1 Chapter 11 Stoichiometry Objectives Perform Mole-Mole Calculations Perform Calculations involving mass, gas volume or particles Identify the Limiting Reagent Calculate the Amount of Product Based on the Limiting Reagent Calculate Theoretical Yield and Percent Yield

Chemical Equations are recipes that tell chemists what amounts of reactants to mix and what amounts of products to expect. Stoichiometry is based on the law of conservation of mass. The mass of reactants equals the mass of the products. Chemical reactions stop when one of the reactants is used up. Stoichiometry is the study of quantitative relationships between the amounts of reactants used and amounts of products formed in a chemical reaction.

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Mole-Mole Calculations N 2 (g) + 3H 2 (g)  2NH 3 (g) The coefficients from the balanced equation are used to write conversion factors called mole ratios. 1 mol N 2 3 mol H 2 2 mol NH 3 3 mol H 2 2 mol NH 3 1 mol N 2 (and three more: switch top and bottom) [nx(n-1) 3x2=6] A mole ratio is a ratio between the numbers of moles of any two substances in a balanced equation.

5 Which of the following is a correct mole ratio for the following equation? 2Al(s) + 3Br 2 (l) → 2AlBr 3 (s) A.2 mol Al : 3 mol Br B.3 mol Br 2 : 2 mol Al C.2 mol AlBr 3 : 1 mol Br 2 D.2 mol Br : 2 mol Al

6 How many mole ratios can be written for the following reaction? 4H 2 (g) + O 2 (g) → 2H 2 O(l) A.6 B.4 C.3 D.2

7 Mole-Mole Calculations N 2 (g) + 3H 2 (g)  2NH 3 (g) 1 mol N 2 3 mol H 2 2 mol NH 3 3 mol H 2 2 mol NH 3 1 mol N 2 How many moles NH 3 can be produced from 0.60 mol of N 2 ? 0.60 mol N 2 x 2 mol NH 3 = 1.2 mol NH 3 1 mol N 2

8 Ex. The carbon dioxide exhaled by astronauts can be removed from a spacecraft by reacting with lithium hydroxide as follows: CO 2(g) + LiOH (s) → Li 2 CO 3(s) An average person exhales about 20 moles of CO 2 per day. How many moles of LiOH would be required to maintain 2 astronauts in a Space- craft for three days? + H 2 O(l)

9 1. Balance equation: CO 2 + 2LiOH → Li 2 CO 3 + H 2 O 2. Determine moles of known substance: 20moles per person, 2 people = 40moles x 3 days = 120moles of CO 2 3. Convert moles of known substance to moles of unknown: 120 mol CO 2 x 2 mol LiOH = 240moles of LiOH 1 mol CO 2

10 A chemical equation must be ____ in order to perform stoichiometric calculations. A.measured B.controlled C.balanced D.produced

11 How many moles of CO 2 will be produced in the following reaction if the initial amount of reactants was 0.50 moles? 2NaHCO 3 → Na 2 CO 3 + CO 2 + H 2 O A.0.25 B.0.3 C.0.5 D.1.0

12 Volume of gas (STP) 1.00 mol 22.4L 1.00 mol molar mass Particles 1.00 mol 6.02 x10 23 p. Mass Mole SubstanceA Substance A Volume of gas (STP) 1.00 mol 22.4L 1.00 mol molar mass Particles 1.00 mol 6.02 x10 23 p. Mass Mole Substance B Mole ratio X moleA X mole A Y mole B

13 Mass-Mass Calculations N 2 (g) + 3H 2 (g)  2NH 3 (g) How many grams NH 3 can be produced from 5.40g of H 2 ?convert g H 2  mol H 2  mol NH 3  g NH 3 g H 2  mol H 2  mol NH 3  g NH 3 2 mol NH g H 2 x 1 mol H 2 x 2 mol NH 3 x 17.0 g NH 3 = 30.6 g NH 3 3 mol H g H 2 3 mol H 2 1 mol NH 3 known  1 mol H 2  mole ratio  molar mass  unknown molar 1 mol NH 3 molar 1 mol NH 3 mass H 2 mass H 2

