Reaction Rates

1. Reaction Rates

2. Thermodynamically stable: a reaction which has an overall positive energy change (non-spontaneous, endothermic) Reaction Rates

3. Reaction Rates

4. Kinetically stable: a reaction that proceeds so slowly that no change is detectable. Reaction Rates

5. Some reactions go to completion. That is, it proceeds until one of the reactants is used up and then stops.Most do not. Reaction Rates

6. Many reactions are reversible. Reactants produce products, while products may decompose back into reactants. A + B AB Reaction Rates

7. When the rate at which the products form, equals the rate at which the products decompose, the reaction has reached dynamic equilibrium. Reaction Rates

8. NOTE: chemical equilibrium says nothing about the amount of products produced (some reactions only produce a small amount of product). Why? Reaction Rates

9. Reversibilty implies that there is always some reactants present, even after products have formed and equilibrium has been established.Why? Reaction Rates

10. Reaction rates are measured in terms of the appearance of the products, or the disappearance of the reactants.Which do you choose? Reaction Rates

11. As a reaction proceeds, samples can be taken and the concentrations of the reactants and products determined. Rates are then computed. Reaction Rates

12. What is concentration? How is it measured? Which units? What would be the units for reaction rates? Reaction Rates

13. Meaning: rate = conc. of species/time aA + bB --> cC + dD Rate = C]/ct = -A]/at Why is rate, computed by species]/t also divided by the molar coefficient? Reaction Rate

14. The minus sign is used because concentrations of reactants decrease with time. The concentration will be expressed in molarity; time may be in seconds, years, etc. Reaction Rate

15. Reaction Rates Concentration vs Time

16. Reaction Rates Concentration vs Time

17. Factors: Nature of the reactants, concentration of the reactants, temperature, amount of reactant surface area, and the presence of a catalysis. Reaction Rates

18. Reactions with bond rearrangements take the longest. Covalent? Ionic reactions proceed almost instantaneously. Nature of Reactants

19. Collisions must have enough energy to rearrange bonds to form products. Nature of Reactants

20. Molecules sometimes have to form activated complexes. Activation energy is the energy required to do this. Nature of Reactants

21. Concentrations: increasing the concentration of a reactant increases the reaction rate. Why? How does pressure effect a reaction? Concentration

22. How does temperature effect reaction rate? Temperature

23. Homogeneous reaction: a reaction where all the reactants are in the same phase. Heterogeneous reactions: reactants are in differentphases (interface). Phases

24. The reaction must take place on the surface of the solid or liquid. How does the amount of exposed surface area affect the rate of the reaction? How can it be increased? Heterogeneous rx

25. Catalysts: a chemical substance that lowers the activation energy and speeds up a reaction but is not consumed in the reaction. Catalyst

26. Homogeneous vs heterogeneous catalysts. Activated complex vs surface catalysts. Inhibitors Catalyst

27. Heterogeneous catalysts: uses adsorption to the surface to speed the production of a product. Catalyst

28. Homogeneous catalysts: 1. reduce activation energy, 2. assist in bringing together reactants. Catalyst

29. 3. Low activation energy = more successful collisions = faster reaction. Catalyst

30. Homogeneous catalyst = activated complex Heterogeneous catalysts = surface catalysts. Catalyst

31. Inhibitors = do not slow reactions, rather they compete for and tie-up reactants. Catalyst

32. Rate expression / rate constants Reaction mechanisms Rate determining step Rate Expressions

33. The reaction rate varies directly as the product of the concentrations of the reactants. Rate Expressions

34. Rate expression: rate = k[reactants]n The rate expression is always determined experimentally. Rate Expressions

35. The specific rate constant is dependent on the size, speed, and kind of molecules involved in the reaction. Rate Expressions

36. There is one k for each reaction at a given temperature. Rate Expressions

37. The exponents of the concentrations determines the ‘order’ of the reaction. Ie. A squared concentration in the expression is considered second order. Rate Expressions

38. Rate dependence on concentration: single reactant, A: rate = k[A]m; k = rate constant, m = order Two reactants, A, B; rate = k[A]m[B]n, overall order = m+n Rate and Concentration

39. In general, m and n are usually positive integers. However, they can be 0 or a fraction. Rate and Concentration

40. Determine m and k from the following rate-conc. data: CH3CHO(g) --> CH4(g) + CO(g); Rate/Concentration

41. Sample problem Rate/Concentration

42. Determine m and k from the following rate-conc. data: CH3CHO(g) --> CH4(g) + CO(g) m = 2, k = 2.0 (M.s)-1 Rate/Concentration

43. First order reactions: ln ([A]o / [A]) = kt [A]o = original conc. of reactant A [A] = conc. of A at any time t. First Order Kinetics

44. Suppose k = 0.250/s, [A]o = 1.00 M. What is the conc. of A after 10.0s? [A] = 0.0819 M First Order Kinetics

45. How long does it take to drop to one half its original value? t1/2 = 2.77s First Order Kinetics

46. Note that, for a first order reaction: t1/2 is independent of the initial conc. It takes just as long to drop from 2.0M to 1.0M as from 1.0M to 0.50M. First Order Kinetics

47. Note that, for a first order reaction: t1/2 is inversely related to k. If t1/2 is small, then k is large and vice versa. First Order Kinetics

48. Rate = k; [A] = [A]o - kt. The plot of [A] versus time is linear. Zero Order Kinetics

49. Rate = k[A]2; 1/[A] - 1/[A]o = kt. The plot of 1/[A] versus time is linear. Second Order Kinetics

50. A series of reactions may have to take place for a reaction to go to completion. The steps are considered the reaction mechanism. Reaction Mechanisms