7.2 Limiting Reactant and Percent Yield Obj S3

Chemistry

Essential Questions

 What is a limiting reactant?

 How do you determine the limiting reactant?

 What is the theoretical and actual yield?

 What is percent yield and how do you calculate it?

Limiting Reactants

 What controls how much you produce in a chemical reaction?

 Amount of the reactants 

Limiting reactant/reagent:

reactant that limits the amount of product that can form in a chemical reaction

Limiting Reactants (cont)

 What do you think the other reactant is called?

 Excess reactant: the reactant that is leftover after the reaction has finished  How would you determine which reactant is the limiting reactant?

 The reactant that produces the smallest amount of product will be the limiting reactant

Determining the Limiting Reactant

1.

2.

3.

Start with a balanced equation.

In two separate problems calculate how many moles of same product will be produced for each of the reactants The reactant that produces the smallest amount of product is the limiting reactant. The smallest amount of product is how much can be produced.

Limiting Reactant Example

 If you have 6.0 mol HF and 4.5 mol SiO 2 which one is the limiting reactant?

SiO 2 (s) + 4HF(g) → SiF 4 (g) + 2H 2 O(l) 2 4  2 4   HF is the limiting reactant 4 4  Which is limiting if you have 5.7 mol HF and 1.3 mol SiO 2 ?

Percent Yield

 You prepared enough cookie dough to make 5 dozen cookies. You burn one batch which you throw out, but the rest of the cookies are okay.

 How many cookies could you have made?

 5 dozen  How many cookies do you still have?

 4 dozen

Percent Yield (cont)

 Scientists use percent yield to measure how efficient a reaction was 

Percent yield:

ratio of the actual yield to the theoretical yield, multiplied by 100.

 The yield for the cookies would be 80%

Percent Yield (cont)



Actual yield:

experimental amount of product formed 

Theoretical yield:

amount of product formed when all of the limiting reactant reacts

Percent Yield (cont)

 When 36.8 g C 6 H 6 react with an excess of Cl2, the actual yield of C 6 H 5 Cl is 38.8 g. What is the percentage yield of C 6 H 5 Cl?

C 6 H 6 + Br 2 → C 6 H 5 Br + HBr 73.2%  A piece of Copper with a mass of 5.0 g is placed in a solution of silver nitrate. The silver metal produced has a mass of 15.2 g. What is the percent yield?

Cu + 2 AgNO 3 → 2 Ag + Cu(NO 3 ) 2

Limiting Reactant and Percent Yield Practice Problem  C 6 H 6 is reacted Br 2 to produce C 6 H 5 Br and HBr as shown below. When 40.0 g of C 6 H 6 are reacted with 95.0 g of Br 2 , 65.0 g of C 6 H 5 Br is produced.

C 6 H 6 + Br 2 → C 6 H 5 Br + HBr a. Which compound is the limiting reactant?

b. What is the theoretical yield of C c. What is the reactant in excess?

d. What is the percent yield?

6 H Br 2 5 Br?

C 6 H 6 80.4 g 80.8%

Limiting Reactant and Percent Yield Practice Problem  Answer the following questions for the combustion of 5.00 g octane (C 8 H 18 ) in 17.5 L O 2 .

2 C 8 H 18 + 25 O 2 → 16 CO 2 + 18 H 2 O  Which compound is the limiting reactant?

 What is the percent yield if 14.6 g of CO 2 produced?

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Essential Questions

 What is a limiting reactant?

 How do you determine the limiting reactant?

 What is the theoretical and actual yield?

 What is percent yield and how do you calculate it?

7.2 Tracked Assignment

 P 375 #33-35  P 379 #45-48(not 47c), 52, 64