14 Limiting Reagent Any reactant that is used up first in a chemical reaction; it determines the amount of product that can be formed in the reaction. The reactant(s) not completely used up is/are the Excess Reagent(s) 11.3

15 You have 12 ‘empty’ frames for sunglasses and 20 lenses. How many complete sunglasses can you put together? Which is your limiting “item” (runs out first and determines how many sunglasses can be made)?

mol Na react with 3.20 mol Cl 2 1) What is the limiting reagent and 2) how many moles of NaCl are produced? Na (s) + Cl 2 (g)  NaCl (balance equation first) + 

17 Calculate each amount separately. The result with less product is the most you can produce from the limiting reagent mol Na react with 3.20 mol Cl 2 2 Na (s) + Cl 2 (g)  2 NaCl 1)6.70 mol Na x 2 mol NaCl = 6.70 mol NaCl 2 mol Na 2) 3.20 mole Cl 2 x 2 mol NaCl = 6.40 mol NaCl 1 mol Cl 2 Limiting reagent = Cl 2 (runs out first, less product) Max. amount the can be made is 6.40 mol NaCl

18 Another approach compares Na + Cl 2 first: 6.70 mol Na react with 3.20 mol Cl 2 2 Na (s) + Cl 2 (g)  2 NaCl 1) The known amount of any one reactant is multiplied by the mole ratio (often can be done in your head) 6.70 mol Na x 1 mol Cl 2 = 3.35 mol Cl 2 2 mol Na 3.35 mol Cl 2 is needed to react 6.70 mol Na. Since only 3.20 mol Cl 2 are available, Cl 2 is the limiting reagent. 2) Limiting reagent = 3.20 mol Cl mole Cl 2 x 2 mol NaCl = 6.40 mol NaCl 1 mol Cl 2 (mole-mole calculation)

19 Calculating the Product when a Reactant is Limiting gram-gram Ex. S 8 (l) + 4Cl 2 (g) → 4S 2 Cl 2 (l) If 200.0g of sulfur reacts with 100.0g of chlorine, what mass of disulfur dichloride is produced? Calculate each! Lesser product is correct according to limiting reactant …

20 2. Determine whether the two reactants are in the correct mole ratio, as given in the balanced chemical equation. Only mol of chlorine is available for every 1 mol sulfur, instead of the 4mol of chlorine required by the balanced chemical equation, thus chlorine is the limiting reactant. 3. Calculate the amount of product formed. 1.Determine moles of reactants Or as in your book:

21 What about the excess reactant, sulfur? How much of it reacted? 1.You need to make a mole-to-mass calculation to determine the mass of sulfur needed to react completely with mol of chlorine. 2. Next, obtain the mass of sulfur needed:

22 3. Knowing that 200.0g of sulfur is available and only 90.42g is needed, you can calculate the amount of sulfur left unreacted when the reaction ends. The mass of the final product in a chemical reaction is based on what? A.the amount of excess reactant B.the amount of limiting reactant C.the presence of a catalyst D.the amount of O 2 present

23 What is the excess reactant in the following reaction if you start with 50.0g of each reactant? P 4 (s) + 5O 2 (g) → P 4 O 10 (s) A.O 2 B.P 4 C.Both are equal. D.unable to determine

24 Theoretical Yield Theoretical Yield: Maximum amount of product that will form from given amount of reactants Actual Yield Actual Yield: Amount of product actually formed when the reaction is carried out. Percent Yield Percent Yield: The ratio of the actual yield to the theoretical yield expressed as a percent Percent Yield = Actual Yield x 100 % Theoretical Yield Theoretical Yield Percent Yield 11.4

25 CaCO 3 (s)  CaO (s) + CO 2 (g) a) What is the theoretical yield of CaO if 24.8g CaCO 3 is heated? (mass-mass-problem) knownunknownmole ratiomolar masses … b) What is the percent yield if 13.1g CaO is produced? (percent calculation) Percent Yield = Actual Yield x 100 % Theoretical Yield Theoretical Yield