2. Kinetics & Equilibrium Dr. Ron Rusay Fall 2001 © Copyright 2001 R.J. Rusay

3. Reaction Diagram of O 3 decomposing

7. Chemical Equilibrium  Most of the reactions considered until now have had reactants react completely to form products. These reactions “went” only in one direction. [  ]  Some reactions can react in either direction. They are “reversible”. [ ]When this occurs some amount of reactant(s) will always remain at the end of change. Weak acids and bases follow this type.

8. Chemical Equilibrium (Definitions)  A chemical system where the concentrations of reactants and products remain constant over time.  On the molecular level, the system is dynamic: The rate of change is the same in both the forward and reverse directions.

9.  For a reaction: jA + kB lC + mDjA + kB lC + mD  The law of mass action is represented by the Equilibrium Expression: where K is the Equilibrium Constant. (Units for K will vary.) Law of Mass Action ( The Equilibrium Expression)

10. Equilibrium Expression  4 NH 3 (g) + 7 O 2 (g)  4 NO 2 (g) + 6 H 2 O(g)  The following Equilibrium Expression can have the amounts uniformly expressed as units of concentration mol/L (M), or as units of pressure (atm).

11. Equilibrium Expressions  If a reaction is re-written where the reactants become products and products-reactants, the new Equilibrium Expression is the reciprocal of the old. K new = 1/K original  When the entire equation for a reaction is multiplied by a molar factor n, K new = (K original ) n

12. Heterogeneous Equilibria  Equilibria that involve more than one phase. CaCO 3 (s)  CaO(s) + CO 2 (g)CaCO 3 (s)  CaO(s) + CO 2 (g) K = [CO 2 ]  The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present.

13. Le Châtelier’s Principle ... If change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.

14. Le Châtelier’s Principle

15. Changes on the System  1.Concentration: The system will shift concentrations away from the added component. K remains the same.  2.Temperature: K will change depending upon the reaction. If endothermic, heat is a reactant, exothermic, heat is a product. Increasing T will increase or decrease K. Endo- > K increases; Exo- > K decreases.

16. Temperature Effects on the NO 2  N 2 O 4 Equilibrium

17. Changes on the System (continued)  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. K p = K c (RT)  n As the volume is decreased pressure increases. [ Le Châtelier’s Principle: if pressure is increased the system shifts to minimize the increase.]As the volume is decreased pressure increases. [ Le Châtelier’s Principle: if pressure is increased the system shifts to minimize the increase.]

18. Changes on the System (continued) 4. The Effect of Catalysts  A catalyst lowers the activation energy barrier for any reaction….in both forward and reverse directions!  A catalyst will decrease the time it takes to reach equilibrium.  A catalyst does not effect the composition of the equilibrium mixture.

19. Catalysts Lower the Energy of Activation

20. Electromagnetic Radiation: Light, Energy, Sunburn

21. http://chemistry.beloit.edu/Stars/EMSpectrum/index.html Ozone, sunscreen and SPF.

22. Catalytic Destruction of Ozone

23. Satellite Images of the Ozone Hole