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Stoichiometry Chapter 12

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The Arithmetic of Equations Essential Question: What kind of information is contained in a balanced chemical equation, and how do you determine the amount of reactants needed, or the amount of products made, in a chemical reaction?

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Balanced Equations In the manufacturing of a tricycle, you must have a frame (F), a seat (S), wheels (W), handlebars (H) and pedals (P). F + S + 3W + 2H + 2P FSW 3 H 2 P 2 How many pedals would be required to make 640 tricycles?

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A “Chemical Recipe” Balanced chemical equations are like a chemical recipe. You can determine the quantities of reactants and products in a chemical reaction from a balanced chemical equation. The calculations are known as stoichiometry.

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Interpreting Chemical Equations The quantities of reactants and products include: Number of atoms, molecules and formula units Moles Mass Volume

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Do These Quantities Look Familiar? Number of atoms, molecules and formula units Mass Volume Moles

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Two Quantities Are Conserved During a Chemical Reaction The number of each kind of atom is conserved. The mass is conserved. This is always true in any chemical reaction.

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Consider the Production of Ammonia N 2 (g) + 3H 2 (g)2NH 3 (g) Are molecules conserved? Are moles conserved? Is volume conserved? Are atoms conserved? Is mass conserved?

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Conserved Quantities

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Chemical Calculations Essential Question: How are mole ratios constructed, and how are they used for solving stoichiometric problems?

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Quantities in Chemical Reactions

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Mole-Mole Calculations The coefficients from a balanced equation are used to write conversion factors called mole ratios. With these mole ratios, you can relate moles of reactants to moles of products.

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N 2 (g) + 3 H 2 (g) 2 NH 3 (g) What mole-to-mole ratios can you see here? One mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia. Are there other relationships? How can these be written as conversion units?

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N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 1 mol N 2 2 mol NH 3 3 mol H 2 3 mol H 2 1 mol N 2 2 mol NH 3 1 mol N 2 2 mol NH 3 3 mol H 2 These mole-to-mole ratios are conversion units

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Sample Problem 12.2 page 360 How many moles of ammonia are produced when 0.60 moles of nitrogen reacts with hydrogen? 0.60 mol N 2 X =1.2 mol NH 3 Does this answer seem reasonable? 2 mol NH 3 1 mol N 2

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Chemistry in Air Bags 2 NaN 3 (s)2 Na(s) + 3 N 2 (g) How can stoichiometry be used to predict the volume of a gas produced in this reaction?

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4 Al(s) + 3 O 2 (g)2 Al 2 O 3 (s) Write the six mole ratios that can be derived from this equation. How many moles of aluminum are needed to form 3.7 mol of Al 2 O 3 ? How many moles of oxygen are required to react completely with 14.8 mol Al? How many moles of Al 2 O 3 are formed when 7.8 mol O 2 reacts with aluminum?

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Mass-Mass Calculations No laboratory scale can measure quantities in moles. Instead, the number of moles in determine by measuring mass in grams and converting to moles.

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Other Stoichiometric Calculations Recall from previous classes the three methods of describing the amount of something you have. With the coefficients in a balance chemical equation (the mole-to-mole ratio) any quantity can be calculated.

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2 KClO 3 (s)2 KCl(s) + 3 O 2 (g) How many molecules of oxygen are produced by the decomposition of 6.54 g of potassium chlorate (KClO 3 )? 4.82 x 10 22 molecules of O 2

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3 NO 2 (g) + H 2 O(l) 2 HNO 3 (aq) + NO(g) How many grams of nitrogen dioxide must react with water to produce 5.00 x 10 22 molecules of nitrogen monoxide? 11.5 g NO 2

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2 CO(g) + O 2 (g)2 CO 2 (g) How many liters of oxygen are required to burn 3.86 L of carbon monoxide? 1.93 L O 2

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P 4 (s) + 6 H 2 (g)4 PH 3 (g) Phosphorus and hydrogen can be combined to form phosphine (PH 3 ). How many liters of phosphine are formed when 0.42 L of hydrogen reacts with phosphorus? 0.28 L PH 3 What is unique about volume-volume calculations?

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Limiting Reagents and Percent Yield Essential Question: What determines the amount of product formed when two or more substances react together, and what does the percent yield of a reaction indicate?

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Limiting Reagent (Reactant) Let’s have a peanut butter and jelly sandwich party! What is the limiting ingredient to the making of our sandwiches?

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Key Terms Limiting Reagent Excess Reagent Theoretical Yield Actual Yield Percent Yield

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Determining Limiting Reagent 2 Cu(s) + S(s)Cu 2 S(s) What is the limiting reagent when 80.0 g of Cu reacts with 25.0 g of S? There are two equally valid methods for determining the limiting reagent –

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Determining Limiting Reagent 2 Cu(s) + S(s)Cu 2 S(s) Method 1: Calculate the amount of product for EACH of the reactants…the smallest amount will be from the limiting reagent. Method 2: Pick one and calculate how much of the other reactant is needed, and compare that with the amount you actually have. Let’s try each of these methods --

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2 Cu(s) + S(s)Cu 2 S(s) 80.0 g 25.0 g ? g ? Method 1:

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2 Cu(s) + S(s)Cu 2 S(s) 80.0 g 25.0 g ? g ? Method 2:

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2C 2 H 2 (g) + 5O 2 (g)4CO 2 + 2H 2 O(g) If 2.70 mol C 2 H 2 is reacted with 6.30 mol O 2, what is the limiting reagent?

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Mg(s) + 2HCl(aq)MgCl 2 (aq) + H 2 (g) Identify the limiting reagent when 6.00 g HCl reacts with 5.00 g Mg.

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H 3 PO 4 + 3NaOHNa 3 PO 4 + 3H 2 O If 1.75 mol H 3 PO 4 is made to react with 5.00 mol NaOH, identify the limiting reagent and calculate the grams of Na 3 PO 4 that can theoretically be produced.

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Percent Yield How do you calculate any percentage? How would one calculate percent yield? What is the part? What is the whole?

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Percent Yield The whole is the “theoretical yield,” the amount we would get in a perfect world. The part that we are focused on is the amount that we actually produced – the actual yield. Part ÷ Whole x 100%

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CaCO 3 (s)CaO(s) + CO 2 (g) What is the theoretical yield of CaO if 24.8 g CaCO 3 is decomposed? What is the percent yield if the actual yield is 12.6 g ∆

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Fe 2 O 3 + 3CO2Fe + 3CO 2 When 84.8 g of iron(III) oxide reacts with an excess of carbon monoxide, what is the theoretical yield of iron? What is the percent yield if that actual yield is 51.4 g?

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