  NaCr(OH)4 If you begin with 66.0g of Cr(OH)3 and obtain 38.4g of NaCr(OH)4, what is the percentage yield? 41.9%

Two other types of percent yield problems In both of these types of problems you will be given the percent yield. Given the percent yield determine the amount of a reactant you need. You use more. Given the percent yield determine the amount of a product you can produce. You make less.

You use more. A bakery produces cookies at a percent yield of 92%. A customer places an order for 2150 cookies. The bakers will need enough ingredients for how many cookies. 2150 = 2337 cookies 0.92 Divide by percent yield. “You use more”

You make less A bakery produces cookies at a percent yield of 92%. The bakers add enough ingredients to make 1525 cookies. How many cookies do they produce? 1525 (0.92) = 1403 cookies Multiply by the percent yield. “You make Less”

A chemical engineer needs to produce 212 g of zinc oxide by the process below: 2ZnS + 3O2  2ZnO + 2SO2 By doing trials runs he expects the process to run at a 78% yield. How much zinc sulfide should he use to prepare the 212 g of zinc oxide? 326g ZnS

Ammonia is synthesized from hydrogen and nitrogen according to the following equation: N2 + 3H2  2NH3 If an excess of nitrogen is reacted with 3.41g of hydrogen, how many grams of ammonia can be produced assuming the reaction has a percentage yield of 48.8%? 9.42g NH3

Homework “Mass-Mass Problems” Worksheet (due tomorrow). Lab Summary: Percent Yield Lab (due Thursday). Percentage Yield Worksheet two sided Due Friday