Chemical EquilibriumChemical Equilibrium

Chemical Equilibrium

Heterogeneous and homogeneous equilibrium

Law of Mass Action

Acids and Bases

The pH Scale

Buffers

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.

Chemical Equilibrium

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 ActionFor

jA + kB lC + mD

The law of mass action is represented by the equilibrium expression:

Kl m

j k C DA 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

C C

O

O

H

H

H H

O HH

C C

O

O-

H

H

H

O+

HH

H

Ionic Equilibria

Weak acids, bases in dilute solution

HA(aq) H+

(aq)+A-(aq)

[HA]Kd =

[H+][A-]

Dissociation constant

Relationship between and Kd

(1-)CHA

( x CHA)(x CHA) 2 x C2HAKd= =

(1-)CHA

[H+]= x CHA [A-]= x CHA

([H+]=[A-])

[HA]=(1-)CHA

Kd=2 x C

1-Ostwald’s dilution law (1888)

Ionic Equilibria