General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 6.8 Percent Yield and Limiting Reactants Chapter 6 Chemical Reactions and Quantities

General, Organic, and Biological Chemistry

Fourth EditionKaren Timberlake

6.8Percent Yield and

Limiting Reactants

Chapter 6Chemical Reactions

and Quantities

© 2013 Pearson Education, Inc.Lectures

© 2013 Pearson Education, Inc. Chapter 6, Section 8 2

Theoretical, Actual, and Percent Yield Theoretical yield is the maximum amount of

product, which is calculated using the balanced equation.

Actual yield is the amount of product obtained when the reaction takes place.

Percent yield is the ratio of actual yield to theoretical yield.


Guide to Calculations for Percent Yield


Calculating Percent Yield

On a space shuttle, LiOH is used to absorb exhaled CO2 from breathing air to form LiHCO3. What is the percent yield of the reaction if 50.0 g of LiOH gives 72.8 g of LiHCO3?



On a space shuttle, LiOH is used to absorb exhaled CO2 from breathing air to form LiHCO3. What is the percent yield of the reaction if 50.0 g of LiOH gives 72.8 g of LiHCO3?

Step 1 State the given and needed quantities. Analyze the Problem.



What is the percent yield of the reaction if 50.0 g of LiOH gives 72.8 g of LiHCO3?

Step 2 Write a plan to calculate the theoretical yield and the percent yield. Calculation of theoretical yield:

grams molar moles mole-mole moles molar grams LiOH mass LiOH factor LiHCO3 mass LiHCO3

Calculation of percent yield:




Step 3 Write the molar mass for the reactant and the mole–mole factor from the balanced equation.




Step 3 Write the molar mass for the reactant and the mole–mole factor from the balanced equation.




Step 4 Solve for the percent yield ratio by dividing the actual yield (given) by the theoretical yield and multiplying the result by 100%.

Calculation of theoretical yield:




Step 4 Solve for the percent yield ratio by dividing the actual yield (given) by the theoretical yield and multiplying the result by 100%.

Calculation of percent yield:


Limiting Reactant

A limiting reactant in a chemical reaction is thesubstance that is used up first and limits the amount of product that can form. The other reactant, called the excess reactant, is left over.


Example: Limiting Reactant

How many peanut butter sandwiches could be madefrom 8 slices of bread and 1 jar of peanut butter?

With 8 slices of bread, only 4 sandwiches could bemade. The bread is the limiting item.


Example: Limiting Reactant

How many peanut butter sandwiches could be madefrom 8 slices of bread and 1 tablespoon of peanut butter?

With 1 tablespoon of peanut butter, only 1 sandwichcould be made. The peanut butter is the limiting item.


Calculating Moles of Product from Limiting ReactantCalculate the moles of product (CH3OH) that can form given 3.00 moles of CO reacts with 5.00 moles of H2. What is the limiting reactant?

a. What mole–mole equalities will be needed in the . calculation?

b. What are the mole–mole factors from these . equalities?

c. What is the number of moles of CH3OH from each . reactant?

d. What is the limiting reactant for the reaction?



a. What mole–mole equalities will be needed in the calculation?

1 mole of CO = 1 mole of CH3OH

2 moles of H2 = 1 mole of CH3OH



b. What are the mole–mole factors from these equalities?



c. What is the number of moles of CH3OH from each reactant?



d. What is the limiting reactant for the reaction?

The limiting reactant is H2, which produces the smaller number of moles of product, 2.50 moles of CH3OH.




Calculating Mass of Product from Limiting Reactant


Calculating Mass of Product from Limiting ReactantHow many grams of CO can be produced from a mixture of 70.0 g of SiO2 and 50.0 g of C?

Step 1 State the given and needed quantities.

Analyze the Problem.



Step 2 Use coefficients to write mole–mole factors; write molar mass factors, if needed.






Step 3 Calculate the number of moles of product from each reactant and determine the limiting reactant.



Step 4 Use the molar mass to convert the smaller number of moles of product to grams.

1 mole of CO = 28.0 g of CO


Learning Check

If 48.2 grams of Ca are mixed with 31.0 grams of N2, how many grams of Ca3N2 can be produced?


Solution


Step 1 State the given and needed quantities.

Analyze the Problem.


Solution



1 mole of Ca = 40.1 g of Ca

1 mole of N2 = 28.0 g of N2


Solution




Solution


Step 3 Calculate the number of moles of product from each reactant and determine the limiting reactant.

The limiting reactant is Ca, which produces 0.402 mole Ca3N2.


Solution


Step 4 Use the molar mass to convert the smaller number of moles of product to grams.

Documents

General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 6.8 Percent Yield and Limiting Reactants Chapter 6 Chemical Reactions and Quantities