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Chapter 10 Stoichiometry Or One plus One isn’t always Two What you will need to know how to do to be successful at stoich: 1. Dimensional analysis 2. Calculating molar mass 3. Balancing equations 4. Using coefficients to determine mole ratios. Learning Objectives: 1. Give the mass or amount of any substance in a chemical reaction, calculate the mass or amount of any other substance in the reaction. 2. Solve limiting reactant problems. 3. Solve percent yield problems. Intro : Stoichiometry is the method used by chemists to calculate the amounts of substances needed or produced in chemical reactions. It is an application of dimensional analysis using: A. Molar masses to convert between grams and moles. B. The coefficients from a balanced equation to convert between moles of different substances. • One plus one isn’t always two + = In Math class you often use Pi But not that kind of Pie in chemistry → + 5 Apples ↑ + 1 Crust ↑ → 1 Pie ↑ The coefficients are ratios of amounts Ratio of apples to crusts = Ratio of apples to pies = Ratio of crusts to pies = It works the same in chemistry. The coefficients are mole ratios. 2NaN3 → 2Na + 3N2 6Na + Fe2O3 → 2Fe + 3Na2O Na2O + 2CO2 + H2O → 2NaHCO3 But why would we need to know exact amounts of reactants or products? The Reactions Prevent This: The gas in an airbag is not air, but nitrogen • Not Enough N2 Too much N2 The nitrogen is produced by this reaction. 2NaN3 → 2Na + 3N2 To get the correct amount of N2 we must start with the correct amount of NaN3 . Stoich is used to calculate this amount. Back to Pies! → + 5 Apples + 1 Crust → 1 Pie I want 2 pies! How many crusts? How many apples? # crusts = 2 pies 1 crust = 2 crusts 1 pie #Apples = 2 pies 5 apples 1 pie = 10 Apples Let’s get chemical! Any balanced chemical reaction will give you the mole ratios between all substances in the reaction. N2 + 3H2 → 2NH3 What is the mole ratio of N2 to H2? What is the mole ratio of N2 to NH3? What is the mole ratio of H2 to NH3? We may use these ratios as conversion factors to convert between any two substances in the reaction – just as we converted between pies and crusts and apples. N2 + 3H2 → 2NH3 If we have 4 moles of H2 , how much NH3 will be produced? 4 moles H2 2 moles NH3 Moles NH3 = 3 moles H2 If we start with 0.2 moles of N2 ,how much H2 is needed? Moles H2 = 0.2moles N2 3 mole H2 1 moles N2 Practice: Given the following reaction, 4Al + 3O2 → 2Al2O3 If we start with 1 mole of Al, how much Al2O3 will be produced? How much Al will react with 0.4 moles of O2? How much O2 will be required to produce 5 moles of Al2O3? Note that it does not matter whether you are given an amount of a product or reactant. The coefficients give you mole ratios for all substances! The number of moles of a substance is usually difficult to measure directly in the lab. It is much easier to measure the amount of a substance by mass. Therefore, in stoich problems you are usually given the mass of a substance rather than the number of moles and asked to calculate the mass of another substance in the reaction. These are called massmass problems. Solving mass-mass problems: You will be given the mass of one substance in a reaction (substance A) and asked to calculate the mass of a different substance in the reaction (substance B). This will be a 3-step dimensional analysis conversion. 1. Convert grams of A to moles of A using the molar mass of A. 2. Convert moles of A to moles of B using the coefficients from the balanced equation. 3. Convert moles of B to grams of B using the molar mass of B. Flow chart for mass-mass problems Given grams of substance A Convert to grams of A to moles of A using molar mass of A Convert moles of A to moles of substance B using coefficients from the balanced equation Convert moles of B to grams of B using molar mass or of B Ex: Given the following reaction, how many grams of N2 will be produced by the reaction of 5.0g of NaN3? 2NaN3 → 2Na + 3N2 Given: 5.0 grams of NaN3 molar mass = 65 g/mol Looking for: ? grams of N2 molar mass = 28 g/mol Conversion: 2 mol NaN3 / 3 mol N2 5.0g NaN3 1 mole NaN3 3 mole N2 28 g N2 65 g NaN3 2 mole NaN3 1 mole N2 Ex2: If 12.5 g Of Na are present, how many grams of iron (III) oxide will be required for a complete reaction? 6Na + Fe2O3 → 2Fe + 3Na2O Given: 12.5g Na Molar mass=23 g/mol Need: ? g Fe2O3 Molar mass=159.8 g/mol Conversion: 6 mol Na / 1 mol Fe2O3 12.5g Na 1 mole Na 23 g Na 1 mole Fe2O3 6 mole Na 159.8g Fe2O3 1 mole Fe2O3 There are only 4 possible variations to this type of problem: 1. Given moles of A : looking for moles of B - 1 step conversion using mole ratio. 2. Given moles of A : looking for grams of B - 2 step conversion Use mole ratio to convert moles of A to moles of B. Use molar mass of B to convert moles of B to grams of B. 3. Given grams of A : looking for moles of B - 2 step conversion Use molar mass of A to convert grams of A to moles of A. Use mole ratio to convert moles of A to moles of B. 4. Given grams of A : looking for grams of B - 3 step conversion This is a mass-mass problem Practice: For the reaction: Na2O + 2CO2 + H2O → 2NaHCO3 If you start with 0.35 moles of CO2, how many grams of H2O are needed? If you have 100g of Na2O, how many moles of CO2 react? If 37.5g of NaHCO3 are produced, how many grams of Na2O reacted? Limiting Reactants: Are reactants in a chemical equation that are used up before any other reactant. When this reactant is used up, the reaction must stop. Think about making pies! 5 apples + 1 crust → 1 pie What if you have 16 apples and 2 crusts. How many pies can be made? The ingredient (reactant) that runs out and stops the reaction is the limiting reagent. The reactant that is left over is said to be “in excess”. It works the same way in chemistry. In any real reaction, you will be adding real amounts of reactants to each other. One of them will nearly always run out first. How to solve limiting reactant problems: In this type of problem, you will be given masses of two different reactants and asked to determine which is the limiting reagent and how much of a given product is produced. Do a mass-mass problems starting with reactant 1 and looking for the mass of the asked for product. Do a 2nd mass-mass problems starting with reactant 2 and looking for the mass of the same product. Compare the calculated masses. The smaller of the 2 masses is the amount produced and the reagent that gave that answer is limiting. Ex: For the reaction: N2 + 3H2 → 2NH3 If you start with 5.0g of N2 and 3.0g of H2 , what is the limiting reactant and how much NH3 is produced? Mass-mass 1: Mass-mass 2 Compare: Practice: 6Na + Fe2O3 → 2Fe + 3Na2O Given the above reaction, if you start with 20.0g of Na and 15.0g of Fe2O3 , how much Fe will be formed? Percent yield problems. Not all chemical reactions go to completion. Even for those that do, it is often difficult to recover all of the product of interest. Ex: Percent yield is a way to express the amount of product that is actually obtained in a lab compared to the theoretical amount predicted by a mass-mass calculation. It is defined as: %yield = Actual mass x 100 Theoretical mass Practice: For the reaction: 2Al2O3 → 4Al + 3O2 , the amount of Al expected from a mass-mass calculation was 24.5g. The amount collected in the lab was 22.7g. What is the %yield? If you started with 50g of Al2O3 and obtained 8g of Al, what is the % yield?