1. Chapter 9 Stoichiometry

2. Definition of “Stoichiometry”: the mathematics of chemical equations Important Concepts: 1. You MUST have a balanced equation! 2. The coefficients of the equation describe the amounts of each substance in moles. Ex: 2 H 2 + O 2  2 H 2 O 2 mol H 2 + 1 mol O 2  2 mol H 2 O 3. A balanced equation is a recipe for the reaction. It MUST be followed!

3. Mole Ratios A mole ratio is a comparison between the amounts of two substances in a chemical reaction. Ex: 2 H 2 + O 2  2 H 2 O The mole ratio of H 2 to O 2 is _________. The mole ratio of H 2 to H 2 O is ________. Ex: C 3 H 8 + 5 O 2  3 CO 2 + 4 H 2 O The mole ratio of C 3 H 8 to O 2 is ___________. The mole ratio of CO 2 to H 2 O is __________. The mole ratio of O 2 to H 2 O is ____________. **A mole ratio MUST be used to solve ANY type of reaction stoichiometry problem.

4. Types of Stoichiometry Problems 1) Mole – Mole 2) Mole – Mass 3) Mass – Mole 4) Mass – Mass

5. Mole– Mole Calculations mol A  mol B Ex: 2 Mg + O 2  2 MgO How many moles of O 2 are needed to react with 0.80 moles Mg to make MgO? Ex: How many moles of NH 3 will be produced from the reaction of 1.35 moles H 2 with N 2 ?

6. Mole – Mass Calculations mol A  mol B  g B Mg + 2 HCl  H 2 + MgCl 2 How many grams of H 2 are produced when 1.25 moles Mg react? How many grams of S are needed to react with 0.395 moles Al to make aluminum sulfide?

7. Mass – Mole Calculations g A  mol A  mol B C 3 H 8 + 5 O 2  3 CO 2 + 4 H 2 O How many moles of O 2 are needed to produce 25.6 grams CO 2 ? Cu + 4 HNO 3  Cu(NO 3 ) 2 + 2 H 2 O + 2 NO 2 How many moles of HNO 3 are needed to completely react with 4.56 grams Cu?

8. Mass – Mass Calculations g A  mol A  mol B  g B 2 Al(OH) 3  Al 2 O 3 + 3 H 2 O How many grams of H 2 O are produced when 250.0 grams of Al(OH) 3 are decomposed? How many grams of Al 2 O 3 are produced by the same reaction?

9. Percentage Yield % yield = actual yield x 100 theoretical yield Actual yield (“Experimental Yield”) – the amount of product collected from an experiment. Theoretical yield – the amount of product that is expected to be produced.

10. Ex: 1. A chemical reaction is performed in which 115 g salt can be produced. If 100. g is collected, what is the percent yield of the experiment? 2. If a reaction has a theoretical yield of 355 g, but a percentage yield of 68.5%, what will be the actual amount of product collected?

11. 3. Determine the percent yield of the reaction below if 55.0 g P is reacted and 110. g P 4 O 10 is produced. 4 P + 5 O 2  P 4 O 10

12. Limiting Reactants Definition: the reactant that runs out first in a chemical reaction; causes the reaction to stop; limits the amount of product made.

13. Example: To build a car…. 1 car body + 4 tires  1 car If you had 1 car body+ 6 tires…. How many cars could you make? _____ The __________________is the limiting reactant. You will use up the car body and you will still only be able to make 1 car (according to the recipe). The __________________ are the excess reactant. 4 of the tires would be used up, but you still have 2 tires left over.

14. To make a peanut butter and jelly sandwich… 2 slices of bread + 2 Tbsp PB + 1 Tbsp. jelly  1 PB&J sandwich If you had a whole jar of PB and a whole jar of jelly, but only 1 slice of bread…. How many PB&J sandwiches could you make? ____ What’s the limiting reactant? ________

15. The same idea can be applied to a chemical reaction: 2 H 2 + O 2  2 H 2 O If you had 3 moles H 2 + 1 mole O 2, The __ is in excess. The __ is the limiting reactant. If you had 2 moles H 2 + 3 moles O 2, The __ is in excess. The __ is the limiting reactant. If you had 4 moles H 2 + 2 moles O 2, Both reactants will be used up completely. Nothing is left over. You will make double the product.

16. Calculating Limiting and Excess Reactants The easiest way to determine the limiting reactant of a chemical reaction is to calculate the mass of the product produced. least The reactant that produces the least amount of product is the limiting reactant.

17. a) How many grams of Fe 2 S 3 can be produced from the reaction of 15.0 g Fe and 12.0 g S ? b) Which reactant is the Limiting Reactant? c) Which reactant is in excess? d) How many grams of the excess reactant will be left over?