dr. s. m. condren chapter 14 chemical equilibrium
TRANSCRIPT
Dr. S. M. Condren
Chapter 14
Chemical Equilibrium
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.
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 ...
Dr. S. M. Condren
Catalytic Methanation Reaction
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.
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
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
Dr. S. M. Condren
Analogy in Semiconductors | | | |-Si:Si- <=> -Si+:Si- + e-
| | | |
| | -Si:Si- <=> h+ + e-
| |
K = h+ * e-
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
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
Dr. S. M. Condren
e- and h+ in Semiconductors
+h
Si
Si SiSi SiSi
SiSiSi
SiSi SiSi
Si
e–+h
e–
(–)(+)
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
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)
Dr. S. M. Condren
Which dopant will act as an acceptor for Si?
B, Ge, As
As a donor?
B, Ge, As
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.
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.
Dr. S. M. Condren
Gas Phase Equilibrium
catalysis
N2(g) + 3 H2(g) <=====> 2 NH3(g) + heat
high pressure and temperature
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
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
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
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
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
Dr. S. M. Condren
Heterogeneous Equilibrium
CaCO3(s) + heat <===> CaO(s) + CO2(g)