Transcript Document 7919316

Stoichiometry
Limiting Reactions and Percentage Yield
Stoichiometry

Objectives

Describe a method for determining which of two
reactants is a limiting reactant

Calculate the amount in moles or mass in grams of a
product, given the amounts in moles or masses in
grams of two reactants, one of which is in excess

Distinguish between theoretical yield, actual yield,
and percentage yield

Calculate percentage yield, given the actual yield and
quantity of a reactant
Stoichiometry
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Limiting Reactions

The limiting reactant is the reactant that limits the amount
of the other reactant that can combine and the amount of
product that can form in a chemical reaction
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The excess reactant is the substance that is not used up
completely in a reaction
Stoichiometry

Limiting Reactants
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Sample Problem

Silicon dioxide (quartz) is usually quite unreactive but reacts readily
with hydrogen fluoride according to the following equation
SiO2(s) + 4HF(g) → SiF4(g) + 2H2O(l)

If 6.0 mol HF is added to 4.5 mol SiO2, which is the limiting
reactant?
Stoichiometry

Limiting Reactants
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Sample Problem Solution


Given: amount of HF = 6.0 mol
amount of SiO2 = 4.5 mol
Unknown: limiting reactant
mol SiF4
mol HF 
 mol SiF4 produced
mol HF
mol SiF4
mol SiO2 
 mol SiF4 produced
mol SiO2
Stoichiometry

Limiting Reactants
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Sample Problem Solution
1 mol SiF4
4.5 mol SiO2 
 4.5 mol SiF4 produced
1 mol SiO2
1 mol SiF4
6.0 mol HF 
 1.5 mol SiF4 produced
4 mol HF
HF is the limiting reactant.
Stoichiometry

Percentage Yield

The theoretical yield is the maximum amount of product
that can be produced from a given amount of reactant

The actual yield of a product is the measured amount of
that product obtained from a reaction

The percentage yield is the ratio of the actual yield to the
theoretical yield, multiplied by 100
actual yield
percentage yield 
 100
theorectical yield
Stoichiometry

Percentage Yield

Sample Problem

Chlorobenzene, C6H5Cl, is used in the production of many
important chemicals, such as aspirin, dyes, and disinfectants. One
industrial method of preparing chlorobenzene is to react benzene,
C6H6, with chlorine, as represented by the following equation
C6H6 (l) + Cl2(g) → C6H5Cl(l) + HCl(g)


When 36.8 g C6H6 react with an excess of Cl2, the actual yield of
C6H5Cl is 38.8 g.
What is the percentage yield of C6H5Cl?
Stoichiometry

Percentage Yield
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Sample Problem Solution
Given: mass of C6H6 = 36.8 g
mass of Cl2 = excess
actual yield of C6H5Cl = 38.8 g
 Unknown: percentage yield of C6H5Cl
Theoretical Yield

g C6H6 
mol C6H6
g C6H6
36.8 g C6H6 

mol C6H5Cl
g C6H5Cl

 g C6H5Cl
mol C6H6
mol C6H5Cl
1 mol C6H6
78.12 g C6H6

1 mol C6H5Cl
112.56 g C6H5Cl

1 mol C6H6
1 mol C6H5Cl
 53.0 g C6H5Cl
Stoichiometry
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Percentage Yield
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Sample Problem Solution
Percentage Yield
actual yield
percentage yield C6H5Cl 
 100
theorectical yield
38.8 g
percentage yield 
 100  73.2%
53.0 g
Stoichiometry
Stoichiometry
Sodium carbonate reacts with nitric acid according to
the following equation.
Na2CO3(s) + 2HNO3  2NaNO3 + CO2 + H2O
How many moles of Na2CO3 are required to produce
100.0 g of NaNO3?
Stoichiometry
Phosphorus burns in air to produce a phosphorus oxide
in the following reaction:
4P(s) + 5O2(g)  P4O10(s)
What mass of phosphorus will be needed to produce
3.25 mol of P4O10?
Stoichiometry
Aluminum will react with sulfuric acid in the following
reaction.
2Al(s) + 3H2SO4(l)  Al2(SO4)3(aq) + 3H2(g)
How many moles of H2SO4 will react with 18 mol Al?
Stoichiometry
In the commercial production of the element arsenic,
arsenic(III) oxide is heated with carbon, which
reduces the oxide to the metal according to the
following equation:
2As2O3 + 3C  3CO2 +4As
If 8.87 g of As2O3 is used in the reaction and 5.33 g of
As is produced, what is the percentage yield?
Stoichiometry
A process by which zirconium metal can be produced
from the mineral zirconium(IV) orthosilicate, ZrSiO4,
starts by reacting it with chlorine gas to form
zirconium(IV) chloride.
ZrSiO4 + 2Cl2  ZrCl4 + SiO2 + O2
What mass of ZrCl4 can be produced if 862 g of ZrSiO4
and 950. g of Cl2 are available?
Stoichiometry
Calculate the mass of silver bromide produced from
22.5 g of silver nitrate in the following reaction:
2AgNO3(aq) + MgBr2(aq) 2AgBr(s) + Mg(NO3)2(aq)
Stoichiometry
Use the following equation for the oxidation of
aluminum in the following problems.
4Al + 3O2  2Al2O3
Which reactant is limiting if 0.32 mol Al and 0.26 mol
O2 are available?
Stoichiometry
Hydrogen peroxide breaks down, releasing oxygen, in
the following reaction:
2H2O2(aq)  2H2O(l) + O2(g)
What mass of oxygen is produced when 1.840 mol of
H2O2 decomposes?
Stoichiometry
Hydrogen is generated by passing hot steam over iron,
which oxidizes to form Fe3O4, in the following
equation.
3Fe(s) + 4H2O(g)  4H2(g) + Fe3O4(s)
If 625 g of Fe3O4 is produced in the reaction, how many
moles of hydrogen are produced at the same time?
Stoichiometry
Aluminum oxidizes according to the following
equation:
4Al + 3O2  2Al2O3
Powdered Al (0.048 mol) is placed into a container
containing 0.030 mol O2. What is the limiting
reactant?