Chemical 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 Action For jA + kB  lC + mD The law of mass action is represented by the equilibrium expression:

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.

N 2 + 3H 2 ↔ 2NH 3

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 CH 3 COOH – acetic acid CH 3 COOH ↔ H + + CH 3 COO – ~10% dissociated ions ~90% molecular form Reaction reaches equilibrium

Important Facts CH 3 COOH & NH 3 are weak electrolytes Incomplete ionization Undergoes equilibrium K

Ionic Equilibria Weak acids, bases in dilute solution HA (aq) H + (aq) +A - (aq) [HA] K d = [H + ][A - ] Dissociation constant

Relationship between  and K d (1-  )C HA (  x C HA )(  x C HA )  2 x C 2 HA Kd=Kd= = (1-  )C HA [H + ]=  x C HA [A - ]=  x C HA ([H + ]=[A - ]) [HA]=(1-  )C HA Kd=Kd=  2 x C 1-  Ostwald’s dilution law (1888) Ionic Equilibria