Transcript No Slide Title

Chemical Equilibrium
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.
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 ...
...
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.
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
Acid-Base Equilibrium in
Aqueous Solution
Acid Dissociation Constant
HC2H3O2 + H2O <=> H3O+ + C2H3O2[H3O+][C2H3O2-]
K = ---------------------[H2O][HC2H3O2]
-
[H3O+][C2H3O2-]
Ka = K*[H2O] = --------------------[HC2H3O2]
Acid-Base Equilibrium in
Aqueous Solution
Base Dissociation Constant
NH3 + H2O <=> NH4+ + OH[NH4+][OH-]
K = ----------------[H2O][NH3]
[NH4+][OH-]
Kb = K*[H2O] = ---------------[NH3]
Autoionization of Water
H2O + H2O <=> H3O+ + OH+
-
[H3O ][OH ]
K = ----------------[H2O]2
Kw = K [H2O]2 = [H3O+][OH-] = 1.0 x 10-14
Analogy in Semiconductors
| |
| |
-Si:Si- <=> -Si+:Si- + e| |
| |
| |
-Si:Si- <=> h+ + e| |
K = h+ * e-
e
and
+
h
in Semiconductors
Production
Si
Si
Si
Si
Si
e–
conduction
band
Si
electrons (e –)
Electron
energy
Si
+
Si h Si
Si
0 Kelvin
room temperature
Si
+
Si h Si
Si
Si
Si
Si
Si
valence
band
conduction
band
Recombination
Si e– Si
holes (h+ )
Si
Eg
Si
Si
Si
valence
band
Autoionization Equilibria
H:O:H 
 H+ + OH–
| |
| |
—Si:Si— 
. Si— + e–
 —Si
| |
| |
or
| |
—Si:Si— 
 h+ + e–
| |
K = [h+] [e–] = p n
carrier (h + or H + ) concentration (cm –3 )
Kw = [H+] [OH –]
Temperature (°C)
400
200
100
0
18
10
Ge
water
10 14
Si
GaAs
10
10
6
10
0.001
0.002
0.003
1/T (Kelvin –1 )
0.004
e
and
Si
e – Si
(+)
Si
Si
h+
+
h
Si
Si
Si
Si
in Semiconductors
e – Si
h+
Si
Si
Si
Si
Si
(–)
Doping
Addition of Al to Si
Addition of P to Si
Si
Si
Conduction Band
P
Si
Si
Si
Conduction Band
Al
Donor
Level
Si
Si
Si
E
E
Si
(e– )
+
Si
+
Al
P
Si
Si
Si
Valence Band
Si
Si
Si
Acceptor
Level
(h + )
Valence Band
Donors and Acceptors in Silicon
conduction band
}
}
valence band
Ionization energy in parentheses (eV),
measured from nearest band edge.
donors
+
M  M + e–
acceptors
M  M – + h+
Which dopant will act as an acceptor for Si?
B, Ge, As
As a donor?
B, Ge, As
Fermi Level
undoped
semiconductor
metal
Ef
Ef
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.
n-type
semiconductor
p-type
semiconductor
Ef
Ef
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.
Gas Phase Equilibrium
catalysis
N2(g) + 3 H2(g) <=====> 2 NH3(g) + heat
high pressure and temperature
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
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
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
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
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
Heterogeneous Equilibrium
CaCO3(s) + heat <===> CaO(s) + CO2(g)
Gibbs Free Energy
and Equilibrium
DG
Reaction
------------------------------------Negative Spontaneous
Positive Non-Spontaneous
Zero
Equilibrium
-------------------------------------
The Influence of Temperature on
Free Energy
DG, DH, & DS
DG = DH - T DS
DH
negative
DS
positive
DG
negative
spontaneous at all temperatures
The Influence of Temperature on
Free Energy
DG, DH, & DS
DG = DH - T DS
DH
positive
DS
negative
DG
positive
non-spontaneous at all temperatures
The Influence of Temperature on
Free Energy
DG, DH, & DS
DG = DH - T DS
DH
negative
DS
negative
DG
--------
spontaneous at low temperatures,
nonspontaneous at high temperatures
The Influence of Temperature on
Free Energy
DG, DH, & DS
DG = DH - T DS
DH
positive
DS
positive
DG
--------
spontaneous at high temperatures,
nonspontaneous at low temperatures
Phase Transitions
H2O(s) -----> H2O(l)
DH > 0; DS > 0
H2O(l) -----> H2O(g)
DH > 0; DS > 0
spontaneous at high temperatures
Phase Transitions
H2O(l) -----> H2O(s)
DH < 0; DS < 0
H2O(g) -----> H2O(l)
DH < 0; DS < 0
spontaneous at low temperatures