1.3 Reacting Masses and Volumes Stoichiometry, Limiting & Excess Reactants, Theoretical & Percent Yields Mrs. Page - IB SL Chemistry

Understandings Reactants can be either limiting or excess. The experimental yield can be different from the theoretical yield. Applications & Skills Solution of problems relating to reacting quantities, limiting and excess reactants, theoretical, experimental and percentage yields.

Stoichiometry Oxygen gas can be produced by decomposing potassium chlorate using the reaction below. If g of KClO 3 is heated and decomposes completely, what mass of oxygen gas is produced? Write equation KClO 3 (s)  KCl (s) + O 2 (g) First Balance the Equation 2 KClO 3 (s)  2 KCl (s) + 3 O 2 (g) What is the mole ratios? 2 : 2 : 3

Stoichiometry

YOU TRY

THEORETICAL YIELD

Limiting Reactant The limiting reactant in a chemical reaction is the reactant that determines the theoretical yield of a product. This is the reactant that will run out first in an equation and therefore determine how much product can be produced. A substance that is not used up completely in a reaction. It’s what is left over after you run out of the limiting reactant is said to be in excess.

Limiting & Excess Reactants How many s’ mores can you make? 10 YIELD You can make 10 s’mores based on the chocolate chips You can make 15 s’mores based on the marshmallows. You can make 40 s’mores based on the cookies. S’mores Recipe 20 choc. chips 30 marshmallows 40 cookies What do you run out first? Chocolate chips LIMITING REACTANT If you make 10 s’mores, you use 20 marshmallows and have 10 left. You use 10 cookies and have 30 left. EXCESS REACTANT How much of the other substance are left over?

Limiting Reactant A reaction vessel is filled with 4.04 g of hydrogen gas and g of oxygen gas and the mixture is exploded. Identify the limiting reactant and deduce the mass of the water produced. Looking at the equation we can see that we need 2 moles of H 2 to react with 1 mole of O 2. Based on the calculations we have 2.00 moles of H 2 but only moles of O 2 Therefore O 2 is limiting and H 2 is in excess

Limiting Reactant A reaction vessel is filled with 4.04 g of hydrogen gas and g of oxygen gas and the mixture is exploded. Identify the limiting reactant and deduce the mass of the water produced. 2 H 2 (g) + O 2(g)  2 H 2 O (l)

YOU TRY When a mixture of silver metal and ssulfur is heated, silver sulfide is formed: 16 Ag (s) + S 8(s)  8 Ag 2 S (s). 2.0 grams of silver and 4.0 grams of sulfur are combined. What is the theoretical yield of silver sulfide? How much excess is left over? Looking at the equation we can see that we need 16 moles of Ag to react with 1 mole of S 8. Using ratios we can see that with mol of S 8 we would need mol of Ag Therefore Ag is limiting and S 8 is in excess

YOU TRY When a mixture of silver metal and sulfur is heated, silver sulfide is formed: 16 Ag (s) + S 8(s)  8 Ag 2 S (s). 2.0 grams of silver and 4.0 grams of sulfur are combined. What is the theoretical yield of silver sulfide in grams? How much excess is left over?

YOU TRY When a mixture of silver metal and sulfur is heated, silver sulfide is formed: 16 Ag (s) + S 8(s)  8 Ag 2 S (s). 2.0 grams of silver and 4.0 grams of sulfur are combined. What is the theoretical yield of silver sulfide in grams? How much excess is left over?

PERCENTAGE YIELD

Aluminum burns in bromine, producing aluminum bromide: 2 Al (s) + 3 Br 2(l)  2 AlBr 3(s) In an experiment, 6.0 g of aluminum was reacted with an excess of bromine to yield 50.3 g of aluminum bromide. Calculate the theoretical and percentage yields for this experiment. THEORETICAL YIELD: PERCENTAGE YIELD:

YOUR TURN Aspirin, C 9 H 8 O 4, is made by reacting ethanoic anhydride C 4 H 6 O 3, with 2-hydroxybenzoic acid, C 7 H 6 O 3, according to the equation: 2C 7 H 6 O 3 + C 4 H 6 O 3  2 C 9 H 8 O 4 + H 2 O g of 2-hydroxybenzoic acid is reacted with g of ethanoic anhydride. a)Determine the limiting reagent. b)The mass obtained in the experiment was g. Calculate the theoretical and percentage yield of aspirin.