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Reaction Rates

Reaction Rates

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Reaction Rates

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  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