1. Equilibrium Topic 7

2. 7.2.1 THE EQUILIBRIUM CONSTANT (K c or K) For any type of chemical equilibrium of the type a A + b B  c C + d D a A + b B  c C + d D the following is a CONSTANT (at a given T) If K c is known, then we can predict concs. of products or reactants.

3. Determining K c 2 NOCl(g)  2 NO(g) + Cl 2 (g) [NOCl] [NO] [Cl 2 ] Initial2.000 0 Change-0.66+0.66+0.33 Equilibrium1.340.660.33

4. Chemical Equilibrium from the lab Fe 3+ + SCN -  FeSCN 2+  After a period of time, the concentrations of reactants and products are constant.  The forward and reverse reactions continue after equilibrium is attained.

5. Writing and Manipulating K Expressions Solids and liquids NEVER appear in equilibrium expressions. S(s) + O 2 (g)  SO 2 (g) S(s) + O 2 (g)  SO 2 (g)

6. Writing and Manipulating K Expressions Solids and liquids NEVER appear in equilibrium expressions. NH 3 (aq) + H 2 O(liq)  NH 4 + (aq) + OH - (aq) NH 3 (aq) + H 2 O(liq)  NH 4 + (aq) + OH - (aq)

7. The Meaning of K 1.Can tell if a reaction is product- favored or reactant-favored. For N 2 (g) + 3 H 2 (g)  2 NH 3 (g) Conc. of products is much greater than that of reactants at equilibrium. The reaction is strongly product-favored.

8. The Meaning of K For AgCl(s)  Ag + (aq) + Cl - (aq) K c = [Ag + ] [Cl - ] = 1.8 x 10 -5 K c = [Ag + ] [Cl - ] = 1.8 x 10 -5 Conc. of products is much less than that of reactants at equilibrium. The reaction with small K is strongly reactant-favored. Ag + (aq) + Cl - (aq)  AgCl(s)  AgCl(s) is product-favored. Ag + (aq) + Cl - (aq)  AgCl(s)  AgCl(s) is product-favored.

9. Product- or Reactant Favored Product-favored K > 1 Reactant-favored K < 1

10. EQUILIBRIUM AND EXTERNAL EFFECTS  Temperature, catalysts, and changes in concentration affect equilibria.  The outcome is governed by LE CHATELIER’S PRINCIPLE LE CHATELIER’S PRINCIPLE  “...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”

11. Le Chatelier’s Principle  Change in temperature  change in K c  therefore change in P or concentrations at equilibrium  Use a catalyst: reaction comes more quickly to equilibrium. K c not changed.  Add or take away reactant or product:  K c does not change  Reaction adjusts to new equilibrium “position”

12. 7.2.5 Apply the concepts of kinetics and equilibrium to industrial processes.  A good example of this is the production of ammonia.  Fertilizers and many plastics are derived from ammonia.  High pressure is used to increase yield.  The reaction is exothermic so a lower temperature is used to increase yield.  A catalyst is used to speed up the reaction, but it doesn’t increase the yield.

13.  N 2 (g) + 3 H 2 (g)  2 NH 3 (g) + heat  K = 3.5 x 10 8 at 298 K Haber-Bosch Process for NH 3 Haber-Bosch Process for NH 3