Equilibrium

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Equilibrium
Its about water vapor and rock candy
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Chemical Equilibrium Reactions Dont Just Stop
They find Balance
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Chemical Equilibrium Reactions Dont Just Stop
They find Balance
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Equilibrium is Attained When the Rates of the
Forward and Reverse Reactions are the Same.
Forward Rate kf A
Reverse Rate kr B
kf A kr B
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Equilibrium is Attained When the Rates of the
Forward and Reverse Reactions are the Same.
kf
B
Because kf A kr B is true
A constant


kr
A

• It makes no difference whether we start with A or
  B, or even with some
• mixture of the two. At equilibrium the ratio of
  their concentrations
• equals a definite value

• Once equilibrium is established, the
  concentrations of A and B do not
• change

Equilibrium is described as being dynamic
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Equilibrium The Haber Process and Nitrogen
Fixation
N2(g) 3H2(g) ? 2NH3(g)
Chemistry at work, p. 521
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Equilibrium The Haber Process and Nitrogen
Fixation
Note that equilibrium can be reached from either
the forward or reverse direction
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Equilibrium The Haber Process and Nitrogen
Fixation
kf
B
Because kf A kr B is true, and
A constant is true


kr
A
We can mathematically describe what happens to a
reaction at equilibrium
Suppose we have the reaction jA kB ? pR q S
The LAW OF MASS ACTION allows us to express the
relative concentrations of reactants and products
at equilibrium in terms of a quantity called
the equilibrium constant, K such that
Rp Sq
K
Aj Bk
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The Haber Process and the Law of Mass Action
N2(g) 3H2(g) ? 2NH3(g)
Rewriting the nitrogen fixation reaction used by
Haber as the Law of Mass Action, we get
Rp Sq
NH32
K
K
Aj Bk
N2H23
The equilibrium constant expression depends only
on the stoichiometry of the reaction, not on its
mechanism
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Suppose we discover that the equilibrium
concentrations of N2O4 and NO2 are 0.0172 M and
0.00140 M, respectively
Remember N2O4 ? 2NO2?
0.01722
NO22
0.221
Kc

N2O4
0.00140
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Equilibrium The Magnitude of Equilibrium
Constants
0.01722
NO22
0.221
Kc

N2O4
0.00140
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Equilibrium Evaluating Equilibrium Constants
In one of their experiments, Haber and co-workers
introduced a mixture of hydrogen and nitrogen
into a reaction vessel and allowed the system to
attain chemical equilibrium at 472 C. The
equilibrium mixture of gases was analyzed and
found to contain 0.01207 M H2, 0.0402 M N2, and
0.00272 M NH3. From these data, calculate the
equilibrium constant for
N2(g) 3H2(g) ? 2NH3(g)
Now you try it!
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Equilibrium Evaluating Equilibrium Constants
Using the ICEBOX
A mixture of 5.00 x 10-3 mol of H2 and 1.00 x
10-2 mol of I2 is place in a 5.00 L container at
448C and allowed to come to equilibrium.
Analysis of the equilibrium mixture shows that
the concentration of HI is 1.87 x 10-3 M.
Calculate Kc at 448 C for the reaction
H2(g) I2(g) ? 2HI
H2(g) I2(g) ? 2HI
2.00 x 10-2 M
O M
Initial
1.00 x 10-3 M
Change
Equilibrium
1.87 x 10-3 M
HI2
Kc
H2 I2
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Equilibrium Evaluating Equilibrium Constants
Using the ICEBOX
H2(g) I2(g) ? 2HI
2.00 x 10-3 M
O M
Initial
1.00 x 10-3 M
Change
1.87 x 10-3 M
-0.935 x 10-3 M
-0.935 x 10-3 M
Equilibrium
1.87 x 10-3 M
1.065 x 10-3 M
0.065 x 10-3 M
H2 5.00 x 10-3 mol /5.00 L 1.00 x 10-3 M
I2 1.00 x 10-2 mol/5.00 L 2.00 x 10-3 M
1.87 x 10-32
Kc
0.065 x 10-3 M 1.065 x 10-3 M
Kc 51
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Equilibrium Evaluating Equilibrium Constants
Using the ICEBOX
A 1.000-L flask is filled with 1.000 mol of H2
and 2.000 mol I2 at 448C. The value of the
equilibrium constant for the following reaction
is 50.5. What are the equilibrium
con- centrations of H2, I2, and HI? H2(g)
I2(g) ? 2HI
H2(g) I2(g) ? 2HI
1.000 M
2.000 M
0 M
Initial
Change
Equilibrium
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Equilibrium Evaluating Equilibrium Constants
Using the ICEBOX
H2(g) I2(g) ? 2HI
1.000 M
2.000 M
0 M
Initial
2x M
- x M
Change
- x M
(2.000 - x)M
2x M
(1.000 - x)M
Equilibrium
HI2
(2x)2
50.5

