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Limiting Reactant & % Yield

(mostly from Chapter 4 Sec 5 & 6of Jespersen 6th ed)

Dr. C. Yau

Spring 2014

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Limiting Reactant

"Limiting Reactant" refers to the reactant that limits the amount of product formed.

It is the reactant in shortest supply (based on moles, not grams).

It is therefore the reactant that will run out first.

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Limiting ReactantThe analogy is manufacturing a tricycle,

requiring 3 tires and 2 handles and 1 seat.

If we have 90 tires, 40 handles and 40 seats, how many tricycles can we make?

How do you go about answering this question?

Can we just base it on the # of tires?

Limiting ReactantIf we have 90 tires, 40 handles and 40 seats, how many tricycles can we make?

Here are 2 ways to figure it out…

1) Calculate how many tricycles you can make from 90 tires, from 40 handles and from 40 seats. The smallest number of tricycles would be the answer.

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Limiting ReactantIf we have 90 tires, 40 handles and 40 seats, how many tricycles can we make?

2) The other way is to take 90 tires and determine how many tricycles can be made, then see whether there are enough handles.

In this case, 30 tricycles can be made from 90 tires, and that would require 60 handles. But, we have only 40 handles, so that tells us handles may be the limiting factor. 5

Limiting ReactantIf we have 90 tires, 40 handles and 40 seats, how many tricycles can we make?

We now look at the handles… 20 tricycles can be made from the 40 handles, are there enough seats to make 20 tricycles?

From 40 seats we can make 40 tricycles, so the seats are in surplus. We can conclude the handle is still the limiting factor. Ans. 20 tricycles

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You should understand BOTH lines of reasoning:1) Determine how much “product” can be made based on each starting material (tires, handles and seats). The one that makes the smallest number of product is the “limiting reactant.”2) Based on one starting material, determine whether there is enough of the other starting material available. This will tell you which is limiting.

Limiting ReactantExample 4.18 p.137

Gold(III) hydroxide is used for electroplating gold onto other metals.2KAuCl4 + 3Na2CO3 + 3H2O 2Au(OH)3 + 6NaCl

+ 2KCl + 3CO2

To prepare a fresh supply of Au(OH)3, a chemist has mixed 20.00 g of KAuCl4 with 25.00 g of Na2CO3 (both dissolved in a large excess of water). What is the maximum number of grams of Au(OH)3 that can be formed?

First, note that this is a “limiting reactant” question. How do you tell?????

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Limiting ReactantExample 4.18 p.137

Gold(III) hydroxide is used for electroplating gold onto other metals.2KAuCl4 + 3Na2CO3 + 3H2O 2Au(OH)3 + 6NaCl

+ 2KCl + 3CO2

To prepare a fresh supply of Au(OH)3, a chemist has mixed 20.00 g of KAuCl4 with 25.00 g of Na2CO3 (both dissolved in a large excess of water). What is the maximum number of grams of Au(OH)3 that can be formed?

MM=377.9 g/mol 106.0 g/mol 248.0 g/mol

Ans.13.13 g Au(OH)3Do Practice Exercises 30, 31 p.139 9

Percent Yield"Yield" refers to how much product is obtained, usually in grams.

"% yield" refers to the percent of the theoretical amount that is actually obtained.

100 x yield ltheoretica

yield actual yield % LEARN THIS!

Theoretical yield is what we calculate, based on the balanced chemical equation.Do not confuse this with % error.

How does this % eqn fit into “part over whole x 100”? 10

Percent Yield

If the reaction went perfectly, as expected, the actual yield would equal to theoretical yield and % yield would be 100%.

Reasons why actual yield is often NOT the same as the theoretical yield:

• Unavoidable experimental error (some product cannot be transferred completely from one container to another).

100 x yield ltheoretica

yield actual yield %

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Percent Yield• Reaction may be slow and product was

collected before reaction was finished.

• Reaction may be reversible and stops at an equilibrium rather than going to all products.

• Reactants may be impure, of poor quality. You started with less reactant than you thought.

• Reaction may have side reactions where other products are formed, thus using up A and not making as much B as expected.A B

C 12

Example 4.19 p. 140A chemist set up a synthesis of phosphorus

trichloride by mixing 12.0 g of phosphorus with 35.0 g of chlorine gas and obtained 42.4 g of liquid phosphorus trichloride. calculate the percentage yield of this compound.

Do we need to consider which is the limiting reactant?

Ans. 93.8% yieldDo Practice Exercise 32 & 33 p.141 13

Example 4.19 p. 140 (continued)

A chemist set up a synthesis of phosphorus trichloride by mixing 12.0 g of phosphorus with 35.0 g of chlorine gas and obtained 42.4 g of liquid phosphorus trichloride. calculate the percentage yield of this compound.

ADDED QUESTION #1 (similar in homework)

Which reactant will be left over and how much is left over?

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Example 4.19 p. 140 (continued)

ADDED QUESTION #2:

A chemist set up a synthesis of phosphorus trichloride by mixing 12.0 g of phosphorus with 35.0 g of chlorine gas

In this reaction, if the yield is 85.5% how much phosphorus trichloride should we get?

(In the last problem we determine the theoretical yield to be 45.2 g.)

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