Chemical
Equilibrium
Chapter 18
Modern Chemistry
Sections 1 & 2
The Nature of Chemical Equilibrium
Shifting Equilibrium
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
1
Section 18.1
The Nature of
Chemical
Equilibrium
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
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Vocabulary
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Reversible Reaction
Chemical Equilibrium
Equilibrium Expression
Equilibrium Constant
LeChatelier’s Principle
Insert Holt Disc 2
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
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Reversible Reactions
Insert Holt Disc 2
Insert Glencoe Disc 1
Chapter 18 Section 1 ChemicaL Equilibrium
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Reversible Reactions
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Products can react to re-form the reactants.
Must occur in a “closed” system
2HgO(s)  2Hg(l) + O2(g)
2Hg(l) + O2(g) 2HgO(s)
Both of these reactions
occur simultaneously
2HgO(s)  2Hg(l) + O2(g)
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
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Chemical Equilibrium
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Rate of its forward reaction equals the
rate of its reverse reaction ….
and the concentrations of its products and
reactants remain unchanged
Eventually all reversible reactions will reach
eq. if the system is closed and conditions
don’t change.
Eq. is dynamic – always in motion.
Chapter 18 Section 1 ChemicaL Equilibrium
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Reaction Rate vs time
p. 591
Chapter 18 Section 1 ChemicaL Equilibrium
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Equilibrium Demonstration
Chapter 18 Section 1 ChemicaL Equilibrium
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Reaction Rate vs. Time
p. 591
Rateforward = Ratereverse
Chapter 18 Section 1 ChemicaL Equilibrium
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What is “favored” at Eq?

At equilibrium
equal rates!
reactants are favored
neither is favored
products are favored
Chapter 18 Section 1 ChemicaL Equilibrium
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Equilibrium Expression
n A + m B  x C + y D
[ ] = concentration
in mol/L
x
y
[C] [D]
Keq =
[A]n [B]m
=
products
reactants
x, y, n, m = coefficients


Dependant on temperature
Independent of initial concentrations
Chapter 18 Section 1 ChemicaL Equilibrium
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Equilibrium Constant

If Keq is large (>1)
then products are
favored at eq.
Keq =

If Keq is small (<1)
then reactants are
favored at eq.
Keq
=
PRODUCTS
REACTANTS
PRODUCTS
REACTANTS
Pure liquids and solid are omitted.
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
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Equilibrium Constants Table
Chapter 18 Section 1 ChemicaL Equilibrium
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Sample Problem p. 594

An equilibrium mixture of N2, O2 , and NO
gases at 1500 K is determined to consist of
6.4x10–3 mol/L of N2, 1.7x10–3 mol/L of O2,
and 1.1x10–5 mol/L of NO. What is the
equilibrium constant for the system at this
temperature?
Keq = 1.1 x 10−5
Chapter 18 Section 1 ChemicaL Equilibrium
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Practice Problems p. 595 #1

At equilibrium a mixture of N2, H2, and NH3
gas at 500°C is determined to consist of
0.602 mol/L of N2, 0.420 mol/L of H2, and
0.113 mol/L of NH3.What is the equilibrium
constant for the reaction
N2(g) + 3H2(g)  2NH3(g) at this temperature?
0.286
Chapter 18 Section 1 ChemicaL Equilibrium
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Practice Problems p. 595 #2

The reaction AB2C(g)  B2(g) + AC(g)
reached equilibrium at 900 K in a 5.00 L
vessel. At equilibrium 0.084 mol of AB2C,
0.035 mol of B2, and 0.059 mol of AC were
detected. What is the equilibrium constant
at this temperature for this system? (Don’t
forget to convert amounts to
concentrations.)
4.9 x 10−3
Chapter 18 Section 1 ChemicaL Equilibrium
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Practice Problems p. 595 #3

