Chapter 17 Reaction Kinetics

1. Chapter 17Reaction Kinetics 17-1 The Reaction Process

2. Can you remember the first time you ever made a friend? How did you meet? What had to happen before the friendship could begin? Eye Contact Mutual Friend Accidentally Bumped into each other

3. Collision Theory • In order for a reaction to occur particles must collide in: • A specific orientation and • with enough energy

4. Activation Energy • The amount of energy required for a reaction to occur

5. Activation Energy • Activation energy - the amount of energy the particles must have when they collide to force a reaction to occur. Activation Energy Reactants Products

6. ∆H will be negative since energy has left the system Reaction Pathways The products have less energy than the reactants. The rxn released energy (heat) = exothermic

7. ∆H will be positive since energy has been added to the system Reaction Pathways The products have more energy than the reactants. The rxn absorbed energy (heat) = endothermic

8. Practice • Draw and label the energy diagram for a reaction in which ΔE = 30 kJ/mol, Ea = 40 kJ/mol. Place reactants at energy level zero. Indicate determined values of ΔEforward, ΔEreverse & Ea’

9. Reaction Mechanisms • Step-by-step sequence of rxns in order to obtain a final product Proposed Mechanism for Ozone Depletion via Free Chlorine Atoms Created by Decomposition of CFCs Step 1) Cl + O3 → ClO + O2 Step 2) 2 ClO → ClOOCl Step 3) ClOOCl → ClOO + Cl Step 4) ClOO → Cl + O2

10. Mechanisms overall rxn Intermediates

11. Mechanisms overall rxn Rate Determining Step Slow Fast Fast Fast

12. Catalysts vs. Intermediates overall rxn Catalysts appear 1st as a reactant and then as a product during a mechanism. Intermediates appear 1st as a product and then as a reactant during a mechanism.

13. Chapter 17Reaction Kinetics 17-2 Reaction Rate

14. How can we increase the rate of a reaction? • Increase Surface Area • Increase Temperature • Increase Concentration • Increase in Pressure • Add a Catalyst

15. Surface Area • Increase the surface area allows for a greater chance for effective collision

16. Temperature • An increase in temperature will cause particles to move at a higher velocity resulting in more effective collisions

17. Concentration • An increase in concentration will also cause an increase in the chance that effective collisions will occur

18. Pressure • Increasing the pressure of a gas system will cause more frequent collisions

19. Catalysts • Adding a catalyst lowers the amount of activation energy required

20. Catalysts Reactants Catalyst

21. Rate Laws • An equation that relates the rxn rate and the concentration of reactants Rate Determining Step Slow Rate = k[HBr][O2]

22. Rate Laws • If no mechanism is given, then… 2H2 + 2NO  N2 + 2H2O Rate = k[H2]2[NO]2

23. Rate Orders • 0, 1st and 2nd order rates • Order is dependent upon what will yield a straight line 2nd order 0 order 1/[reactants] [reactants] 1st order ln [reactants]

24. Rate Orders For Individual Components: • 1st order: reaction rate is directly proportional to the concentration of that reactant • 2nd order: reaction rate is directly proportional to the square of that reactant • 0 order: rate is not dependant on the concentration of that reactant, as long as it is present.

25. Rate Orders For Overall Order: • Overall reaction orders is equal to the sum of the reactant orders. • Always determined experimentally!

26. Calculating for k A + 2B  C Rate = k[A][B]2 What is the value of k, the rate constant?

27. Calculating for k Rate = k[A][B]2 2.0 x 10-4 = k[0.20][0.20]2 2.0 x 10-4 = k(0.008) k = 2.50 x 10-2 min-1 M-2

28. Practice 1. In a study of the following reaction: 2Mn2O7(aq) → 4Mn(s) + 7O2(g) When the manganese heptoxide concentration was changed from 7.5 x 10-5 M to 1.5 x 10-4 M, the rate increased from 1.2 x 10-4 to 4.8 x 10-4. Write the rate law for the reaction. 2. For the reaction: A + B → C When the initial concentration of A was doubled from 0.100 M to 0.200 M, the rate changed from 4.0 x 10-5 to 16.0 x 10-5. Write the rate law & determine the rate constant for this reaction. Rate = k[Mn2O7]2 Rate = k[A]2 Constant = 4.0 x 10-3 M/s

29. More Practice 3. The following reaction is first order: CH3NC(g) → CH3CN(g) The rate of this reaction is 1.3 x 10-4 M/s when the reactant concentration is 0.040 M. Predict the rate when [CH3NC] = 0.025. 4. The following reaction is first order: (CH2)3(g) → CH2CHCH3(g) What change in reaction rate would you expect if the pressure of the reactant is doubled? New Rate = 8.1 x 10-5M/s An increase by a factor of 2

30. Even More Practice 5. The rate law for a single step reaction that forms one product, C is R = k[A][B]2. Write the balanced reaction of A & B to form C. 6. The rate law of a reaction is found to be R = k[X]3. By what factor does the rate increase if the concentration of X is tripled? 7. The rate of reaction, involving 2 reactants, X & Z, is found to double when the concentration of X is doubled, and to quadruple when the concentration of Z is doubled. Write the rate law for this reaction. A + 2B → C The rate will increase by a factor of 27 R = k[X][Z]2