Stoichiometry II Dr Ron Rusay Spring 2008 Copyright

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Stoichiometry II Dr. Ron Rusay Spring 2008 © Copyright 2008 R. J. Rusay

Stoichiometry II Dr. Ron Rusay Spring 2008 © Copyright 2008 R. J. Rusay

Chemical Equations / Stoichiometric Calculations • Another Example: C 8 H 18(l)+ O 2(g)

Chemical Equations / Stoichiometric Calculations • Another Example: C 8 H 18(l)+ O 2(g) CO 2(g) + H 2 O(l) • For the combustion of octane which produces carbon dioxide and water. • The equation must conserve mass , i. e. account for the mass in product and reactant by having a “balance” which is an equal number of individual atoms in reactant and product. © Copyright 1995 -2008 R. J. Rusay

Chemical Equations ð The equation is “balanced” as before by placing stoichiometric factors before

Chemical Equations ð The equation is “balanced” as before by placing stoichiometric factors before each of the molecules in the reaction so that the atoms equal ? C 8 H 18(l) + ? O 2(g) 1 25/2 ? CO 2(g) + ? H 2 O(l) 8 9 Remove fraction: 2 C 8 H 18(l)+ 25 O 2(g) © Copyright 1995 -2008 R. J. Rusay 16 CO 2(g)+18 H 2 O(l)

Mass Calculations ð The Balanced Equation is a balance on a molar basis which

Mass Calculations ð The Balanced Equation is a balance on a molar basis which can be related to mass. 2 C 8 H 18(l)+ 25 O 2(g) 16 CO 2(g) +18 H 2 O(l) 228 g of octane (2 moles)* will react with 800 g of oxygen (25 moles) to produce 704 g of carbon dioxide and 324 g of water. *(2 moles octane x 114 g/mol = 228 g ) © Copyright 1995 -2008 R. J. Rusay

Mass Calculations: Products Reactants 1. 2. 3. 4. 5. Balance the chemical equation. Convert

Mass Calculations: Products Reactants 1. 2. 3. 4. 5. Balance the chemical equation. Convert mass of reactant or product to moles. Identify mole ratios in balanced equation: They serve as the “Gatekeeper”. Calculate moles of desired product or reactant. Convert moles to grams. © Copyright 1995 -2008 R. J. Rusay

Mass Calculations: Products Reactants © Copyright 1995 -2008 R. J. Rusay

Mass Calculations: Products Reactants © Copyright 1995 -2008 R. J. Rusay

Mass Calculations: Reactants Products � � How many grams of salicylic acid are needed

Mass Calculations: Reactants Products � � How many grams of salicylic acid are needed to produce 1. 80 kg of aspirin? Balanced Equation:

Mass Calculations: Reactants Products grams (Aspirin) grams (Salicylic Acid) Avogadro's Number Atoms Molecules Stoichiometry

Mass Calculations: Reactants Products grams (Aspirin) grams (Salicylic Acid) Avogadro's Number Atoms Molecules Stoichiometry grams (SA) 1800 grams (A) 1 mol SA grams (A) 1 mol A (Molecular Weight A) "Gatekeeper" (Molecular Weight SA) ? (SA) 1 mol (SA) Calculation: http: //ep. llnl. gov/msds/Stoichiometry/Aspirin-Calc. html © Copyright 1995 -2000 R. J. Rusay

Mass Calculations: How many grams of salicylic acid are needed to produce 1. 80

Mass Calculations: How many grams of salicylic acid are needed to produce 1. 80 kg of aspirin? � Balanced Equation: 1 1 ? g C 7 = 1. 80 x 103 g. C 9 x [mol. C 9 /180. 15 g. C 9 ] x [1 mol. C 7/ 1 mol. C 9] x 138. 12 g. C 7/mol. C 7 ? g C 7 = 1380 grams

Limiting Reagent An Ice Cream Sundae ? ? ?

Limiting Reagent An Ice Cream Sundae ? ? ?

Limiting Reagent An Ice Cream Sundae What’s left? What’s totally consumed?

Limiting Reagent An Ice Cream Sundae What’s left? What’s totally consumed?

Limiting Reagent

Limiting Reagent

QUESTION

QUESTION

ANSWER 5) none of these Section 3. 10 : Calculations Involving a Limiting Reactant

ANSWER 5) none of these Section 3. 10 : Calculations Involving a Limiting Reactant (p. 106) The limiting reactant in a reaction is the reactant that produces the least number of grams of any product.

Mass Applications: Limiting Reagent ð How do masses of reactants relate? Is there enough

Mass Applications: Limiting Reagent ð How do masses of reactants relate? Is there enough mass of each reactant for the reaction to consume all of both of them or will there be some left of one of them? 2 C 8 H 18(l)+ 25 O 2(g)-----> 16 CO 2(g)+18 H 2 O(l) What would happen if only 600. g of O 2 were available for the reaction of 228 g of octane? © Copyright 1995 -2008 R. J. Rusay

Mass Applications: Determining a Limiting Reagent � Does one of the reactants have fewer

Mass Applications: Determining a Limiting Reagent � Does one of the reactants have fewer stoichiometrically adjusted moles than the other reactant? If so, the reactant with the smaller value is the limiting reagent. Calculation: � Divide the mass of each reactant by its respective Molar Mass and by its Stoichiometric factor from the balanced equation; then compare the results.

