Transcript Limiting Reagents/% Yield Chapter 9 Notes, part II What are limiting reagents? • Up until now, we have assumed that all reactants are used.

Limiting Reagents/% Yield
Chapter 9 Notes, part II
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?
• Set up as many sets of
dimensional analysis as pieces of
data you are given.
• Solve each, and the one with the
lowest amount of product formed
is the correct answer.
• The chemical you started with 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 a real world setting, you never
form 100% of the products that
you think you should based on
theoretical calculations.
• By comparing how much of a
product forms and how much
should formed based on
stoichiometry, you can find the
percent yield.
Percent Yield
• %Yield= actual yield
theoretical yield
x100
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?
Practice Problem
• 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
many grams of lead chromate is
produced?
Practice Problem
• 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?
Tougher Practice Problem
• If 12.2mL of ethanol (C2H5OH)
are added to 3.78L of O2 and
the ethanol undergoes
combustion, how many
milliliters of water are
formed? Density of water is
1.00g/mL and density of
ethanol is 0.779g/mL.
Practice Problem
• In the reaction:
Mg3N2 + 6H2O a 3Mg(OH)2 + 2NH3
• If the initial amount of Mg3N2 is 58.1g
and the initial amount of H2O is 20.4g,
and 26.2g of Mg(OH)2 are produced,
what is the percent yield of the
reaction?
In Review:
What is a limiting reagent?
What is the excess reagent?
How does a limiting reagent control the
amount of product?
What is percent yield?