Chemical Equilibrium
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Transcript Chemical Equilibrium
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)