Ppt 12 Plan(PS5, 12-18 material)

1. Limiting Reactant Situations2. % Yield3. Molarity (done in lab—Exp. 24; worksheet in next Ppt)

1Ppt 12

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

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

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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

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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!

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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

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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

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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.

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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)?

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PS Sign-Posting

• The concepts and skills related toproblems 12-17 on PS5 were covered in the prior section of this PowerPoint. Give those problems a try now!

10Ppt 12

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

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”

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% Yield: How well did you do?!

• “% Yield” means “percent of the maximum amount that you actually got”

partial% x 100

total

partial % x 100

total (possible)

yieldyield

yield

yield % yield x 100

yield

actual

theoretical

• 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

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: Actual yield (“to form”); Asked for: % Yield

Needed: Theoretical yield

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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:

P4O10 + 5 CCl4 5 CO2 + 4 PCl3 + 4 Cl2

a. How many moles of CO2 will be formed in theory if 3 moles of P4O10 react?

b. How many moles of P4O10 would be used up if 1.9 moles of Cl2 were produced?

c. How many moles of PCl3 form in theory if 2.4 moles of CCl4 react?

d. How many moles of CO2 will be formed in theory if 0.247 moles of P4O10 react?

e. How many grams of Cl2 would be formed in theory if 3.2 moles of P4O10 were reacted?

f. 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!***

15Ppt 12

PS Sign-Posting

• The concepts and skills related toproblem 18 on PS5 were covered in the prior section of this PowerPoint. Give this problem a try now!

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