A reaction between gaseous sulfur dioxide
and oxygen gas to produce gaseous sulfur
trioxide takes place at 600°C.At that
temperature, the concentration of SO2 is
found to be 1.50 mol/L, the concentration of
O2 is 1.25 mol/L, and the concentration of
SO3 is 3.50 mol/L. Using the balanced
chemical equation, calculate the equilibrium
constant for this system.
4.36
Chapter 18 Section 1 ChemicaL Equilibrium
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Equilibrium Concentrations
and Keq values
p. 593
Chapter 18 Section 1 ChemicaL Equilibrium
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Eq. Concentration Problem

For the decomposition reaction of ammonia,
0.75 M of ammonia are added to an empty 1
L flask. When the reversible reaction has
achieved equilibrium the concentration of
nitrogen in the flask is 0.15 M. Find the
equilibrium concentrations of hydrogen and
ammonia. Also find the Keq.
Chapter 18 Section 1 ChemicaL Equilibrium
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Determining Eq. Concentrations
N2 (g)+3 H2 (g)  2NH3(g)
I
initial
C
change
E
equil.
1/3 = 0.15/x
0M
0M
+0.15 M
+0.45 M
0.15 M
0.45 M
x = 0.45M
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
0.75 M
20
Determining Eq. Concentrations
N2 (g)+3 H2 (g)  2NH3(g)
I
initial
C
change
E
1/2 = 0.15/x
0M
0M
0.75 M
+0.15 M
+0.45 M
-0.30 M
0.15 M
0.45 M
0.45 M
x = 0.30M
equil.
If this side is + then the other side is -.
Chapter 18 Section 1 ChemicaL Equilibrium
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Eq. Concentration Problem

For the synthesis reaction of hydrogen and
iodine, 0.20M of hydrogen and 0.30M of
iodine are added to an empty 1 L flask.
When the reversible reaction has achieved
equilibrium the concentration of hydrogen in
the flask is 0.10 M. Find the equilibrium
concentrations of iodine and hydrogen
iodide. Also find the Keq.
Chapter 18 Section 1 ChemicaL Equilibrium
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Determining Eq. Concentrations
H2 (g) + I2 (g) 
I
initial
C
change
E
equil.
2HI(g)
1/1 = 0.10/x
0.20 M
0.30 M
-0.10 M
-0.10 M
0.10 M
0.20 M
x = 0.10M
Chapter 18 Section 1 ChemicaL Equilibrium
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0M
23
Determining Eq. Concentrations
H2 (g) + I2 (g) 
I
initial
C
change
E
1/2 = 0.10/x
2HI(g)
0.20 M
0.30 M
0M
-0.10 M
-0.10 M
+0.20 M
0.10 M
0.20 M
0.20 M
x = 0.20M
equil.
If this side is - then the other side is +.
Chapter 18 Section 1 ChemicaL Equilibrium
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Eq Concentration Problem

A 0.20 M solution of HC2H3O2 is 5.0%
ionized. Find the equilibrium
concentrations of H+, C2H3O21- and
HC2H3O2, also find the Keq.
answer
Chapter 18 Section 1 ChemicaL Equilibrium
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Eq Conc. & Keq Problems


At a specific temperature and pressure, 1.2
moles of hydrogen, 0.40 moles of nitrogen and
1.3 moles of ammonia are put into a closed
one liter flask. When allowed to reach
equilibrium the amount of ammonia is 1.6
moles. Find the Keq for this system.
N2 (g) + 3H2 (g)  2NH3 (g)
answer
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
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Eq Conc. & Keq Problems

When .56 moles of SO3 is placed in a liter
container, some of it decomposes. The
equilibrium concentrations of SO2 is 0.42
moles / liter. Calculate the equilibrium
concentration of O2 and SO3 and the Keq.
O2 = 0.21 M SO3 = 0.14 M Keq = .53
Chapter 18 Section 1 ChemicaL Equilibrium
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Ch 18 Sec 1 Homework
Page 595 # 1-9
Chapter 18 Section 1 ChemicaL Equilibrium
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Ch 18 Sec 1 Homework
Equilibrium Concentrations and Keq
Worksheet
Chapter 18 Section 1 ChemicaL Equilibrium
p. 589-597
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