Limiting Reagent Calculation ð The reactant present in the smallest molar amount considering stoichiometry

Limiting Reagent Calculation ð The reactant present in the smallest molar amount considering stoichiometry limits the mass basis of any reaction. 2 C 8 H 18(l)+ 25 O 2(g)-----> 16 CO 2(g)+18 H 2 O(l) 228 g octane / 114 g/mol = 2 mol octane 600. g oxygen / 32 g/mol = 18. 75 mol oxygen 2 mol octane / 2 mol (stoich. ) = 1 18. 75 mole oxygen / 25 mol (stoich. ) = 0. 75 © Copyright 1995 -2002 R. J. Rusay

Mass Effects of the Limiting Reagent What amount of octane remains unreacted in the

Mass Effects of the Limiting Reagent What amount of octane remains unreacted in the reaction of 600. g of O 2 with 228 g of octane? 600. g O 2 x mol O 2 /32 g O 2 x [2 mol C 8 H 18 /25 mol O 2] x 114 g / mol C 8 H 18 = 171 g C 8 H 18 are reacted á 228 g - 171 g = 57 g C 8 H 18 remain unreacted á © Copyright 1995 -2002 R. J. Rusay

Limiting Reagent / Theoretical Yield The limiting reagent governs theoretical yield of products. For

Limiting Reagent / Theoretical Yield The limiting reagent governs theoretical yield of products. For the reaction of 228 g of octane with 600. g of oxygen, what is theoretical yield of carbon dioxide? 2 C 8 H 18(l)+ 25 O 2(g)-----> 16 CO 2(g)+18 H 2 O(l) 600. g O 2 x mol O 2 /32 g O 2 x 16 mol CO 2 /25 mol O 2 x 44 g / mol CO 2 = 528 g CO 2 © Copyright 1995 -2002 R. J. Rusay

Thoughts to Consider How much CO 2 do you produce per gallon of gasoline

Thoughts to Consider How much CO 2 do you produce per gallon of gasoline (octane, d= 0. 70 g/ml) when gasoline is combusted? How much CO 2 do you personally produce from driving every week? …. every month? …. every year? …. from other uses and sources? Why do people in developed nations, like the U. S. , Japan & in the EU, produce tons more of CO 2 person than people in under developed or developing nations? Does the increase in “man-made” CO 2 relate to global warming?

Greenhouse Gases: The Chemistry of Warming

Greenhouse Gases: The Chemistry of Warming

What is a greenhouse gas? The sun’s energy & the molecule’s shape decide. •

What is a greenhouse gas? The sun’s energy & the molecule’s shape decide. • Our atmosphere (air) is 78% nitrogen and 21% oxygen. • Neither are greenhouse gases. They do not absorb infrared radiation (heat). • However, H 2 O and CO 2 can absorb infrared energy. Without them earth would be very chilly. http: //zebu. uoregon. edu/1998/es 202/l 13. html

QUESTION

QUESTION

ANSWER 2) 16. 4 g Section 3. 9 Stoichiometric Calculations: Amounts of Reactants and

ANSWER 2) 16. 4 g Section 3. 9 Stoichiometric Calculations: Amounts of Reactants and Products (p. 102) The balanced equation is Ca(OH)2 + 2 HNO 3 Ca(NO 3)2 + 2 H 2 O. Water is the other product for an acid/base reaction.

Percent Yield In synthesis, the actual yield (g) is measured and compared to theoretical

Percent Yield In synthesis, the actual yield (g) is measured and compared to theoretical yield (g). This is the percent yield: % = actual / theoretical x 100 If a reaction produced 2. 45 g of Ibogaine, C 20 H 26 N 2 O, a natural product with strong promise in treating heroin addiction, and theoretical yield was 3. 05 g, what is the % yield? % yield = 2. 45 g / 3. 05 g x 100 = 80. 3% © Copyright 1995 -2002 R. J. Rusay

QUESTION The amount of tungsten metal, used to make many lightbulb filaments, starting with

QUESTION The amount of tungsten metal, used to make many lightbulb filaments, starting with 25. 0 grams of WO 3 in the following reaction was 18. 0 grams. What percent yield does this represent? WO 3 + 3 H 2 W + 3 H 2 O 1. 2. 3. 4. 25. 8% 110% 90. 8% I think I may have confused one of the steps in this calculation

ANSWER Choice 3 has properly converted the initial grams of WO 3 to moles

ANSWER Choice 3 has properly converted the initial grams of WO 3 to moles and used the mole ratio of the equation to determine the maximum moles of W that would be produced if all the WO 3 reacted (theoretical yield – 19. 8 g). The division of the actual yield by theoretical yield 100 produces the answer. Section 3. 8: Stoichiometric Calculations: Amounts of Reactants and Products

Percent Yield �A reaction was conducted that theoretically would produce 0. 0025 moles of

Percent Yield �A reaction was conducted that theoretically would produce 0. 0025 moles of quinine, C 20 H 24 N 2 O 2. The actual amount of isolated quinine was 780 mg. What is the percent yield of quinine? � 324 g/mol x 0. 0025 mol = 81 g = 810 mg(theoretical) � % Yield = 780 mg/ 810 mg x 100 � % Yield = 96%

Mass Calculations: How many grams of salicylic acid are needed to produce 1. 80

Mass Calculations: How many grams of salicylic acid are needed to produce 1. 80 kg of aspirin if the process produces an 85. 0% yield? � Balanced Equation: 1 1 ? g C 7 = 1. 80 x 103 g. C 9 x [mol. C 9 /180. 15 g. C 9 ] x [1 mol. C 7/ 1 mol. C 9] x 138. 12 g. C 7/mol. C 7 1. 80 x 103 g. C 9 /85/100 = 2. 12 x 103 g. C 9 ? g C 7 = 1380 grams ? g C 7 = 2. 12 x 103 g. C 9 x [mol. C 9 /180. 15 g. C 9 ] x [1 mol. C 7/ 1 mol. C 9] x 138. 12 g. C 7/mol. C 7 ? g C 7 = 1624 grams = 1380 grams /85/100