Ch4.Stoichiometry
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Transcript Ch4.Stoichiometry
Chemical
Reactions
Chapter 4
Stoichiometry
1
Balancing Chemical Equations
“Matter is conserved in
chemical change”
Antoine Lavoisier, 1789
An equation must be balanced:
It must have the same number
of atoms of each kind on both sides
4
Combustion Reactions
In combustion, a hydrocarbon or C–H–O fuel
combines with O2 to form CO2 and H2O
__ CH4 + __ O2 __ CO2 + __ H2O
1 CH4 + 2 O2 1 CO2 + 2 H2O
Balanced equation shows 1 C, 4 H, and 4 O on each side
If N or S are in the formula for the fuel, assume it
is oxidized to NO2 or SO2
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Example 4-2
Write a balanced equation for the complete
combustion of glycerol, C3H8O3
Write a balanced equation for the complete
combustion of thiosalicylic acid, C7H6O2S
8
Stoichiometry
Stoichiometry is chemical accounting
The heart of stoichiometry is the mole ratio given
by the coefficients of the balanced equation
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Stoichiometry
Stoichiometry is chemical accounting
The heart of stoichiometry is the mole ratio given
by the coefficients of the balanced equation
moles A
mole ratio
moles B
moles A
moles B
10
Example 4-3B
How many moles of Ag are produced in the
decomposition of 1.00 kg of silver (I) oxide:
2 Ag2O (s) 4 Ag (s) + O2 (g)
12
Example 4-6B
The model problem describes
an Al-Cu alloy composed of
93.7% Al and 6.3% Cu by
mass, with a density of 2.85
g/cm3. The Al (but not the Cu)
reacts with HCl:
2 Al (s) + 6 HCl (aq) 2 AlCl3 (aq) + 3 H2 (g)
How
many grams of Cu are present in a sample of
alloy that yields 1.31 g H2 when it reacts with HCl?
18
Example 4-7B
A vinegar contains 4.0% HC2H3O2 by mass and
has a density of 1.01 g/mL. It reacts with sodium
hydrogen carbonate:
HC2H3O2 (aq) + NaHCO3 (s)
NaC2H3O2 (aq) + H2O (l) +
CO2 (g)
How many grams of CO2 are produced by the
reaction of 5.00 mL of this vinegar with NaHCO3?
20
Chemical Reactions in Solution
Most reactions occur in aqueous solution
SOLUTE is the substance to be dissolved in solution
SOLVENT is the substance (often a liquid) the solute
dissolves in
The concentration of the solution is
Molarity (M) = moles solute
L solution
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Example 4-8B
15.0 mL of concentrated acetic acid, HC2H3O2
(d = 1.048 g/mL), are dissolved in enough water to
produce 500.0 mL of solution. What is the
concentration of the solution?
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Example 4-9B
How many grams of Na2SO4 • 10 H2O are needed
to prepare 355 mL of 0.445 M Na2SO4?
25
Dilution problems
It is common to prepare a solution by diluting a
more concentrated solution (the stock solution).
The moles of solute taken from the stock solution
are given by moles solute = volume x molarity
All the solute taken from the stock appears in the
diluted solution, so moles solute are constant:
VstockMstock = VdiluteMdilute
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Example 4-10A
15.00 mL of 0.450 M K2CrO4 solution are diluted
to 100.00 mL. What is the concentration of the
dilute solution?
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Example 4-10B
After being left out in an open beaker, 275 mL of
0.105 M NaCl has evaporated to only 237 mL.
What is the concentration of the solution after
evaporation?
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Stoichiometry in Solution
Stoichiometry in solution is just the same as for
mass problems, except the conversion into or out
of moles uses molarity instead of molar mass:
grams A
grams B
moles A
mL A
mole ratio
moles B
moles A
moles B
mL B
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Example 4-11B
K2CrO4 (aq) + 2 AgNO3 (aq) Ag2CrO4 (s) + 2 KNO3 (aq)
How many mL of 0.150 M AgNO3 must react with
excess K2CrO4 to produce exactly 1.00 g
Ag2CrO4?
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Limiting reactant
In a given reaction, often there is not enough of
one reactant to use up the other reactant completely
The reactant in short supply LIMITS the quantity
of product that can be formed
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33
Goldilocks Chemistry
Imagine reacting different amounts of Zn with
0.100 mol HCl:
Zn (s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)
Mass Zn
Moles Zn
Moles HCl
Ratio mol HCl
mol Zn
Rxn 1
6.54 g
0.100 mol
0.100 mol
1.00
Rxn 2
3.27 g
0.0500 mol
0.100 mol
2.00
Rxn 3
1.31 g
0.0200 mol
0.100 mol
5.00
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Limiting reactant problems
The easiest way to do these is to do two
stoichiometry calculations
Find the amount of product possible from each reactant
The smaller answer is the amount of product you
can actually make (you just ran out of one reactant)
The reactant on which that answer was based is the
limiting reactant
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Example 4-13A
When 215 g P4 react with 725 g Cl2
P4 (s) + 6 Cl2 (g) 4 PCl3 (l) (example 4-12A)
which reactant is in excess and what mass of that
reactant remains after the reaction is finished?
