Graded Concept Check:

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1 Today: Stoichiometric Analysis: Mole to Mole Conversions: Use stoichiometric coefficients from balanced equations Gram to Gram Conversions: Use MOLAR MASS to get to moles Theoretical Yield: How much product can form in a reaction? Limiting Reagents: Method 1 Method 2 PERCENT YIELD Schedule an appointment or stop by office hours to pick up QUIZ 2 Next Meeting Concept Check: Covers today s material Check out Canvas for Stoichiometry Practice Problems Reading for Monday: Start Chapter 9, pp Graded Concept Check: Stoichiometric Analysis: Relating one reactant to another How many moles of nitrogen gas would be needed to react with 15 moles of hydrogen gas? A. 45 moles N 2 N 2(g) + 3 H 2(g) 2 NH 3(g) B. 30 moles N 2 C. 15 moles N 2 D. 10 moles N 2 E. 5 moles N 2 The expansion of the world's population from 1.6 billion people in 1900 to today's seven billion [in 2012] would not have been possible without the [industrial] synthesis of ammonia. MIT Press 1

2 # of grams NaN 3 Molar Mass of NaN 3 : x = g/mol # of moles NaN 3 Mass-to-Mass Conversions: How many grams of sodium metal can be produced from the decomposition of 130 grams of sodium azide? 2 NaN 3(s) 2 Na (s) + 3 N 2(g) Moles cannot be measured directly, but MASS can be measured on a balance & tied to the number of moles with MOLAR MASS Mole-to-mole ratios relate reactants & products: 2 NaN 3 produce 2 Na # of grams Na # of moles Na Molar Mass Na: g/mol What volume of water could form from the combustion of 20. ml of C 3 H 8 O? 2 C 3 H 8 O + 9 O 2(g) 6 CO 2(g) + 8 H 2 O (g) # of moles C 3 H 8 O 2 mol of C 3 H 8 O make 8 mol H 2 O # of moles H 2 O MOLAR MASS = 60.1 g/mol MOLAR MASS = 18.0 g/mol Volume of C 3 H 8 O Mass of C 3 H 8 O DENSITY = g/ml DENSITY = 1.00 g/ml Mass of H 2 O Volume of H 2 O 2

3 How many grams of oxygen would be needed to fully burn 1.00 gram of Magnesium metal? 2 Mg + O 2 2 MgO Molar Masses (g/mol): Remember: the stoichiometric coefficients are only used to relate one substance to another. They are NOT used to calculate mole quantities. Nitrogen Dioxide: A major component of smog Nitrogen monoxide (aka nitric oxide, NO) is produced in internal combustion engines. Nitric oxide released into the atmosphere can react with O 2 to form nitrogen dioxide (NO 2 ). 2 NO + O 2 2 NO 2 3

4 Determining the Limiting Reagent: METHOD 1 While calculating the THEORETICAL YIELD 1. Convert the starting quantity of each reactant to the quantity of product that could potentially form. 2. The limiting reagent is the reactant that would produce LESS product and determines the THEORETICAL YIELD. 4 mol O 2 4 mol NO Nitric oxide is LIMITING (it is consumed & limits the formation of product) Animations adapted from McGraw Hill Publishers O 2 is in EXCESS (some remains unreacted in the end) Determining the Limiting Reagent: METHOD 2 Comparing one reactant to another 1. Start with Reactant A. Given the starting quantity of A, calculate the quantity of the other reactant (B) needed to FULLY react with A. 2. Compare the available quantity of B to the amount needed to react with A. If more is available than is needed, B is in EXCESS & A is the LIMITING. 2 mol O 2 6 mol NO Nitric oxide is in EXCESS (some remains unreacted in the end) Oxygen is LIMITING (it is consumed & limits the formation of product) Animations adapted from McGraw Hill Publishers 4

