Lecture 22 Equilibrium I

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Lecture 22 Equilibrium I
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Three things of interest in a chemical reaction

• What happens? (Stoichiometry, Chapters 3 and 4)
• How fast does it happen? (Kinetics, Chapter 12)
• How extensively does it happen (Equilibrium,
  Chapter 13)

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for example,

• Put some Cl2(aq) and NO(aq) in a flask.
• What happens?
• Cl2(aq) 2 NO(aq) 2 NOCl(aq)

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• Cl2(aq) 2 NO(aq) 2 NOCl(aq)

concentrations constant!
concentration
time
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Most reactions are reversible
A B
C D
and
C D
A B
or,
A B ? C D
A and B are in equilibrium with C and D
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Definition of Equilibrium
A B ? C D
If the concentrations are not changing, forward
rate reverse rate
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Equilibrium is a Dynamic Process...
radioactive NaCl(s)
NaCl(s) ? NaCl(aq) equilibrium is attained
when the solution is saturated
NaCl(aq)
NaCl(s)
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Equilibrium is a Dynamic Process...
NaCl(aq)
NaCl(aq)
NaCl(s)
NaCl(s)
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The concentration equilibrium constant, Kc

• for a reaction
• aA bB ? cC dD

CcDd
Kc
(at equilibrium)
AaBb
X (mol X) / L
Kc is a function of temperature!
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for example,

• Cl2(aq) 2 NO(aq) ? 2 NOCl(aq)

NOCl(aq)2
Kc
Cl2(aq) NO(aq)2
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Heterogeneous Equilibria

• C(s) H2O(g) ? CO(g) H2(g)

C(s) initial
C(s) final
same CONCENTRATION of C(s)
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C(s) H2O(g) ? CO(g) H2(g)
CO(g) H2(g)
X
Kc
C(s) H2O(g)
CO(g) H2(g)
Kc
H2O(g)
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HF(aq) H2O(l) ? H3O(aq) F-(aq)

• In a dilute solution,
• H2O(l) does not change
• In general, leave out condensed phases (liquids
  and solids)

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For gases, use Kp, the pressure equilibrium
constant

• CS2(g) 3 O2(g) ? CO2(g) 2 SO2(g)

(at equilibrium)
pX pressure of X (atm)
Kp is a function of temperature!
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Kc and Kp are related

• CS2(g) 3 O2(g) ? CO2(g) 2 SO2(g)

n 4 molesof gas
n 3 moles of gas
?n -1
R 0.082 L atm K-1 mol-1
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The Physical Meaning ofthe Equilibrium Constant

• H2(g) ½ O2(g) ? H2O(g)

1.2 x 1040
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The Physical Meaning ofthe Equilibrium Constant

• CO(g) H2O(g) ? CO2(g) H2(g)

2.1
(roughly the same pressures of reactants and
products at equilibrium)
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The Physical Meaning ofthe Equilibrium Constant

• H2O(l) ? H2O(g) at 5oC

0.0086
(very little water in the gas phase at this
temperature)
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Using the Equilibrium Constant

• Testing for equilibrium
• Calculating equilibrium concentrations
• Predicting the effects of changing a
  concentration, the pressure or temperature

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N2(g) 3 H2(g) ? 2 NH3(g)
NH32
Kc
N2 H23
0.0596
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N2(g) 3 H2(g) ? 2 NH3(g)

• suppose H2 1.20 M,
• N2 1.40 M, NH3 0.20 M

NH32
(0.20)2

Q
(1.40)(1.20)3
N2 H23
0.0170
Q is the Reaction Quotient
Q lt Kc !!
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N2(g) 3 H2(g) ? 2 NH3(g)
system moves in this direction until Q
Kc NH3 increases, N2 and H2 decrease
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Using the Equilibrium Constant

• Testing for equilibrium
• Calculating equilibrium concentrations
• Predicting the effects of changing a
  concentration, the pressure or temperature

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The concentrations can be calculated...

• e.g. COCl2(g) ? CO(g) Cl2(g)
• Kc 8.3 x 10-4 (at 360oC)
• Put 5.0 mol COCl2 in a 10 L flask. What happens?

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• COCl2(g) ? CO(g) Cl2(g)
• initially (M) 0.500 0.0 0.0
• change (M) - x x x
• equilibrium (M) 0.500 - x x x

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at equilibrium,
COCl2
Kc
8.3 x 10-4
COCl2
(x) (x)
thus,
8.3 x 10-4
(0.500 - x)
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x2
8.3 x 10-4
(0.500 - x)
x2 8.3 x 10-4 (0.500 - x)
x2 (8.3 x 10-4) x - (4.15 x 10-4) 0
(a x2 b x c 0)
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0.0200 or -0.0208
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thus, at equilibrium,

• COCl2 0.500 - x
• 0.500 - 0.0200
• 0.480
• CO Cl2 x 0.0200
• Check