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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 FSW3H2P2 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**

N2(g) + 3H2(g) 2NH3(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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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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**N2 (g) + 3 H2 (g) 2 NH3 (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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**N2 (g) + 3 H2 (g) 2 NH3 (g) 1 mol N2 2 mol NH3 3 mol H2**

3 mol H2 1 mol N2 2 mol NH3 3 mol H2 1 mol N2 2 mol NH3 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 N2 X =1.2 mol NH3 Does this answer seem reasonable? 2 mol NH3 1 mol N2

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**Chemistry in Air Bags 2 NaN3(s) 2 Na(s) + 3 N2(g)**

How can stoichiometry be used to predict the volume of a gas produced in this reaction?

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4 Al(s) O2(g) 2 Al2O3(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 Al2O3? How many moles of oxygen are required to react completely with 14.8 mol Al? How many moles of Al2O3 are formed when 7.8 mol O2 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 KClO3(s) 2 KCl(s) O2(g) How many molecules of oxygen are produced by the decomposition of 6.54 g of potassium chlorate (KClO3)? 4.82 x molecules of O2

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**3 NO2(g) + H2O(l) 2 HNO3(aq) + NO(g)**

How many grams of nitrogen dioxide must react with water to produce 5.00 x 1022 molecules of nitrogen monoxide? 11.5 g NO2

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2 CO(g) + O2(g) 2 CO2(g) How many liters of oxygen are required to burn 3.86 L of carbon monoxide? 1.93 L O2

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P4(s) H2(g) 4 PH3(g) Phosphorus and hydrogen can be combined to form phosphine (PH3). How many liters of phosphine are formed when 0.42 L of hydrogen reacts with phosphorus? 0.28 L PH3 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) Cu2S(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) Cu2S(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) Cu2S(s) 80.0 g g ? g ? Method 1:

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2 Cu(s) + S(s) Cu2S(s) 80.0 g g ? g ? Method 2:

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2C2H2(g) + 5O2(g) 4CO2 + 2H2O(g) If 2.70 mol C2H2 is reacted with 6.30 mol O2, what is the limiting reagent?

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**Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)**

Identify the limiting reagent when 6.00 g HCl reacts with 5.00 g Mg.

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H3PO4 + 3NaOH Na3PO4 + 3H2O If 1.75 mol H3PO4 is made to react with 5.00 mol NaOH, identify the limiting reagent and calculate the grams of Na3PO4 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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CaCO3(s) CaO(s) + CO2(g) ∆ What is the theoretical yield of CaO if 24.8 g CaCO3 is decomposed? What is the percent yield if the actual yield is 12.6 g

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Fe2O3 + 3CO 2Fe + 3CO2 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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