2. Created by C. Ippolito February 2007 Chapter 18 Chemical Equilibrium Objectives: 1.Distinguish between a reversible reaction at equilibrium and one that is not 2.Derive mass-action expressions 3.Calculate equilibrium constants and apply them 4.Explain and apply LeChatelier’s principle 5.Derive solubility product expressions and determine solubilities

3. Created by C. Ippolito February 2007 Reversible Reactions Theoretically all reactions reversible Chemical Equilibrium –forward and reverse reactions –proceed under same conditions –proceed at same rate –use “=“ or “double arrow” between reactants and products

4. Created by C. Ippolito February 2007 Characteristics of Equilibrium Rate forward = Rate reverse

5. Created by C. Ippolito February 2007 Mass-Action Expression describes chemical change –derived from balanced equation mA + nB pC + qD –Mass-Action Expression

6. Created by C. Ippolito February 2007 Equilibrium Expression –mass expression when set equal to an experimentally determined constant, K eq (Figure 18-5 on page 519) Law of Chemical Equilibrium –at a particular temperature, the mass-action expression for a reversible reaction is a constant when equilibrium concentrations are used in the expression

7. Created by C. Ippolito February 2007 Size of Equilibrium Constant (K eq ) Equilibrium Constant larger than 1 –favors formation of products Equilibrium Constant less than 1 –favors formation of reactants

8. Created by C. Ippolito February 2007 Applications of K eq Given the equation and concentrations, find the constant. Given the equation and constant, find the missing concentration.

9. Created by C. Ippolito February 2007 LeChatelier’s Principle when equilibrium experiences stress –change in concentration –change in temperature –change in pressure it will shift direction to counteract or relieve the effect of stress

10. Created by C. Ippolito February 2007 Effect of Temperature Change A + B  C + D + heat –reverse reaction will be favored with  temperature [A] and [B] will increase [C] and [D] will decrease A + B  C + D + heat –forward reaction will be favored with  temperature [C] and [D] will increase [A] and [B] will decrease

11. Created by C. Ippolito February 2007 Effect of Pressure Change little or no effect on reacting solids or liquids large effect on reacting gases –N 2 (g) + 3H 2 (g)  2NH 3 (g) 4 moles of reactants for 2 moles of products –forward reaction decreases # of molecule –increases with pressure increase

12. Created by C. Ippolito February 2007 Effect of Catalyst no effect catalysts increase rate of reaction equally reach equilibrium more rapidly but do not shift it

13. Created by C. Ippolito February 2007 Solubility Equilibrium Solubility Product Constant –constant value at a particular temperature equal to the product of the concentration of the ions in a saturated solution indicates relative solubility –smaller values are MORE insoluble

14. Created by C. Ippolito February 2007 Common Ion Effect dissociation of a compound is decreased by dissolving a readily soluble compound with an ion in common with the less soluble compound. –Example: CuI(s)  Cu + (aq) + I - (aq) has low solubility –adding soluble NaI adds I - ions that will remove even more Cu + ions from solution!