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Ppt 12 Plan (PS5, 12-18 material) 1.Limiting Reactant Situations 2.% Yield 3.Molarity (done in lab—Exp. 24; worksheet in next Ppt) 1Ppt 12

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Important Realization (about Stoichiometry Problems thus far) In all stoichiometry problems thus far, you have been TOLD either how much of a – reactant reacts (g or mol), or – product forms (g or mol) A B You were ASKED: – How much of somebody else was formed or used? You then used a mole ratio (from the balanced equation) to get ─ moles of B that reacted or formed from moles of A that reacted or formed 2

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Example (from earlier “Quiz” worksheet) If 32.6 g of CCl 4 reacts according to the equation below, how many grams of Cl 2 will form? P 4 O 10 + 5 CCl 4 5 CO 2 + 4 PCl 3 + 4 Cl 2 3Ppt 12

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How is this problem different? If 32.6 g of CCl 4 and 11.6 g of P 4 O 10 are combined, and as much reaction occurs as is possible according to the equation below, how many grams of Cl 2 will form? P 4 O 10 + 5 CCl 4 5 CO 2 + 4 PCl 3 + 4 Cl 2 4Ppt 12

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“Limiting Reactant” Situations You are given amounts of TWO (or more) reactants that are present in a container. Can all of both reactants react? You need to figure out who runs out “first” and who has some leftover! – Only if they happen to be present (initially) in the same ratio in which they react! VERY RARE! – Usually there will be “leftovers” of one. – So you cannot assume that what is present all reacts “limiting reactant” it limits the amount of reaction that can occur! 5Ppt 12

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How do you figure out which reactant is limiting? 1) Ratio method. Directly compare mole ratios: “ratio PRESENT” to “ratio in which they REACT” – Ratio PRESENT > ratio of REACTION “too much of substance in NUMERATOR” numerator is in excess; denominator is LR – Ratio PRESENT < ratio of REACTION “too little of substance in NUMERATOR” numerator is LR ; denominator is in excess 6Ppt 12

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How do you figure out which reactant is limiting? 2) Assumption method. Assume one of the reactants (A) ALL REACTS. Calculate how much of the other reactant (B) would be NEEDED (using mole ratio from equation). – moles B PRESENT > moles B NEEDED “too much of B” (to react with all the A present) B is in excess; A is LR – moles B PRESENT < moles B NEEDED “too little of B” (to react with all the A present) B is LR ; A is in excess **Could go “grams A” to “grams of B needed” also 7Ppt 12

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How do you figure out which reactant is limiting? 3) “Which amount of reactants makes the least amount of product?” method a) Assume one of the reactants (A) ALL REACTS. Calculate how much product would be made. b) Do the same thing assuming all of the other reactant (B) ALL REACTS. Whichever results in fewer products being made is the L.R. 8Ppt 12

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Example (from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) ) CO (g) + 2 H 2 (g) CH 3 OH (l) (a) Starting with 12.0 g H 2 and 74.5 g CO, which reactant is limiting? (b) What mass of the excess reactant is left over? (c) What mass of methanol can be obtained (in theory)? 9Ppt 12

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PS Sign-Posting The concepts and skills related to problems 12-17 on PS5 were covered in the prior section of this PowerPoint. Give those problems a try now! 10Ppt 12

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If masses of both reactants are given (“Limiting Reactant” situation) Compare the mole ratio of reactants PRESENT to the mole ratio of REACTION (from equation) [or use “assumption” or “which makes the least amount of products” method] Because it all reacted!Formed, in theory “Theoretical yield” (Maximum possible assuming all of the LR reacted in the way you assumed) OR moles of other reactant that reacted!! 11Ppt 12

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Actual vs. Theoretical Yield: Theory calculates the max, but you rarely get that in the real world! "yield" means "amount of a product“ - it could be in grams or moles “theoretical yield" means "maximum amount of product” - Assumes that as much reaction as possible occurs according to a given balanced chemical equation). no “side” reactions occur no losses during isolation (stuck to filter paper, etc.) - Obtained by doing a stoichiometry calculation using the balanced equation “actual yield" means “amount of product you actually isolated in the lab after doing a reaction” 12Ppt 12

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% Yield: How well did you do?! “% Yield” means “percent of the maximum amount that you actually got” Warning: Read problems carefully! You might not be asked for % yield! - If you have any 2 of the 3 “variables” you can calculate the third! 13

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Example (from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) ) CO (g) + 2 H 2 (g) CH 3 OH (l) What is the percent yield of a reaction in which 5.0 x 10 3 g of H 2 reacts with excess CO to form 3.5 x 10 3 g CH 3 OH? Given: Actual yield (“to form”); Asked for: % Yield Needed: Theoretical yield 14Ppt 12

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Stoichiometry Quiz-Revisited (Chemistry 121 Quiz Used as PRACTICE WORKSHEET on "Early" Stoichiometry) 1. (8 pts) Given the following chemical equation: P 4 O 10 + 5 CCl 4 5 CO 2 + 4 PCl 3 + 4 Cl 2 a.How many moles of CO 2 will be formed in theory if 3 moles of P4O10 react? b.How many moles of P 4 O 10 would be used up if 1.9 moles of Cl 2 were produced? c.How many moles of PCl 3 form in theory if 2.4 moles of CCl 4 react? d.How many moles of CO 2 will be formed in theory if 0.247 moles of P 4 O 10 react? e.How many grams of Cl 2 would be formed in theory if 3.2 moles of P 4 O 10 were reacted? f.If 32.6 g of CCl 4 reacts, how many grams of Cl 2 form in theory? ***It is implied in all these problems that you are determining a theoretical yield!*** 15Ppt 12

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PS Sign-Posting The concepts and skills related to problem 18 on PS5 were covered in the prior section of this PowerPoint. Give this problem a try now! 16Ppt 12

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