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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.

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Chemical Equilibrium

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  1. Chemical Equilibrium

  2. Chemical Equilibrium • Heterogeneous and homogeneous equilibrium • Law of Mass Action • Acids and Bases • The pH Scale • Buffers

  3. 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.

  4. Dynamic Equilibriumin 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!

  5. Chemical Equilibrium

  6. The Law of Mass Action • For • jA + kB «lC + mD • The law of mass action is represented by theequilibrium expression:

  7. 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.”

  8. 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

  9. 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).

  10. 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.

  11. N2 + 3H2 ↔ 2NH3

  12. 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 Strong Acid vs. Weak Acid

  13. Important Facts • CH3COOH & NH3 are weak electrolytes • Incomplete ionization • Undergoes equilibrium • K

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

  15. [H+]= a x CHA [A-]= a x CHA ([H+]=[A-]) [HA]=(1-a)CHA a2 x C • Kd= 1-a (a x CHA)(a x CHA) a2 x C2HA = • Kd= (1-a)CHA (1-a)CHA Ionic Equilibria Relationship between a and Kd Ostwald’s dilution law (1888)

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