Transcript Motion

Section 3: Gas Stoichiometry
When gases react, the coefficients in the balanced chemical
equation represent both molar amounts and relative volumes.
K
What I Know
W
What I Want to Find Out
L
What I Learned
• 9(B) Perform stoichiometric calculations, including
determination of mass and volume relationships between
reactants and products for reactions involving gases.
• 8(E) Perform stoichiometric calculations, including determination of
mass relationships between reactants and products, calculation of
limiting reagents, and percent yield.
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Gas Stoichiometry
Essential Questions
• What stoichiometric ratios can be determined for gaseous
reactants and products from balanced chemical
equations?
• How are the amounts of gaseous reactants and products
in a chemical reaction calculated?
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Gas Stoichiometry
Vocabulary
Review
• coefficient
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Gas Stoichiometry
Stoichiometry of Reactions Involving Gases
The gas laws can be applied to calculate the stoichiometry of reactions in
which gases are reactants or products.
2H2(g) + O2(g) → 2H2O(g)
2 mol H2 reacts with 1 mol O2 to produce
2 mol water vapor.
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Gas Stoichiometry
Stoichiometry and Volume-Volume Problems
Coefficients in a balanced equation represent volume ratios for gases.
•
A balanced chemical equation allows you to find ratios for only moles and
gas volumes, not for masses.
•
All masses given must be converted to moles or volumes before being
used as part of a ratio.
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Gas Stoichiometry
Stoichiometry and Volume-Volume Problems
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Gas Stoichiometry
VOLUME–VOLUME PROBLEMS
SOLVE FOR THE UNKNOWN
Use with Example Problem 7.
Problem
What volume of oxygen gas is needed for
the complete combustion of 4.00 L of
propane gas (C3H8)? Assume that pressure
and temperature remain constant.
Response
•
KNOWN
UNKNOWN
VC3H8= 4.00 L
V02 = ? L
Write the balanced equation.
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O(g)
•
ANALYZE THE PROBLEM
You are given the volume of a gaseous
reactant in a chemical reaction. Remember
that the coefficients in a balanced chemical
equation provide the volume relationships of
gaseous reactants and products.
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Use the balanced equation for the
combustion of C3H8. Find the volume ratio
for O2 and C3H8, then solve for VO2.
Find the volume ratio for O2 and C3H8
5 volumes O2
1 volume C3H8
•
Multiply the known volume of C3H8 by
the volume ratio to find the volume of O2.
VO2 = (4.00 L C3H8) ×
5 volumes O2
1 volume C3H8
= 20.0 L O2
Gas Stoichiometry
VOLUME–VOLUME PROBLEMS
EVALUATE THE ANSWER
The coefficients in the combustion
equation show that a much larger volume
of O2 than C3H8 is used up in the
reaction, which is in agreement with the
calculated answer. The unit of the answer
is liters, a unit of volume, and there are
three significant figures.
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Gas Stoichiometry
VOLUME–MASS PROBLEMS
Use with Example Problem 8.
Problem
Ammonia is synthesized from hydrogen
and nitrogen. N2(g) + 3 H2(g)→2NH3(g)
If 5.00 L of nitrogen reacts completely
with hydrogen at a pressure of 3.00 atm
and a temperature of 298 K, how much
ammonia, in grams, is produced?
Response
ANALYZE THE PROBLEM
You are given the volume, pressure, and
temperature of a gas sample. The mole and
volume ratios of gaseous reactants and
products are given by the coefficients in the
balanced chemical equation. Volume can be
converted to moles and thus related to mass
by using molar mass and the ideal gas law.
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KNOWN
UNKNOWN
VN2 = 5.00 L
mNH3 = ? g
P = 3.00 atm
T = 298 K
SOLVE FOR THE UNKNOWN
Determine how many liters of gaseous
ammonia will be made from 5.00 L of
nitrogen gas.
•
Find the volume ratio for N2 and NH3
using the balanced equation.
1 volume N2
2 volumes NH3
•
Multiply the known volume of N2 by the
volume ratio to find the volume of NH3.
5.00 L N2
2 volumes NH3
= 10.0 L NH3
1 volume N2
Gas Stoichiometry
VOLUME–MASS PROBLEMS
•
Find the molecular mass of NH3.
M=
1 N atom ×14.01 amu
3 H atoms ×1.01 amu
+
1 N atom
1 H atom
SOLVE FOR THE UNKNOWN (continued)
= 017.04 amu
Use the ideal gas law. Solve for n, and
calculate the number of moles of NH3.
• State the ideal gas law.
PV = nRT
• Solve for n.
𝑃𝑉
𝑛=
R𝑇
• Substitute P = 3.00 atm, VNH3 = 10.0 L,
and T = 298 K.
3.00 atm (10.0 L)
𝑛=
L ∙ atm
(0.0821
298 K
mol ∙ K
• Multiply and divide numbers and units.
M = 17.04 g/mol
𝑛=
3.00 atm (10.0 L)
(0.0821
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L∙atm
mol∙K
298 K
= 1.23 mol NH3
Convert moles of ammonia to grams of
ammonia.
•
Use the molar mass of ammonia as a
conversion factor.
•
1.23 mol NH3 ×
17.04 g NH3
= 21.0 g NH3
1 mol NH3
EVALUATE THE ANSWER
To check your answer, calculate the volume
of reactant nitrogen at STP. Then, use molar
volume and the mole ratio between N2 and
NH3 to determine how many moles of NH3
were produced. The unit of the answer is
grams, a unit of mass. There are three
significant figures.
Gas Stoichiometry
Review
Essential Questions
• What stoichiometric ratios can be determined for gaseous
reactants and products from balanced chemical equations?
• How are the amounts of gaseous reactants and products in a
chemical reaction calculated?
Copyright © McGraw-Hill Education
Gas Stoichiometry