Ch. 9 Notes -- Stoichiometry Stoichiometry refers to the calculations of chemical quantities from_________________ chemical equations. Greek for “________________________” When baking cookies, a recipe is usually used, telling the exact amount of each ingredient If you need more, you can _______________or ______________the amount Thus, a recipe is much like a balanced equation balanced Measuring elements double triple

Interpreting Everyday Equations 2 guards + 2 forwards + 1 center  1 basketball team ___ + ___ + ___  __________ Practice Problems: 1) How many guards does it take to make 7 teams? ______ 2) How many forwards are there in 8 teams? ______ 3) If you have 8 centers, 17 forwards and 14 guards, how many teams can be made? ______ What do you run out of first? _________ G2F2C 2G 2F C 14 16 7 guards

Interpreting Chemical Equations ___N2 (g) + ___H2 (g)  ___NH3 (g) The first thing that must be done is to ______________ the equation! Here are the kinds of information you can get from the equation: ____ mole N2 + ____ moles H2  ____ moles NH3 ____ molecule N2 + ____ molecules H2  ____ molecules NH3 ____ liter N2 + ____ liters H2  ____ liters NH3 ____ grams N2 + ____ grams H2  ____ grams NH3 1 3 2 balance 1 3 2 1 3 2 1 3 2 28.0 6.0 34.0

Mole-Mole Conversions The mole conversion factor comes from the _________________ of the balanced chemical equation. Step 1: Write down the “given”. This becomes “x” Step 2: Set up a conversion factor to change from moles of “x” to moles of “y”. Practice Problems: N2 (g) + 3H2 (g)  2NH3 (g) 1) How many moles of ammonia can be made from 7 moles of nitrogen reacting with an excess of hydrogen? 2) How many moles of hydrogen are required to completely react with 8 moles of nitrogen to produce ammonia? 3) How many moles of hydrogen are needed to react with an excess of nitrogen to make 10 moles of ammonia? coefficients 2 mol NH3 7 mol N2 14 moles of NH3 x = 1 mol N2 3 mol H2 8 mol N2 24 moles of H2 x = 1 mol N2 3 mol H2 15 moles of H2 10 mol NH3 x = 2 mol NH3

Mole to Mole conversions continued 4) How many moles of O2 are produced when 3.34 moles of Al2O3 decompose? 2 Al2O3 → 4Al + 3O2 3 mol O2 3.34 mol Al2O3 x = 5.01 moles of O2 2 mol Al2O3 5) If 3.84 moles of C2H2 are burned, how many moles of O2 are needed? 2C2H2 + 5 O2 → 4CO2 + 2 H2O 5 mol O2 3.84 mol C2H2 x = 9.6 moles of O2 2 mol C2H2

Other Conversion Problems Volume-Volume: (treat it exactly like a ____________ conversion) Step 1: Write down the “given”. Step 2: Convert from liters to liters using a mole conversion factor. N2 (g) + 3H2 (g)  2NH3 (g) Practice Problem: How many liters of ammonia can be made from reacting 125 liters of hydrogen with an excess of nitrogen? mole-mole 2 L NH3 125 L H2 83.3 L NH3 x = 3 L H2 …the long way to get the same answer… 1 mol H2 2 mol NH3 22.4 L NH3 125 L H2 83.3 L NH3 x x x = 22.4 L H2 3 mol H2 1 mol NH3

Molar Mass Review Review Practice Problems: c) C3H6O d) H3PO4 100 g (1) Calculate the molar mass of each compound. (Add up all the atomic masses for each atom from the Periodic Table.) a) CaCO3 b) (NH4)2SO4 c) C3H6O d) H3PO4 1 Ca = 1 x 40.0 = 40.0 1 C = 1 x 12.0 = 12.0 3 O’s =3 x 16.0 = 48.0 Add them up! 100 g 2 N’s = 2 x 14.0 = 28.0 8 H’s = 8 x 1.0 = 8.0 1 S = 1 x 32.0 = 32.0 4 O’s = 4 x 16.0 = 64.0 Add them up! 132.0 g 3 C’s = 3 x 12.0 = 36.0 6 H’s = 6 x 1.0 = 6.0 1 O = 1 x 16.0 = 16.0 3 H’s = 3 x 1.0 = 3.0 1 P = 1 x 31.0 = 31.0 4 O’s = 4 x 16.0 = 64.0 Add them up! 58.0 g Add them up! 98.0 g

Stoichiometry Notes Part 2 Mole-Mass: (moles to grams) Step 1: Write down the “given” (start moles of “x”). Step 2: Convert from moles of the “given” (x) to moles of the “unknown” (y) using a mole conversion factor. Step 3: Convert from moles of “y” to grams of “y” using molar mass. N2 (g) + 3H2 (g)  2NH3 (g) Practice Problem: How many grams of ammonia, NH3, can be made from reacting 20.0 moles of hydrogen with an excess of nitrogen? x y 20.0 moles H2 2 mol NH3 17.0 g NH3 226.67 g NH3 x x = 3 mol H2 1 mol NH3