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Example 4-13B
12.2 g H2 and 154 g O2 are allowed to react.
Identify the limiting reactant, which gas remains
after the reaction, and what mass of it is left over.
2 H2 (g) + O2 (g) 2 H2O (l)
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Percent Yield
In real experiments we often do not get the amount
of product we calculate we should, because
the reactants may participate in other reactions (side
reactions) that produce other products (by-products)
The reaction often does not go to completion.
Percent yield tells the ratio of actual to theoretical
amount formed.
40
Percent Yield
Suppose you calculate that a reaction will produce
50.0 g of product. This is the theoretical yield.
The reaction actually produces only 45.0 g of
product . This is the actual yield.
Percent yield = 45.0 g (actual) x 100 = 90.0%
50.0 g (theoretical)
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Example 4-14B
What is the percent yield if 25.0 g P4 reacts with
91.5 g Cl2 to produce 104 g PCl3:
P4 (s) + 6 Cl2 (g) 4 PCl3 (l)
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Example 4-15B
What mass of C6H11OH should you start with to
produce 45.0 g C6H10 if the reaction has 86.2%
yield and the C6H11OH is 92.3% pure:
C6H11OH (l) C6H10 + H2O (l)
45
Exercise 26
Balance these equations by inspection
(NH4)2Cr2O7 (s) Cr2O3 (s) + N2 (g) + H2O (g)
NO2 (g) + H2O (l) HNO3 (aq) + NO (g)
H2S (g) + SO2 (g) S (g) + H2O (g)
SO2Cl2 + HI H2S + H2O + HCl + I2
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Exercise 30
Write balanced equations for these reactions:
Sulfur dioxide gas with oxygen gas to produce sulfur
trioxide gas
Solid calcium carbonate with water and dissolved
carbon dioxide to produce aqueous calcium hydrogen
carbonate
Ammonia gas and nitrogen monoxide gas to produce
nitrogen gas and water vapor
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Exercise 32
3 Fe (s) + 4 H2O (g) Fe3O4 (s) + H2 (g)
How many moles of H2 can be produced from 42.7 g Fe
and excess steam?
How many grams of H2O are consumed in the
conversion of 63.5 g Fe to Fe3O4?
If 7.36 mol H2 are produced, how many grams of Fe3O4
must also be produced?
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Exercise 36
Silver oxide decomposes above 300 °C to yield
metallic silver and oxygen gas. 3.13 g impure
silver oxide yields 0.187 g O2. Assuming there is
no other source of O2, what is the % Ag2O by mass
in the original sample?
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Exercise 42
How many grams of CO2 are produced in the
complete combustion of 406 g of a bottled gas that
consists of 72.7% C3H8 (propane) and 27.3%
C4H10 (butane), by mass?
50
Exercise 45
What are the molarities of these solutes?
150.0 g sucrose (C12H22O11) in 250.0 mL aqueous
solution
98.3 mg of 97.9% pure urea, CO(NH2)2, in 5.00 mL
aqueous solution
12.5.0 mL methanol (CH3OH, density = 0.792 g/mL) in
15.0 L aqueous solution
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Exercise 52
After 25.0 mL of aqueous HCl solution is diluted
to 500.0 mL, the concentration of the diluted
solution is found to be 0.085 M HCl. What was
the concentration of the original HCl solution?
52
Exercise 56
Ca(OH)2 (s) + 2 HCl (aq) CaCl2 (aq) + 2 H2O (l)
How many grams of Ca(OH)2 will react completely with 415 mL
of 0.477 M HCl?
How many kilograms of Ca(OH)2 will react with 324 L of an
HCl solution that is 24.28% HCl by mass, density = 1.12 g/mL?
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Exercise 63
0.3126 g oxalic acid, H2C2O4, is exactly
neutralized by 26.21 mL of a NaOH solution.
What is the concentration of the NaOH solution?
H2C2O4
+ 2 NaOH Na2C2O4 + 2 H2O
54
Exercise 70
Chlorine can be generated by heating calcium
hypochlorite and hydrochloric acid to form
chlorine gas, calcium chloride, and water. If 50.0 g
Ca(OCl)2 and 275 mL 6.00 M HCl react, how
many grams of Cl2 gas form? Which reactant is
left over, and how much (in grams)?
55
Exercise 72
2 C6H5NO2 + 4 C6H14O4 (C6H5N)2 + 4 C6H12O4 + 4 H2O
nitrobenzene
triethylene
glycol
azobenzene
If 0.10 L nitrobenzene (d = 1.20 g/mL) react with
0.30 L triethylene glycol (d = 1.12 g/mL) to form
55 g azobenzene, find
Theoretical yield
Actual yield
Percent yield
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