2. 1 Chemical Kinetics Chapter 11 H 2 O 2 decomposition in an insect H 2 O 2 decomposition catalyzed by MnO 2

3. 2 KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM(steps in the process). We can use thermodynamics to tell if a reaction is product- or reactant-favored. Only KINETICS will tell us HOW FAST the reaction happens! KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM(steps in the process). We can use thermodynamics to tell if a reaction is product- or reactant-favored. Only KINETICS will tell us HOW FAST the reaction happens! Chemical Kinetics

4. 3 Meaning of Rate of Reaction rate – the change per interval of time Ex—speed (distance/ time) is a RATE

5. 4 Types of rates - –initial rate, average rate, and instantaneous rate Reaction Rates

6. 5 Determining a Reaction Rate Blue dye is oxidized with bleach. Its concentration decreases with time. The rate — the change in dye conc with time — can be determined from the plot. Blue dye is oxidized with bleach. Its concentration decreases with time. The rate — the change in dye conc with time — can be determined from the plot. NOTE THE UNITS OF RATE IN CHEMICAL REACTIONS!!!

7. 6 Units [ ] = Molarity = moles/L Time= hr, min, s Overall= M/time or moles/L. time

8. 7 C. Measurement General reaction rate: calculated by dividing rate expressions by stoichiometric coefficients EOS For aA + bB  cC + dD,

9. 8 Example: Calculate the rate of N 2 O 5 decomposition at 1.0 minute using this graph. How are the rates of formation of NO 2 and O 2 related to the rate of decomposition of N 2 O 5 ?

10. 9 Example 2: Sample Calculation of Average Rate Rate of decomposition of N 2 O 5 (g) at 67 o C: Time(min)0 1 2 3 4 [ N2O5]0.160 0.113 0.080 0.056 0.040 What is the average rate of decomposition of dinitrogen pentoxide during the first three minutes?

11. 10 II. Reaction Rate and Concentration A. Reactant concentration and collision theory *Reactions occur when molecules collide to exchange or rearrange atoms *Effective collisions occur when molecules have correct energy and orientation

12. 11 1.Concentrations and physical state of reactants and products 2. Temperature 3. Catalysts Rate Law: relates the rate of the reaction to the concentration of the reactants Factors Affecting Rate:

13. 12 Concentrations & Rates 0.3 M HCl6 M HCl Mg(s) + 2 HCl(aq) ---> MgCl 2 (aq) + H 2 (g) As concentration increases, what happens to rate?

14. 13 Physical state of reactants Factors Affecting Rates Greater surface area means greater rate of reaction!

15. 14 Temperature Factors Affecting Rates Bleach at 54 ˚CBleach at 22 ˚C As Temperature increases, what happens to rate? Why?

16. 15 Catalysts: catalyzed decomp of H 2 O 2 Catalysts are substances that speed up a reaction but are unchanged by the reaction 2 H 2 O 2 --> 2 H 2 O + O 2 Factors Affecting Rates

17. 16 Effect of Concentration on Reaction Rate To propose a reaction mechanism, we study reaction rate and its concentration dependence To propose a reaction mechanism, we study reaction rate and its concentration dependence

18. 17 B. Rate Expression and Rate Constant The rate law is: R = k[A] m [B] n [C] p EOS The exponents in a rate law must be determined by experiment. They are not the stoichiometric coefficients and k is the rate constant.

19. 18 B. continued m, n, and p are the reaction order can be 0, 1, 2 or fractions must be determined by experiment! Overall Order = Sum of m, n and p

20. 19 Rate Constant, k Relates rate and concentration at a given temperature General Formula M (1- overall order) time -1

21. 20 -For one reactant aA  products Rate = k [A] m m= order of reaction with respect to A

22. 21 If m = 0 (zero order) Rate = k [A] 0 rate is independent of [A] If m = 1 (1 st order) Rate = k [A] 1 If [A] doubles, then rate doubles If m = 2 (2 nd order) Rate = k [A] 2 If [A] doubles, then rate quadruples

23. 22 C. Determining Rates Pick any two points from the given data! Use a ratio of rate expressions for the 2 chosen points.

