1. Chapter 14 Chemical Kinetics Chemical Kinetics CH 141

2. 2 Factors affecting chemical reaction Rate of reaction Average rate, Instantaneous rate Rate law Order of reaction First order reaction Second order reaction Half - life time

3. Chemical Kinetics CH 143 Chemical Kinetics: How fast is the chemical reaction, (i.e. studying of rates of chemical processes).

4. Factors That Affect Reaction Rates 1. Reactant concentration: As the concentration of reactants increases, so does that reactant molecules will collide and rate of reaction increases. 2. Temperature: As temperature increases, the reaction rate increases, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. 3. Catalysts: catalyst increases chemical reactions by changing mechanism. Chemical Kinetics CH 144

5. Speed of a reaction is measured by: the change in concentration with time. For a reaction A  B Reaction Rates Chemical Kinetics CH 145

6. Reaction Rates Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time.  [A] /  t Chemical Kinetics CH 146 A  B

7.  At t = 0 (time zero) there is 1.00 mol A (100 red spheres) and no B present = zero.  At t = 20 min, there is 0.54 mol A and 0.46 mol B.  At t = 40 min, there is 0.30 mol A and 0.70 mol B.  Calculating average rate: Reaction Rates Chemical Kinetics CH 147

8. For the reaction: A  B There are two ways of measuring rate: 1.The speed at which the products appear (i.e. change in moles of B per unit time), or 2.The speed at which the reactants disappear (i.e. the change in moles of A per unit time). Reaction Rates Chemical Kinetics CH 148

9. 9 Example: Reaction of butyl chloride to give butanol. Average rate decreases as the reaction goes on.

10. The average rate of the reaction over each interval = the change in concentration divided by the change in time: Average Rate, M/s C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) Chemical Kinetics CH 1410

11. Instantaneous rate defines as The rate at any instant in time and it is the slope of the tangent to the curve. Average rate: is the change in reactant or product concentration to the change of time. Chemical Kinetics CH 1411 Instantaneous Rate & Average Rate

12. Example: C 4 H 9 Cl(aq) + H 2 O(l)  C 4 H 9 OH(aq) + HCl(aq) –If We plot [C 4 H 9 Cl] with respect to t. –The units for average rate are mol/L·s or M/s. Chemical Kinetics CH 1412 Instantaneous Rate & Average Rate

13. Chemical Kinetics CH 1413 Calculate: 1. average rate ? 2. instantaneous rate at Z point ? Average rate= Y 2 -Y 1 X 2 -X 1 Z

14. Reaction Rates and Stoichiometry What if the ratio is not 1:1? H 2 (g) + I 2 (g)  2 HI (g) Only 1/2 HI is made for each H 2 used. Chemical Kinetics CH 1414

15. In General, for the reaction aA + bBcC + dD (-) sign because Reactants (decrease) with time (+) sign because Products (increase) with time Chemical Kinetics CH 1415 Reaction Rates and Stoichiometry

16. For Example C 4 H 9 Cl(aq) + H 2 O(l)  C 4 H 9 OH(aq) + HCl(aq) Chemical Kinetics CH 1416

17. Chemical Kinetics CH 1417 For Example

18. In general rates increase as concentrations increase. NH 4 + (aq) + NO 2 - (aq)  N 2 (g) + 2H 2 O(l) Concentration and Rate Chemical Kinetics CH 1418 Constant increases

19. From previous table, for the reaction NH 4 + (aq) + NO 2 - (aq)  N 2 (g) + 2H 2 O(l) we note:  as [NH 4 + ] doubles with [NO 2 - ] constant the rate doubles,  as [NO 2 - ] doubles with [NH 4 + ] constant, the rate doubles Concentration and Rate Chemical Kinetics CH 1419

20. Chemical Kinetics CH 1420 Concentration and Rates The above equation is called the rate law, and k is the rate constant. For the reaction

21. For a general reaction w i th rate law m: order in reactant 1 and n: order in reactant 2. The total order of reaction = (m + n + ….) The total order of reaction = zero, if m = 0, n = 0. Rate Law Chemical Kinetics CH 1421

22. Chemical Kinetics CH 1422 Concentration and Rate This reaction is First-order in [NH 4 + ] First-order in [NO 2 − ] The overall reaction order: is the sum of the exponents on the reactants in the rate law. The overall order of this reaction= 1+1= 2 ( i.e. second-order).

23. Chemical Kinetics CH 1423 ABAB Differential Rate Law

24. Chemical Kinetics CH 1424 [A] 0 : the initial concentration at t = 0. [A] t : the concentration after time, t >0.

25. Chemical Kinetics CH 1425 When [A] t is plotted as a function of time, a curve results. Slope = - k First Order Reactions

26. Straight Line Equation y = mx + b Slope= + m intercept = b Chemical Kinetics CH 1426 First Order Reactions

27. A plot of ln[A] t vs t is a straight line. slope = -k intercept = ln[A] 0 Chemical Kinetics CH 1427 First Order Reactions ln[A] t

28. Half-life t 1/2 : is the time taken for the concentration of a reactant to drop to half its original value. For a first order process, when t = t ½, so [A] t = ½[A] 0. Chemical Kinetics CH 1428 First Order Reactions  Half- life time doesn’t depend on concentration of reactant

29. For a second order reaction with just one reactant. Second Order Reactions Chemical Kinetics CH 1429 [A] = [A] 0, t=0 Differential Equation

30. Second Order Reactions The Change of Concentration with Time y = mx + b A plot of 1/[A] vs. t is a straight line with a slope of k. Intercept= 1/[A] 0 Chemical Kinetics CH 1430

31. Chemical Kinetics CH 1431 For a second-order process, set [A] t =0.5 [A] 0. Half-Life of Second Order

32. Chemical Kinetics CH 1432 The decomposition of NO 2 at 300°C is described by the equation NO 2 (g) NO (g) + 1/2 O 2 (g) and yields these data: Time (s)[NO 2 ], M 0.00.01000 50.00.00787 100.00.00649 200.00.00481 300.00.00380 Determining the order of chemical reaction Example

33. Chemical Kinetics CH 1433 Graphing ln [NO 2 ] vs. t yields: Time (s)[NO 2 ], Mln [NO 2 ] 0.00.01000-4.610 50.00.00787-4.845 100.00.00649-5.038 200.00.00481-5.337 300.00.00380-5.573 The plot is not a straight line, so the process is not first- order in [A]. Does not fit: Determining the order of chemical reaction

34. Chemical Kinetics CH 1434 A graph of 1/[NO 2 ] vs. t gives this plot. Time (s)[NO 2 ], M1/[NO 2 ] 0.00.01000100 50.00.00787127 100.00.00649154 200.00.00481208 300.00.00380263 This is a straight line. Therefore, the process is second-order in [NO 2 ]. Determining the order of chemical reaction

35. Chemical Kinetics CH 1435 Practice Problems CH.14 in the book