A simple ppt for going over excess and limiting reactants.

1. Limiting Reactants and % Yield

2. Limiting Factors
 Limiting factors are those factors which
control the speed or the outcome of a
reaction or a situation
 Have you ever been behind a school bus in
the morning?
 You can only go as fast as the bus in front of
you.
 The bus one limiting factor preventing you
from getting to school on time.

3. Limiting Reactants
Ingredients for making “S’mores”:
2 graham crackers
1 marshmallow
1 Hershey bar
How many “S’mores” can you make with 14 graham
crackers, 5 marshmallows, and 6 Hershey bars?
5 “S’mores”
How many of each ingredient are left over?
0 marshmallows
4 graham crackers
1 Hershey bar

4. Limiting Reactant
 The limiting reactant is the reactant
which controls the rate of reaction or
the amount of product that can be
made
 The limiting reactant is determined
using stoichiometric relationships

5. Excess
 The reactant that you have more than
you need to perform a chemical
reaction is called the excess reactant
 It is not completely used up in a
chemical reaction.

6. Here’s an example
 You are making ham and cheese sandwiches
and you have:
 5 pieces of ham
 5 pieces of cheese
 8 pieces of bread
 Which of these ingredients do you have more
than enough of (excess reactant)?
 Which of these do you not have enough of
(limiting reactant?

7. Yield
 Yield is how much product that you produce
 In industry, the amount of yield needs to be
calculated so that production schedules can
be made
 You also need to know how much product
you are going to be making so that you will
have the appropriate size collection
container

8. Real or Not?
 Actual yield refers to the amount of
product that is actually generated
 Theoretical yield refers to the amount
of product that you expect to generate

9. Calculating Yield
 In order to calculate the theoretical yield
you need to
 Write a balanced chemical equation.
 Calculate the amount of product produced
from each reactant individually.
 The reactant that produces the least amount
of product is the limiting reactant.
 The reactant that you have enough of is the
excess reactant.

10. Percentage Yield
You recently bought
a new car. The EPA
sticker says that you
should get 28 mpg in
“average driving”.
Your actual miles per
gallon turns out to
be less than 28 mpg.
Why?

11. Theoretical Yield vs. Actual
Yield
You perform a mass-
mass calculation to
determine how much
chemical should be
produced in a reaction.
The actual results of
the experiment
produce less chemical
than calculated.
Why?

12. Calculating Percent Yield
1. Calculate the theoretical yield
(mass-mass calculation).
2. Determine the actual yield.
3. Calculate the % yield
% Yield = Actual yield x 100
Theoretical yield

13. A Percent Yield Problem
A chemist starts with 1.75 g of salicylic acid
(C7H6O3) and excess methanol (CH3OH) and
reports the production of 1.42 g oil of
wintergreen (C8H8O3) in the following reaction:
C7H6O3 + CH3OH  C8H8O3 + H2O
What is the percent yield for this reaction?

14. Solving the Problem
C7H6O3 + CH3OH  C8H8O3 + H2O
1.
2.
1.75 g ?g
3. 1.75 g x 1 mol =
138 g
0.0127 mol C7H6O3
4. 0. 0127 mol C7H6O3 x 1 molC8H8O3 =
1 mol C7H6O3
0.0127 mol C8H8O3
5. 0.0127 mol C8H8O3 x 152 g =
1 mol
1.93 g C8H8O3

15. % Yield cont.
6. (Actual Yield ÷ Theoretical Yield) x 100
(1.42 g ÷ 1.93 g) x 100 =
73.6 % Yield

17. Practice Problems
1. A chemist carried out a reaction that should produce 21.8 g
of a product, according to a mass-mass calculation.
However, the chemist was able to recover only 13.9 g of
the product. What percentage yield did the chemist get?
2. A calculation indicates that 82.2 g of a product should be
obtained from a certain reaction. If a chemist actually gets
30.7 g, what is the percentage yield?
3. Chromium(III) hydroxide will dissolve in sodium
hydroxide according to the following equation:
NaOH + Cr(OH)3  NaCr(OH)4
If you begin with 66.0 g of Cr(OH)3 and obtain 38.4 g of
product, what is your % yield?

18. Solutions
#1. (13.9g / 21.8g) x 100 = 63.8%
63.8%
#2. (30.7g / 82.2g) x 100 = 37.3%
37.3%
#3. NaOH + Cr(OH)3  NaCr(OH)4
66.0g ?g
66.0g Cr(OH)3 x 1mol/103g = 0.641mol Cr(OH)3
= 0.641mol NaCr(OH)4 x 143g / 1mol NaCr(OH)4 =
91.66g NaCr(OH)4
(38.4g / 91.7g) x 100 = 41.9%
41.9%

19. Practice Problem
Identify the limiting reactant and the theoretical
yield of H3PO3 if 225 g of PCl3 is mixed with 125 g of
H2O
PCl3 + 3H2O  H3PO3 + 3HCl
Convert each mass to moles:
225 g PCl3 x 1 mol/137 g = 1.64 mol PCl3
125 g H2O x 1 mol/ 18 g = 6.94 mol H2O
1.64 mol PCl3 requires 4.92 mol H2O
PCl3 is the limiting reactant
1.64 mol PCl3 = 1.64 mol H3PO3
1.64 mol H3PO3 x 82 g/1 mol = 134 g H3PO3
225g 125g ?g