dr. s. m. condren chapter 14 chemical equilibrium
TRANSCRIPT
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Dr. S. M. Condren
Chapter 14
Chemical Equilibrium
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Dr. S. M. Condren
Chemical Equilibrium
The state of a chemical reaction in which its forward and reverse reactions occur at equal rates so that the concentration of the reactants and products does not change with time.
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Dr. S. M. Condren
Law of Mass Action
aA + bB + cC + ... <=> pP + qQ + rR + ...
Equilibrium Constant
[P]p [Q]q [R]r ...
K = --------------------- [A]a [B]b [C]c ...
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Dr. S. M. Condren
Catalytic Methanation Reaction
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Dr. S. M. Condren
Meaning of Equilibrium Constant• K>>1: reaction is product-favored;
equilibrium concentrations of products are greater than equilibrium concentrations of reactants.
• K<<1: reaction is reactant-favored; equilibrium concentrations of reactants are greater than equilibrium concentrations of products.
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Dr. S. M. Condren
If K = 100 = [I2 in CCl4] / [I2 in water] for the equilibrium
I2 in water = I2 in CCl4
What is K for the reverse reaction,
I2 in CCl4 = I2 in H2O?
100, 1, 0.01
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Dr. S. M. Condren
Autoionization of Water
H2O + H2O <=> H3O+ + OH-
[H3O+][OH-]K = -----------------
[H2O]2
Kw = K [H2O]2 = [H3O+][OH-] = 1.0 x 10-14
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Dr. S. M. Condren
Analogy in Semiconductors | | | |-Si:Si- <=> -Si+:Si- + e-
| | | |
| | -Si:Si- <=> h+ + e-
| |
K = h+ * e-
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Dr. S. M. Condren
e- and h+ in Semiconductors
SiSi
Si
SiSi
0 Kelvin
Si
Si Si
Si
Sie–
+h
room temperature
Production
Recombination
electrons (e )–
holes (h )+
Electron energy
conduction band
valence band
Si
Si SiSi SiSi
SiSiSi
SiSi SiSi
Si
e–
+h
Eg
conduction band
valence band
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Dr. S. M. Condren
Autoionization EquilibriaH:O:H H+ + OH–
Kw = [H+] [OH–]
—Si|
|:Si
|
|— —Si
|
|.Si
|
|— + e–
or
—Si|
|:Si
|
|— h+ + e–
K = [h+] [e–] = p n
10
10
10
106
10
14
18
0.001 0.002 0.003 0.004
GaAs
Si
Ge
water
400 200 100
Temperature (°C)0
1/T (Kelvin )–1
carr
ier
(h
or
H
) c
on
cen
tra
tion
(cm
)+
+–
3
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Dr. S. M. Condren
e- and h+ in Semiconductors
+h
Si
Si SiSi SiSi
SiSiSi
SiSi SiSi
Si
e–+h
e–
(–)(+)
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Dr. S. M. Condren
Doping
Conduction Band
Donor Level
Valence Band
E
-
P
Si SiSi
Si+
(e )–
P
Si SiSi
Si
Addition of P to Si Addition of Al to Si
Al
Si SiSi
Si+
-(h )+
AlSi Si
Si
SiConduction Band
Acceptor Level
Valence Band
E
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Dr. S. M. Condren
Donors and Acceptors in Silicon
Ionization energy in parentheses (eV), measured from nearest band edge.
conduction band
acceptors M M + h+–}
valence band
donorsM M + e
+ –}00.10.20.3
0.50.4
0.30.4
0.20.10
0.5
In (0.16)–
Al (0.057)– Ga (0.065)–B (0.045)–
Sb (0.039)
+As (0.049)+P (0.044)+
Mn (0.53)+
Cu –(0.49)
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Dr. S. M. Condren
Which dopant will act as an acceptor for Si?
B, Ge, As
As a donor?
B, Ge, As
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Dr. S. M. Condren
Fermi Level
fE fE
fE
fE
metal
p-typesemiconductor
undopedsemiconductor
n-typesemiconductor
The Fermi level is the energy at which the probability of finding an electron is 50%; below the Fermi level it is more likely that the electronic states are occupied with electrons and above the Fermi level it is more likely they are not occupied.
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Dr. S. M. Condren
Le Chatelier's Principle
If a stress, such as a change in concentration, pressure, temperature, etc., is applied to a system at equilibrium, the equilibrium will shift in such a way as to lessen the effect of the stress.
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Dr. S. M. Condren
Gas Phase Equilibrium
catalysis
N2(g) + 3 H2(g) <=====> 2 NH3(g) + heat
high pressure and temperature
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Dr. S. M. Condren
The Principle of Le Chatelier
Changes in Concentration or Partial Pressure
for N2(g) + 3 H2(g) 2 NH3(g)
an increase in N2 and/or H2 concentration or pressure, will cause the equilibrium to shift towards the production of NH3
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Dr. S. M. Condren
The Principle of Le Chatelier
Changes in Concentration or Partial Pressure
for N2(g) + 3 H2(g) 2 NH3(g)
likewise, a decrease in NH3 concentration or pressure will cause more NH3 to be produced
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Dr. S. M. Condren
The Principle of Le Chatelier
Changes in Temperature
for N2(g) + 3 H2(g) 2 NH3(g) + heat
for an exothermic reaction, an increase in temperature will cause the reaction to shift back towards reactants
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Dr. S. M. Condren
The cobalt complexes participating in the equilibrium below comprise a humidity sensor. From Le Châtelier's principle, when the sensor is moist (excess H2O), what color is the cobalt complex?
pink, blue
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Dr. S. M. Condren
A competition experiment involves O2 and CO vying for hemoglobin (Hb) sites, defined by the equilibrium
Hb(O2)4 + 4 CO = Hb(CO)4 + 4O2
From Le Châtelier's principle, how is CO poisoning reversed?
decrease O2 pressure, increase O2 pressure, remove Hb
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Dr. S. M. Condren
Heterogeneous Equilibrium
CaCO3(s) + heat <===> CaO(s) + CO2(g)