Other Conversion Problems Mass-Mass: (grams to moles to moles to grams) Step 1: Write down the “given” (x) and convert from grams to moles. Step 2: Convert from moles of the “given” (x) to moles of the “unknown” (y) using a mole conversion factor. Step 3: Convert from moles of “y” to grams of “y”. N2 (g) + 3H2 (g)  2NH3 (g) Practice Problem: How many grams of ammonia can be made from reacting 39.0 grams of nitrogen with an excess of hydrogen? 1 mol N2 2 mol NH3 17.0 g NH3 39.0 g N2 47.4 g NH3 x x x = 28.0 g N2 1 mol N2 1 mol NH3

Other Conversion Problems Mass-Mass: (grams to moles to moles to grams) Step 1: Write down the “given” (x) and convert from grams to moles. Step 2: Convert from moles of the “given” (x) to moles of the “unknown” (y) using a mole conversion factor. Step 3: Convert from moles of “y” to grams of “y”. Cu + 2AgNO3  2Ag + Cu(NO3)2 Practice Problem: How many grams of silver are produced from the reaction of 17.0 g of copper with an excess of silver nitrate, AgNO3? 1 mol Cu 2 mol Ag 108 g Ag 17.0 g Cu x 57.38 g of Ag x x = 64 g Cu 1 mol Cu 1 mol Ag

Percent Yield Percent Yield is a ratio that tells us how ________________ a chemical reaction is. The higher the % yield, the more efficient the reaction is. Actual or “experimental” Yield Theoretical or “ideal” Yield The ___________ yield is the amount you experimentally get when you run the reaction in a lab. The _______________ yield is the amount you are ideally supposed to get if everything goes perfectly. You can calculate this amount using stoichiometry! efficient x 100 % Yield = actual theoretical

Percent Yield Practice Problem: 2H2 (g) + O2 (g)  2H2O (g) 1) A student reacts 40 grams of hydrogen with an excess of oxygen and produces 300 grams of water. Find the % yield for this reaction. Step 1: Do a mass-mass conversion starting with the given reactant and converting to the product, (in this example, the water.) The answer you get is how much water you “theoretically” should have produced. Step 2: The other value in the question, (300 grams), is what you actually produced in the lab. Divide them to get your % yield! 1 mol H2 2 mol H2O 18.0 g H2O 40 g H2 360 g H2O x x x = 2.0 g H2 2 mol H2 1 mol H2O 300 % Yield = x 100 = 83.3% 360

Percent Yield Practice Problem 7H202 + N2H4  2HNO3 + 8H2O 2) The reaction of 11 g of N2H4 with excess H2O2 according to the following reaction yields 35 g of HNO3. What is the percent yield? Step 1: Do a mass-mass conversion starting with the given reactant and converting to the product, (in this example, the HNO3.) The answer you get is how much water you “theoretically” should have produced. Step 2: The other value in the question, (35 grams), is what you actually produced. Divide them to get your % yield! 1 mol N2H4 2 mol HNO3 63 g HNO3 43.3 g HNO3 11g N2H4 x x x = 32 g N2H4 1 mol N2H4 1 mol HNO3 35 % Yield = x 100 = 80.8% 43.3

Limiting Reagent (or Limiting Reactant) runs out The limiting reagent is the reactant that ___________ _____ first. The reactant that is in abundance is called the ___________ reagent. excess

Practice Problems: N2 (g) + 3H2 (g)  2NH3 (g) Calculations Step 1: Do a mole-mole conversion for one of the substances. This answer is how much of it you need. Step 2: Compare your answer to how much reactant was given. Do you have enough? If not, this reactant is your limiting reagent! Practice Problems: N2 (g) + 3H2 (g)  2NH3 (g) 1) If 2.7 moles of nitrogen reacts with 6.3 moles of hydrogen, which will you run out of first? 2) If 3.9 moles of nitrogen reacts with 12.1 moles of hydrogen, what is the limiting reagent? Were you given enough H2? ______ Limiting Reagent = ______ 3 mol H2 No ! (6.3<8.1) 2.7 mol N2 8.1 moles of H2 are needed for the reaction. x = 1 mol N2 H2 Were you given enough H2? ______ Limiting Reagent = ______ 3 mol H2 Yes ! (12.1>11.7) 3.9 mol N2 11.7 moles of H2 are needed for the reaction. x = 1 mol N2 N2

Excess Reagent (or Excess Reactant) How many moles of excess reagent do you have? Step 1: Do a mole-mole conversion starting with the limiting reagent as the given. The answer you get is how much of the excess reagent you need to completely react with the limiting reagent. Sometimes you get lucky and you already did this conversion from the previous problem! Step 2: Subtract this answer from the amount given in the original problem, and that is how many moles of excess reagent there are.

Excess Reagent (or Excess Reactant) Practice Problem: Find the number of moles of excess reagent from the previous practice problems. 1 mol N2 6.3 mol H2 2.1 moles of N2 are needed for the reaction. x = 3 mol H2 2.7 moles of N2 were originally given, so the excess will be… 2.7 moles given − 2.1 moles needed = 0.6 moles of N2 excess For the second practice problem, we already started with the limiting reagent, so all you have to do is subtract… 12.1 moles given −11.7 moles needed= 0.4 moles of H2 excess