Chemical Equilibrium
Chemical Equilibrium

Heterogeneous and homogeneous equilibrium

Law of Mass Action

Acids and Bases

The pH Scale

Buffers
Chemical Equilibrium
The
state where the concentrations of all
reactants and products remain constant with
time.
On
the molecular level, there is frantic
activity. Equilibrium is not static, but is a
highly dynamic situation.
Dynamic Equilibrium in Chemical Systems



Dynamic vs. Static
 Movement vs. Stationary
Equilibrium in chemical systems are dynamic.
 Equilibrium occurs at the molecular level.
 Rate of forward rxn = rate of reverse rxn
 Changes do occur!
 At macroscopic level, no discernible change is
apparent.
Kinetics & Equilibrium are not related!
Chemical Equilibrium
The Law of Mass Action
For
jA +
kB  lC + mD
The
law of mass action is represented by the equilibrium
expression:
l
m
C D
K
j
k
A B
LeChatelier’s Principle

“If an outside influence upsets an
equilibrium, then the system undergoes a
change in a direction that counteracts the
disturbing influence and, if possible, returns
the system to equilibrium.”

“For every action, there is an opposite
action.”
Le Chatelier’s Principle


If we disturb a reaction at equilibrium
 Changing the concentration or pressure of a reagent
 Altering the temperature
The reaction rates will shift to try to re-establish
equilibrium concentrations of all reagents
 The rate in one direction will exceed the other
Effects of Changes on the System

1. Concentration: The system will shift away from the
added component.

2. Temperature: K will change depending upon the
temperature (treat the energy change as a reactant).
Effects of Changes on the System

3.
Pressure:

a. Addition of inert gas does not affect the
equilibrium position.

b. Decreasing the volume shifts the equilibrium
toward the side with fewer moles.
N2 + 3H2 ↔ 2NH3
Strong Acid vs. Weak Acid







Strong Acid
HCl – hydrochloric acid
HCl  H+ + Cl–
100 % dissociated
No molecules of HCl
Only H+ and Cl– ions are
present
Reaction goes to
completion.






Weak Acid
CH3COOH – acetic acid
CH3COOH ↔
H+ + CH3COO–
~10% dissociated ions
~90% molecular form
Reaction reaches
equilibrium
Important Facts




CH3COOH & NH3 are weak electrolytes
Incomplete ionization
Undergoes equilibrium
K
H
H
C
H
O
H
H
O H
C
O
H
H
C
O
+
O H
C
-
H
H
O
H
Ionic Equilibria
Weak acids, bases in dilute solution
HA(aq)
H+(aq)+A-(aq)
Kd =
[H+][A-]
[HA]
Dissociation constant
Ionic Equilibria
Relationship between a and Kd
[H+]= a x CHA
[A-]= a x CHA
([H+]=[A-])
[HA]=(1-a)CHA
Kd=
Kd=
a2 x C
1-a
(a x CHA)(a x CHA) =
(1-a)CHA
a2 x C2HA
(1-a)CHA
Ostwald’s dilution law (1888)