(2 x 22.4 L H 2 ) + (1 x 22.4 L O 2 ) (2 OBJECTIVES:

Transcription

1 Chapter 9 The calculations of quantities in a chemical reaction chemical bookkeeping Section 9.1 The Arithmetic of Equations OBJECTIVES: Calculate the amount of reactants required, or product formed, in a nonchemical process. Interpret balanced chemical equations in terms of interacting moles, representative particles, masses, and gas volume at STP. Need to Build a Car? 4 Wheels 2 Seats + 1 Car So we have these fixed ratios If we wanted to build 5 cars, how many wheels would we need? How could we write out the stoichiometry? The Smart-Car A fixed ratio of starting Materials (Reactants) & Products 4 Wheels 2 Seats 1 Car 1 Car If we want to build 5 cars the number of wheels would be determined as: 5 Cars 4 Wheels X = 20 Wheels 1 Car Greek for measuring elements Stoich is the calculations of quantities in chemical reactions that are based on a balanced equation. We can interpret balanced chemical equations several ways. First, We can Think in Term of Particles 2H 2 + O 2 2H 2 O Two molecules of hydrogen and one molecule of oxygen form two molecules of water. 1

2 Why Not Look on a Grander Scale? 2H 2 + O 2 2H 2 O 2 dozen molecules of hydrogen and 1 dozen molecules of oxygen form 2 dozen molecules of water. 2 x (6.02 x ) molecules of hydrogen and 1 x (6.02 x ) molecules of oxygen form 2 x (6.02 x ) molecules of water. 2 moles of hydrogen and 1 mole of oxygen form 2 moles of water. Looking in the Mole Frame of Mind 2 Al 2 O 3 4 Al + 3O 2 2Na + 2H 2 O 2NaOH + H 2 The coefficients tell us how many moles of each substance Can we Consider Reactions by Mass? The Law of Conservation of Mass applies 2H 2 + O 2 2H 2 O 4.04 g H g H 2 O g O 2 Can We think in Terms of Volume?? 2H 2 + O 2 2H 2 O At STP, 1 mol of any gas = 22.4 L (2 x 22.4 L H 2 ) + (1 x 22.4 L O 2 ) (2 x 22.4 L H 2 O) NOTE: mass and atoms are always conserved- however, molecules, moles, and volumes will not necessarily be conserved! The Many Interpretations of atoms molecules mole mass volume Section 9.2 Chemical Calculations OBJECTIVES: Construct mole ratios from balanced chemical equations, and apply these ratios in mole-mole stoichiometric calculations. Calculate stoichiometric quantities from balanced chemical equations using units of moles, mass, representative particles, and volumes of gases at STP. 2

3 Mole to Mole Reaction Stoich 2 Al 2 O 3 4Al + 3O 2 each time we use 2 moles of Al 2 O 3 we will make 3 moles of O 2, and 4 moles of Al The Mole to Mole conversion factors are 2 moles Al 2 O 3 3 mole O 2 & 3 mole O 2 4 mole Al 2 moles Al 2 O 3 4 mole Al Let s try some Mole Mole Problems How many moles of O 2 are produced when 3.34 moles of Al 2 O 3 decompose? 2 Al 2 O 3 4Al + 3O 2 Ans = 5.01 mol O 2 How many moles of Al are produced if 9.3 moles of O 2 are produced? Ans = 12.4 mol O 2 More Mole to Mole Practice!! 2C 2 H O 2 4CO H 2 O If 3.84 moles of C 2 H 2 are burned, how many moles of O 2 are needed? (9.6 mol) How many moles of C 2 H 2 are needed to produce 8.95 mole of H 2 O? (8.95 mol) Mole to Mole Problems Worksheet If 2.47 moles of C 2 H 2 are burned, how many moles of CO 2 are formed? (4.94 mol) Can We Perform Mass to Mass Calculations?? Isn t that really an extension of Mole to Mole?? Remember the Mole to Mass Conversions? So How do we Do these Mass-Mass Calculations? We can convert grams to moles Use the Periodic Table for Molar Masses Then do the stoich with the mole ratios Balanced equation gives mole ratio! Then turn the moles back to grams Use the Molar Masses to convert back to moles. 3

4 Mass to Mass If 10.1 g of Fe are added to a solution of Copper (II) Sulfate, how much solid copper would form? Fe + CuSO 4 Fe 2 (SO 4 ) 3 + Cu Balanced?? 2Fe + 3CuSO 4 Fe 2 (SO 4 ) 3 + 3Cu Another Mass to Mass Problem Given the unbalanced decomposition reaction of baking soda: NaHCO 3 + heat Na 2 CO 3 + CO 2 + H 2 O How many grams of each product are produced by the decomposition of 42.0 grams of baking soda? Answer: Na 2 CO 3 (26.5 grams) Answer = 17.2 g Cu CO 2 (11 grams) H 2 O (4.5 grams) Mass to Mass Problems Worksheets (2) Couldn t We Perform Volume to Volume Calculations at STP? How many liters of CH 4 at STP are required to completely react with 17.5 L of O 2? CH 4 + 2O 2 CO 2 + 2H 2 O Ans = 8.75 L CH 4 Avogadro s Law equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties" A Shortcut to STP Equal volumes of gas, at the same temperature and pressure contain the same number of particles Liter = 6.02 x molecules Since Moles are numbers of particles, you can treat reactions as if they happen liters at a time, as long as you keep the temperature and pressure the same. Instead of Stoichin by Moles, you can Stoich by Liters 4

5 Stoichin a Liter at a Time How many liters of H 2 O at STP are produced by completely burning 17.5 L of CH 4? CH 4 + 2O 2 CO 2 + 2H 2 O 17.5 L CH 4 2 L H 2 O 1 L CH 4 = 35.0 L H 2 O Note: : This only works for Volume- Volume problems. Gram Town Remember Moletown? Molar Mass Mole Town 22.4 L/mole Volume City Avacado s Number Particl e City We Can Stoich Thru Moletown Stoichin through Moletown Given the unbalanced decomposition reaction of baking soda: NaHCO 3 + heat Na 2 CO 3 + CO 2 + H 2 O What volumes of each product are produced by the decomposition of 42.0 grams of baking soda? Answer: CO 2 & H 2 O: 5.6 L Some A to Z How many liters of CO 2 at STP will be produced from the complete combustion of 23.2 g C 4 H 10? Answer = 35.8 L CO 2 What volume of Oxygen would be required? Answer = 58.2 L O 2 Problems Worksheet 1 & Mixed Problems Worksheet What is a Reaction Yield? 5

6 The Yields The Yield is the amount of product made in a chemical reaction. There are three types: 1. Actual yield- what you get in the lab when the chemicals are mixed 2. Theoretical yield- what the balanced equation tells should be made, or the Yield. 3. Percent yield = Actual X 100 Theoretical A Yielding Problem 6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate. Al + CuSO 4 Al 2 (SO 4 ) 3 + Cu 2Al + 3 CuSO 4 Al 2 (SO 4 ) 3 + 3Cu What is the actual yield? What is the theoretical yield? What is the percent yield? 6.78 g Cu 49% g Cu What does the % Yield Tell Us? Percent yield tells us how efficient a reaction is. Percent yield can not be bigger than 100 %. 6