Transcript Chapter 9 Notes Part III Limiting Reagents & Percent Yield What are limiting reagents? • Up until now, we have assumed that all reactants.

Chapter 9 Notes Part III
Limiting Reagents
& Percent Yield
What are limiting reagents?
• Up until now, we have assumed
that all reactants are used up in
a reaction.
• In actuality, usually one
chemical runs out first,
stopping the reaction—that
chemical is called the limiting
reagent.
The limiting reagent limits or
determines the amount of product
that can be formed in a reaction.
On the flip side, the excess reagent
is the one not completely used up.
How do you find what the
limiting reagent is?
• To find the limiting reagent, use
dimensional analysis to determine
how much of the product will be
formed by both reactants.
• The smaller of the two results will
indicate which reactant is the
limiting reagent.
Here’s an example:
In the reaction:
Mg + HCl a MgCl2 + H2
you combine 35.5 grams of
magnesium with 28.1 grams of
hydrochloric acid. Which one is
the limiting reagent, and how
much magnesium chloride is
produced?
Percent Yield
• In the real world, you never form
100% of the products that you think
you should based on theoretical
calculations.
• By comparing how much of a
product forms to how much should
form based on stoichiometry, you
can find the percent yield.
Percent Yield
%Yield= actual yield
theoretical yield
Practice Problem
• In the reaction of nitrogen gas
and hydrogen gas producing
ammonia (NH3), 250.2g H2
produces 1095.1g NH3. What
is the % yield of this reaction?
Rxn #1—A Precipitation Rxn
K2CrO4 + Pb(NO3)2 a KNO3 + PbCrO4
• If 0.947g K2CrO4 are
combined with 0.331g
Pb(NO3)2 what will be the
limiting reagent, and how
much lead chromate is
produced?
Rxn #2—Whoosh!
• C3H8O + O2 a CO2 + H2O
• If 3.78 L of O2 are combined
with 7.9 g of C3H8O, what is the
limiting reagent, and how much
water is produced?
Rxn #3—No, it’s not blood...
K2CrO4 + AgNO3 a KNO3 + Ag2CrO4
• If 1.17g K2CrO4 are combined with
1.23g AgNO3, what is the limiting
reagent and how much Ag2CrO4 is
produced?
Rxn #4—Hindenburg, 2004
• Zn + HCl a ZnCl2 + H2
• What is the limiting reagent and
how many liters of hydrogen
are produced if 15.5 grams of
zinc are combined with 26.3
grams of hydrochloric acid?
Rxn #5—Kaboom!
• Na + H2O a NaOH + H2
• If 2.1g of sodium is added to 3.8g
of water, what is the limiting
reagent, and how many grams of
sodium hydroxide are produced?
In Review:
What is a limiting reagent?
What is the excess reagent?
How does a limiting reagent control
the amount of product?