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)