Ppt 12 Plan (PS5, 2 nd half)

1. Ppt 12 Plan(PS5, 2nd half) • Limiting Reactant Situations • % Yield • Molarity (done in lab—Exp. 24; worksheet in next Ppt) Ppt 12

2. 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 • You were ASKED: • How much of somebody else was formed or used? B • You then used a mole ratio (from the balanced equation) to get • ─moles of B that reacted or formedfrom moles of A that reacted or formed

3. Example (from earlier “Quiz” worksheet) • If 32.6 g of CCl4 reacts according to the equation below, how many grams of Cl2 will form? P4O10 + 5 CCl4 5 CO2 + 4 PCl3 + 4 Cl2 Ppt 12

4. How is this problem different? • If 32.6 g of CCl4 and 11.6 g of P4O10 are combined, and as much reaction occurs as is possible according to the equation below, how many grams of Cl2 will form? P4O10 + 5 CCl4 5 CO2 + 4 PCl3 + 4 Cl2 What if only 0.00000001 g of P4O10 were initially present (instead of 11.6 g)? What if 1,000,000 g of P4O10 were initially present (instead of 11.6 g)? Ppt 12

5. “Limiting Reactant” Situations • You are given amounts of TWO (or more) reactants that are present in a container. • Can all of both reactants react? • 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  You need to figure out who runs out “first” and who has some leftover! • “limiting reactant” • it limits the amount of reaction that can occur! Ppt 12

6. How do you figure out which reactant is limiting? There are actually many ways, not just one! • Try to understand whythese methods “work” • Pick the one best for you or the problem • Ratio method • Assumption method • “Which one make the least amount of product?” method Ppt 12

7. 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 Ppt 12

8. 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 Ppt 12

9. 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. Ppt 12

10. Example(from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) ) CO (g) + 2 H2 (g) CH3OH (l) (a) Starting with 12.0 g H2 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)? Ppt 12

11. If masses of both reactants are given (“Limiting Reactant” situation) “Theoretical yield” (Maximum possible assuming all of the LR reacted in the way you assumed) 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 OR moles of other reactant that reacted!! Ppt 12

12. Actual vs. Theoretical Yield: Theory calculates the max, but you rarely get that in the real world! • "yield" means "amount of aproduct“ - it could be in grams or moles • “theoreticalyield" 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 • “actualyield" means “amount of product you actually isolated in the lab after doing a reaction” Ppt 12

13. % 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!

14. Example(from "Chemistry, the Molecular Science", Moore, Stanitski, and Jurs (2002, Harcourt) ) CO (g) + 2 H2 (g) CH3OH (l) What is the percent yield of a reaction in which 5.0 x 103 g of H2 reacts with excess CO to form 3.5 x 103 g CH3OH? Given: Actualyield(“to form”); Asked for: % Yield  Needed: Theoretical yield Ppt 12

15. Stoichiometry Quiz-Revisited(Chemistry 121 Quiz Used as PRACTICE WORKSHEET on "Early" Stoichiometry) 1. (8 pts) Given the following chemical equation: P4O10 + 5 CCl4 5 CO2 + 4 PCl3 + 4 Cl2 How many moles of CO2 will be formed in theory if 3 moles of P4O10 react? How many moles of P4O10 would be used up if 1.9 moles of Cl2 were produced? How many moles of PCl3 form in theory if 2.4 moles of CCl4 react? How many moles of CO2 will be formed in theory if 0.247 moles of P4O10 react? How many grams of Cl2 would be formed in theory if 3.2 moles of P4O10 were reacted? If 32.6 g of CCl4 reacts, how many grams of Cl2 form in theory? ***It is implied in all these problems that you are determining a theoretical yield!*** Ppt 12