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2/17/2010
Chemical equilibrium
Reading: Chapter 15 (omit 15.7)
As you read ask yourself …
What is meant by chemical equilibrium?
How does the equilibrium constant expression depend on the nature of the
species (liquids, solids, gases) involved in the equilibrium?
How does the magnitude of the equilibrium constant provide information about
the relative amounts of reactants and products at equilibrium?
How can the direction of a reaction (towards products or toward reactants)
be predicted based on the starting amounts?
q
How can the value of the equilibrium
constant be obtained from amounts
of reactants and products present in equilibrium?
How can the equilibrium concentrations of reactants and products be
obtained from the values of the equilibrium constant?
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Chemical equilibrium
we have studied physical equilibria
con
ncentration (mol L-1)
chemical reactions can g
go forward as well as in the reverse direction
concentrations of reactants and products become constant over time
2HI J H2 + I2
H2 + I2 J 2HI
HI
HI
time J
H2, I2
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H2, I2
time J
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dynamic equilibrium
CO(g) + 2 H2(g)
CH3OH(g)
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how can we describe the equilibrium?
a specific ratio of concentrations is constant
law of mass action:
aA+bB
dD+eE
CO(g) + 2 H2(g)
CH3OH(g)
based on the balanced chemical equation
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Expt. 1
CH3OH 0.08
CO
0.9
0.8
H2
Expt. 2
0.25
0.75
1.50
Expt. 3
0.6
1.4
1.8
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K depends on the equilibrium conditions and the nature of the reaction
concentrations can be expressed in molarity (moles/L)
or in partial pressures, PCO
2 SO2(g) + O2(g)
2 SO3(g)
recall: PV = nRT,, so P = (n/V)
(
) RT
when substituted into KP expression:
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The equilibrium constant does not have units
K is related to rates of reactions and to thermodynamics
in thermodynamics the equilibrium constant is defined in terms of
activities
activity is defined as a ratio of actual concentration or
pressure to concentration or pressure of a reference state
reference states:
1 M solutions, 1 atm for partial pressures
activity = 1 for pure solids and pure liquids
aCO = [CO]/co where a is activity and c0 is the reference value 1 M
units cancel in ratios
we will ignore slight numerical differences between
activities and concentrations or pressure and use no
units for the equilibrium constant values
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magnitude of equilibrium constants
value of KC (or KP ) indicates how far the reaction has proceeded towards
products
th equilibrium
the
ilib i
constant
t t says nothing
thi about
b t
very
small
very
large
10-3
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10+3
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K values for related reactions
consider the reaction
multiples
2 SO2(g) + O2(g) ' 2 SO3(g)
KC =
[SO3 ]2
[SO2 ]2 [O2 ]
If you multiply the coefficients of a chemical equation by a factor
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reverse reactions
consider the reaction
2 SO2(g) + O2(g) ' 2 SO3(g)
KC =
[SO3 ]2
[SO2 ]2 [O2 ]
KC’
if yyou reverse the reaction,,
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new reactions
Given:
2 NO(g) + Br2(g) ' 2 NOBr(g)
2 NO(g) ' N2(g) + O2(g)
What is Kc for
KC1=2.0
KC2= 2.1 x 1030
N2(g) + O2(g) + Br2(g) ' 2 NOBr(g) ?
if you add reactions to obtain an overall chemical equation,
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Heterogeneous equilibria (section 15.4)
CaCO3(s) ' CaO(s) + CO2(g)
small amount of CaCO3
large amount of CaO
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large amount of CaCO3
small amount of CaO
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heterogeneous equilibria
pure solids and pure liquids
concentration (mol/L ) of solids and liquids
CaCO3(s) ' CaO(s) + CO2(g)
Ni(s) + 4 CO(g) ' Ni(CO)4(g)
Ag2O(s) + 2 HNO3(aq) ' 2 AgNO3(aq) + H2O(l)
solvents are omitted when
concentrations are low, because they
are effectively a pure liquid
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Calculating equilibrium constants from concentrations
Does the data provide
equilibrium
concentrations?
Use initial amounts and mol
relationships from balanced
chemical equation to calc.
equilibrium concentrations
put equilibrium concentrations
into the equilibrium constant
expression
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Example:
If the equilibrium concentration of O2 in air is 8 x 10-3M and that of O3
is 9 x 10-32M, what is the equilibrium constant for the conversion of
O2(g) to O3(g)?
Chem 102
Example:
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N2O4(g) ' 2 NO2(g)
In an experiment 2.00 mol of N2O4(g) are placed in an empty 5.00 L
flask and heated to 407 K. A dark red-brown gas appears indicating the
formation of NO2. The intensity of the colour indicates that the
concentration of NO2 at equilibrium is 0.525 M. What is the equilibrium
constant?
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Applications of equilibrium constants
Predict the direction of a reaction
Use value of Q defined
2 SO3(g) ' 2 SO2(g) + O2(g)
Q
same way as equilibrium constant
expression except the concentrations
are not equilibrium values
Compare relative magnitude of Q and K
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Example:
A mixture of hydrogen, iodine and hydrogen iodide, each at a
concentration of 0.0020 M, was introduced into a container heated
to 783 K. At this temperature, KP = 46 for the reaction
H2(g) + I2(g) ' 2 HI(g)
predict whether or not more HI has a tendency
p
y to form.
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Calculating equilibrium concentrations:
write the balanced chemical equation
use the equation to make a table of initial concentrations
calculate Q, compare to K and predict direction of reaction
represent the change in one concentration as x and use the mol
relationships to define the changes in all other species in terms of x
sum the initial concentration and the change represented by
values of x to get expressions for the equilibrium concentrations
substitute the equilibrium concentration expressions into the
equation for K and solve for x
use the value of x to calculate the equilibrium concentrations
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Example
In an experiment H2 and CO2 are placed in a flask so that their
concentrations are both 0.050 M. The mixture is heated to 420 °C and
equilibrium is achieved. What are the concentrations of water and carbon
monoxide (the products) at equilibrium?
H2 (g)
( ) + CO2(g)
( ) ' H2O (g)
( ) + CO(g)
CO( )
Kc = 0.10
0 10 (at
( t 420 °C)
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Example:
At 25 °C, the reaction I2(g) + Cl2((g) ' 2 ICl(g) has an equilibrium constant
KP = 81.9. Initially a reaction mixture at this temperature contains
PI2 = 0.100 atm, PCl2 = 0.100 atm and PICl = 0.100 atm.
Calculate the equilibrium partial pressures of I2, Cl2, and ICl.
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Example:
What is the equilibrium concentration of the pollutant NO produced in
automobile engines at high temperature if the reactants N2(g) and O2(g)
are present in their normal ratio in air: 2.24 M N2 and 0.56 M O2?
N2 (g) + O2(g) ' 2 NO(g)
Chem 102
KC= 1.7 x 10-3 at 2300 K
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