Chapter 9, Section 2. Pages 312-319
Limiting Reactants and Oercentage Yield
Objectives
To identify the limiting reactant and use it to calculate the theoretical yield
To perform calculations involving percentage yield.
Limiting Reactant and Theoretical Yield
How much gas you have in your car decides how far you can go. Gas is the limiting
factor.
Making muffins and you have 3 eggs, 100 cups. flour, 200 cups sugar. 24 muffins can be
made with 3 eggs, 1 cup flour, ½ cup sugar. What is the limiting “reactant”? Why?
Likewise the reactants in an chemical reaction are seldom available in the exact quantities
stated in a balanced equation.
Zn + 2 HCl ----------> ZnCl + H2
If you have 0.23 mol Zn and 0.60 mol HCl would they react completely? No, looking at
the balanced equation above you can see that for every 1 mol of Zn you need 2 mol of
HCl. So, for every 0.23 mol Zn you would need 0.46 mol of HCl. In the example given
you have excess HCl (0.60 mol, 0.46 mol is all you need) but not enough 0.23 mol Zn.
Zn is the LIMITING REACTANT.
Limiting Reactant—the substance that controls the quantity of product that can form in a
chemical reaction, Zn
Excess Reactant—the substance that is not used up completely in a reaction. HCl
Determining Theoretical Yield from the Limiting Reactant
Identify the limiting reactant and the theoretical yield of phosphoric acid H3PO3 if 225g
of PCl3 is mixed with 123g of H2O.
PCl3 + 3H2O------------> H3PO3 + 3HCl
1. What do you know? PCl3=137.5g/mol, H2O=18g/mol
2. Plan your work, PCl3 =225/137.5=1.636 mol H2O, 123/18=6.833 mol
3. Calculate ratio is 1:3, 1.66x 3 =4.9, PCl3 limiting, H2O excess, 82 x 1.636=134g
H3PO3
See Sample Problem E page 314
Actual Yield and Percentage Yield
The actual yield of a reaction may be less than the theoretical yield. There are many
reasons. Sometimes the reaction may reverse, the products turn back into the reactants.
See table 1 on page 316
Determining the Percentage Yield
The percentage yield is the ratio of the actual yield to the theoretical yield. In the
reaction noted in Table 1 the percentage yield is,
Percentage Yield =
Actual Yield
Theoretical Yield
X 100
Calculating the Percentage Yield
Determine the limiting reactant, the theoretical yield and the percentage yield if 14.0g N2
are mixed with 9.0 g H2 and 16.1 g NH3
N2 + 3H2---------> 2NH3
1. What do you know?
Mass N2 = 14.0g
molar mass N2 = 28.02 g/mol
Mass H2 = 9.0g
molar masss H2 = 2.02 g/mol
Theoretical yield NH3 = ?
Actual yield of NH3 = 16.1g NH3
1 mol N2 = 2 mol NH3 and 3 mol H2 = 2 mol NH3
2. Plan you work
Set up your equation to solve the problem
3. Calculate
? g NH3 = 14.0g N2 x 1 mol N2 x 2 mol NH3 x 17.04 g NH3 = 17.0g NH3
28.02gN2
1 mol N2
1 mol NH3
? g NH3 = 9.0g H2 x 1 mol H2 x 2 mol NH3 x 17.04 g NH3 = 51g NH3
2.02 g H2 3mol H2
1 mol NH3
The smaller quantity made is 17.0 g so N2 is the limiting reactant
Percentage yield = 16.1 actual yield x 100 = 94.7%
17.0g (theor.yield)
Determining the Actual Yield
How many grams of CH3COOC5H11 should form if 4808g are theoretically possible
and the percentage yield for the reaction is 80.5%?
1. What do you know?
Theor. Yield CH3COOC5H11 = 4808g CH3COOC5H11
Actual Yield “
= ? g CH3COOC5H11
2. Plan your work.
3. Calculate
80.5% = actual yield x 100 = 3.87 x 10 3 g CH3COOC5H11
4808g
Practice Problems pg. 318, 1, 2, 3
Conclusion
Review key concepts
Handout to be given out in class