H2 0.065 M I2 1.065 M HI 1.870 M
H2I
(1.000 - x)
(2.000 - x)
4x2 50.5(x2 - 3.00 x 2.00)
46.5x2 - 151.5x 101.0 0
x -(-151.1) (-151.4)2 - 4(46.5)(101.0)
2(46.5)
x 2.323 or 0.935
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Equilibrium Evaluating Equilibrium Constants
By Converting from Kc to Kp
Solutions are Understood in terms of
Molarities Gas Pressure is Understood in Terms of
Atmospheres
Kp Kc(RT)?n, where n moles of product - moles
of reactant
Using the value of Kc 0.105 for the reaction
for the reaction N2(g) 3H2(g) ? 2NH3 at 472
C. Convert that value to Kp
Kp 0.105 (0.0821 L-atm/mol-K 745 K)-2
0.105
Kp
(0.0821 L-atm/mol-K 745 K)2
Kp 2.81 x 10-5
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Equilibrium Constructing a Law of Mass Action
Equation for Heterogeneous Equilibria
Heterogeneous Equilibria a reaction which may
possess reactants or products which are in
different phases.
CaCO3(s) ? CaO(s) CO2(g)
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Equilibrium Constructing a Law of Mass Action
Equation for Heterogeneous Equilibria
The density of a pure liquid or solid is a
constant at any given temperature and changes
very little with temperature. Thus the effective
concentration of a pure liquid or solid is
constant regardless of how much pure liquid or
solid is present
Given CaCO3(s) ? CaO(s) CO2(g)
CaO(s)CO2
K
CaCO3(s)
CO2
K
Even though they do not appear in the equilibrium
constant expression,pure solids and liquids must
be present for equilibrium to be established
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Equilibrium Applications of Equilibrium
Constants Predicting the Direction of Equilibrium
Le Châteliers principle If a system at
Equilibrium is disturbed by a change in
temperature, pressure or the concentration of a
component, the system will shift its equilibrium
position so as to counteract the effect of the
disturbance.
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Equilibrium Applications of Equilibrium
Constants Predicting the Direction of Equilibrium
Given,
N2(g) 3H2(g) ? 2NH3(g)
NH32
and Kc 0.105
K
N2H23
If the equilibrium concentrations are such that
2.002
Q
0.500
1.002,003
What must happen in order for the 0.500 value,
lets now call it the reaction quotient Q, to
return to Kc 0.105?
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Equilibrium Applications of Equilibrium
Constants Predicting the Direction of Equilibrium
NH32
Q
0.500 and Kc 0.105
N2H23
Equilibrium will re-emerge if the
concentration of NH3 decreases and
the concentrations of N2 and H2 increase. In a
closed system, this would require that the
reaction favor the formation of reactants such
that
N2(g) 3H2(g) 2NH3(g)
If Q gt K, the reaction will shift in favor of the
reactants
If Q K, the reaction is at equilibrium
If Q lt K, the reaction will shift in favor of the
products
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Equilibrium Taking a More In-Depth Look at the
Haber Process
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Equilibrium Taking a More In-Depth Look at the
Haber Process
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Equilibrium Taking a More In-Depth Look at the
Haber Process
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Equilibrium Effects of Pressure and Volume
Changes
If a system is at equilibrium and the total
pressure is increased by the application of an
external pressure, the system will respond by a
shift in equilibrium in the direction that
reduces the pressure
2NO2(g)
N2O2(g)
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Equilibrium Effects of Temperature Changes
When heat is added to a system, the equilibrium
shifts in the direction that absorbs heat. In an
endothermic reaction reactants are converted to
products, and K increases. In an exothermic
reaction, the opposite occurs.
heating
cooling
Room temperature
Co(H2O)62 (aq) 4Cl- (aq) ? CoCl42- (aq)
6H2O(l) ?H gt 0
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