24. 23 The initial rate of decomposition of acetaldehyde, CH 3 CHO, was measured at a series of different concentrations and at a constant temperature. Using the data below, determine the order of the reaction (m) in the equation: Rate = k[CH 3 CHO] m CH 3 CHO(g)  CH 4 (g) + CO(g) CH 3 CHO (mol/L) 0.1620.1950.2730.4100.518 Rate (mol/L*min)3.154.568.9420.235.2

25. 24 Consider the rate data for the decomposition of CH 3 CHO given in the above example. Knowing that the reaction is 2nd order, determine the a. value of the rate constant (k)

26. 25 b. the rate of the reaction when [CH 3 CHO] = 0.452mol/L

27. 26 Order of a Reaction for more than one reactant aA +bB  products Rate = k[A] m [B] n Overall order of reaction = m + n Use one equation to determine each reactant’s order, where the other reactant(s) concentration is held constant.

28. 27 The data below are for the reaction of nitrogen (II) oxide with hydrogen at 800 o C. 2NO(g) + 2H 2 (g)  N 2 (g) + 2H 2 O(g) Determine the order of the reaction with respect to both reactants, calculate the value of the rate constant, and determine the rate of formation of product when [NO]=0.0024M and [H2]=0.0042M.

29. 28 Ex The initial rate of a reaction A + B  C was measured with the results below. State the rate law, the value of the rate constant, and the rate of reaction when [A] = 0.050M and [B] = 0.100M.

30. 29 Ex The following data were collected for this reaction at constant temperature: 2NO(g) + Br 2 (g)  2NOBr(g) State the rate law, and determine the rate of this reaction when [NO]=0.15M and [Br2]=0.25M.

31. 30 Properties of Reactions OrderRate Expression 0 (zero) rate = k 1 (first)rate = k[A] 2 (second)rate = k[A] 2

32. 31 Activation Energy A. definition: the minimum amount of energy required to react, E a. activated complex - a short-lived molecule formed when reactants collide; it can return to reactants or form products. Reaction coordinate diagram

33. 32 Effective Collisions How can the number of effective collisions be increased? Increase concentration of reactants, Increase temperature of reaction, Increase surface area of reactants.

34. 33 Potential Energy Diagrams and Catalysts Energy of Activated Complex

35. 34 CATALYSIS Catalysis LOWERS activation energy! Uncatalyzed reaction Catalyzed reaction MnO 2 catalyzes decomposition of H 2 O 2 2 H 2 O 2 ---> 2 H 2 O + O 2

36. 35 REACTION MECHANISMS mechanism: the elementary steps involved in a chemical reaction. Most chemical reactions DO NOT occur in a single step!

37. 36 Reaction Mechanisms B. Rate Determining Step: the slowest step in the mechanism; thus determining the overall rate

38. 37 C. Intermediates: A substance that is not an original reactant or a final product; it is produced in one step and consumed in another step of the mechanism

39. 38 D. Catalysts: A substance that speeds up the reaction but is not permanently changed by the reaction; both an original reactant and a final product

40. 39 Sample Problem H 2 O 2 (aq) + I 1- (aq)  H 2 O(l) + IO 1- (aq) Slow H 2 O 2 (aq) + IO 1- (aq)  H 2 O(l) + O 2 (g) + I 1- (aq) Fast Identify the following: Rate determining step? Are there any intermediates or catalysts in the reaction? If yes who?

41. 40 Sample Problem O 3 (g) + Cl(g)  O 2 (g) + ClO(g) Slow ClO(g) + O(g)  Cl(g) + O 2 (g) Fast Identify the following: Rate determining step? Are there any intermediates or catalysts in the reaction? If yes who?