5 A Stoichiometric Mixture: Neither reactant is limiting or in excess When the reactants are in the proper stoichiometric ratio, each can FULLY react with the other. Neither starting material remains in the end of the reaction. Neither is considered a limiting reagent. 6 mol NO 3 mol O 2 Animations adapted from McGraw Hill Publishers iclicker Participation Question: Stoichiometric Analysis If 10 moles of iron (III) oxide are mixed with 10 moles of zinc metal and react according to the equation below, which component will be consumed first? A. Fe 2 O 3 In other words, which is the limiting reagent? B. Zn C. ZnO D. Fe Fe 2 O Zn 3 ZnO + 2 Fe E. There are no limiting reagents here. Everything reacts fully 5

6 Galvanized Steel Iron metal corrodes in the presences of oxygen and water to produce rust (Fe 2 O 3 ), which compromises the strength of the metal. Galvanized steel uses a barrier of zinc or magnesium metal to protect the iron from corrosion. Write the balanced equation for magnesium metal reacting with Iron (III) Oxide (aka rust ) to produce Iron metal & magnesium oxide. SINGLE Replacement Reactions Single Replacement: a reaction where one type of element is replaced with another. This most often occurs when an ionic compound reacts with a substance composed of a single element. Single replacement reactions are a type of of REDOX reaction (involving an exchange of electrons) Batteries make use of Redox reactions: Write the balanced equation for Zinc metal reacting with copper (II) sulfate to produce zinc (II) sulfate and copper metal. 6

7 SINGLE Replacement Reactions Magnesium metal can burn with carbon dioxide in a single replacement reaction. Write the balanced equation for this reaction. If 30 g of Mg was mixed with 30 g of carbon dioxide, what is the maximum amount of magnesium oxide that could form? In other words, calculate the theoretical yield. FIRST: calculate the limiting reagent by finding the maximum amount of product (MgO) that could form IF each reactant was fully consumed. THEN: compare the amount of product that could form in each case. The reactant that produces less product is the limiting reagent & LIMITS the formation of product. The Limiting Reagent is used to calculate the Theoretical Yield. If 30 g of Mg was mixed with 30 g of carbon dioxide, what is the maximum amount of magnesium oxide that could form? In other words, calculate the theoretical yield. 7

8 Reactions are not 100% efficient If only a portion of the limiting reagent reacts, only a fraction of the theoretical yield will be obtained. The quantity of the product that is produced in reality is called the ACTUAL YIELD. The PERCENT YIELD measures how much of the limiting reagent actually reacted. PERCENT YIELD = Actual Yield Theoretical Yield x 100 (This number should always be LESS than or equal to 100 %) Given the Actual Yield & the Theoretical Yield, it is possible to calculate the PERCENT YIELD. OR: Given the PERCENT YIELD & the Theoretical Yield, it is possible to calculate the Actual Yield. In the course of history, the discovery of single reactions has repeatedly revolutionized human civilizations. The start of the Iron Age around 1300 B.C. marked the moment we learned to transform brittle iron ores to iron metal. This affected everything from how we grew food to how we waged wars. Write the balanced chemical equation describing iron (III) oxide reacting with carbon atoms to form iron metal and carbon dioxide. How much iron metal could be produced from grams of Fe 2 O 3 & 30. grams of Carbon? What if only 10.3 grams of iron metal were obtained? What would be the percent yield? 8

9 Practice Problems: Urea (CH 4 N 2 O), a common fertilizer, can be synthesized by the reaction of ammonia (NH 3 ) with carbon dioxide. 2 NH 3(aq) + CO 2(aq) CH 4 N 2 O (aq) + H 2 O (l) An industrial synthesis of urea begins with 35.8 kg of aammonia and 89 kg of carbon dioxide. What mass of urea could theoretically be produced from this starting mixture? An emergency reathing apparatus placed in mines or caves works via the chemical reaction below: 4 KO 2(s) + 2 CO 2(g) 2 K 2 CO O 2(g) If the oxygen supply becomes limited, a worker can use the apparatus to breathe while exiting the mine. Notice that the reaction produces O 2, which can be breathed, and absorbs CO 2, a product of respiration. What minimum amount of KO 2 is required for the apparatus to provide enough oxygen to allow the user 15 minutes to exit the mine? Assume 4.4 g of O 2 are needed for 15 minutes of normal breathing. 9