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Ch 15 Rates of Chemical Reactions

Ch 15 Rates of Chemical Reactions

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Ch 15 Rates of Chemical Reactions

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  1. Ch 15 Rates of Chemical Reactions Chemical Kinetics is a study of the rates of chemical reactions. Part 1 macroscopic level what does reaction rate mean? how are reaction rates determined experimentally? how do factors like temp and conc influence rates? Part 2 microscopic level reaction mechanism detailed pathway taken by atoms and molecules in the reaction in order to control the reaction

  2. Ch 15 Rates of Chemical Reactions • The rate of a chemical reaction is the change of concentration of a substance (reactant) per unit time. • Generally D conc reactant D time Slope of the line changes When Conc is large, Time is small, When Conc is small, Time is long.

  3. Ch 15 Rates of Chemical Reactions

  4. Reaction Rate Terms • Concentration in mol/L [ square brackets ] • Changes D X = Xfinal - X initial • Units for reaction rates mol / L . time • Rate Expression for this reaction 2 N2O5 -> 4 NO2 + O2 1 D [N2O5]=1 D [NO2]=D [O2] 4 Dt 2 Dt Dt

  5. Ch 15 Rates of Chemical Reactions • For a chemical reaction to occur, reactants must • physically collide, • with sufficient energy, • With appropriate geometry For the reaction to occur. Rates are dependent on these variables.

  6. Ch 15 Rates of Chemical Reactions • Factors that affect the speed of a reaction • Concentration - greater concentration of reactions results in the faster rate. • Temperature - for endothermic reactions, faster moving molecules react at a faster rate. • Catalyst - substances that accelerate a chemical reaction, but but are not transformed by the reaction.

  7. Effect of Concentration on Reaction Rate • The rate of a reaction is proportional to the concentration of reactants aA + bB -> xX • Rate = k [A]m[B]n k is the proportionality constant called the rate constant [A] and [B] are the concentrations of A and B m and n are determined experimentally

  8. Effect of Concentration on Reaction Rate • Rate = k [A]m[B]n • The order of a reactant is the exponent m or n • The order of a reaction is the sum of the exponents • If the exponent is 1, doubling the concentration, doubles the rate • If the exponent is 2, doubling the concentration, quadruples the rate. • If the exponent is 0, doubling the concentration has no effect on the rate.

  9. Effect of Concentration on Reaction Rate • Rate = k [A]m[B]n • The rate constant is k • k is a proportionality constant that relates rate and concentration at a given temperature.

  10. Effect of Concentration on Reaction Rate

  11. Determining the Rate Equation for a Reaction. • Rate = k [CO]m[NO2]n

  12. Determining the Rate Equation for a Reaction. • Rate = k [NO]m[O2]n

  13. Determining the Rate Equation for a Reaction.

  14. Concentration-Time Relationships:Integrated Rate Law ln [R]t = - kt [R]o

  15. Concentration-Time Relationships:Integrated Rate Law • Second-Order Reactions suppose R -> is second order then - D [R] = k[R]2 D t 1 _ 1 = kt [R]t [R]o

  16. Concentration-Time Relationships:Integrated Rate Law 1 _ 1 = kt [R]t [R]o

  17. Concentration-Time Relationships:Integrated Rate Law 1 _ 1 = kt [R]t [R]o

  18. Concentration-Time Relationships:Integrated Rate Law • Zero-Order Reactions suppose R -> is zero order then - D [R] = k[R]0 D t [R]o - [R]t = kt

  19. Concentration-Time Relationships:Graphical Analysis Method [R]o - [R]t = kt

  20. Concentration-Time Relationships:Graphical Analysis Method ln [R]t = - kt [R]o

  21. Concentration-Time Relationships:Graphical Analysis Method 1 _ 1 = kt [R]t [R]o

  22. Concentration-Time Relationships:Graphical Analysis Method

  23. Concentration-Time Relationships:Integrated Rate Law

  24. Concentration-Time Relationships:Integrated Rate Law

  25. Concentration-Time Relationships:Graphical Analysis • Half-Life and First-Order Reactions

  26. Concentration-Time Relationships:Integrated Rate Law • Half-Life and First-Order Reactions

  27. Concentration-Time Relationships:Integrated Rate Law • Half-Life and First-Order Reactions ln [R]t = - kt [R]o

  28. Concentration-Time Relationships: ln [R]t = - kt [R]o

  29. Particulate View of Reaction RatesCollision Theory • The reacting molecules must collide with one another • The reacting molecules must collide with one another with sufficient energy • The reacting molecules must collide in an orientation that can lead to rearrangement of the atoms.

  30. Particulate View of Reaction RatesCollision Theory • The reacting molecules must collide with one another The rate of reactions is primarily related to the number of collisions which is related to the concentration of molecules. The rate of a reaction is related to the concentration of each reactant.

  31. Collision Theory: Concentration and Reaction Rate. The rate of molecular reactions is related to the number of collisions Which is related to the concentration. Collisions directly related to Concentrations

  32. Collision Theory: Temperature, Reaction Rate and Activation Energy • Recall the Boltzman distribution of molecular energies • The reacting molecules must collide with one another with sufficient energy • Activation Energy, Ea, minimum energy required for molecules to react.

  33. Collision Theory: Temperature, Reaction Rate and Activation Energy • Increasing the Temperature Increases the number of molecules with sufficient energy to react.

  34. Temperature, Reaction Rate and Activation Energy The reacting molecules must collide in an orientation that can lead to rearrangement of the atoms. A graph of this is described as the reaction pathway. Reactant molecules approach each other with Kinetic energy Kinetic energy decreases, potential increases Reactant Molecules collide and bonds rearrange Highest potential energy Transition state Activated complex Product molecules convert potential energy to Kinetic energy as they move apart from each other.

  35. Temperature, Reaction Rate and Activation Energy

  36. Temperature, Reaction Rate and Activation Energy

  37. Reaction Pathway diagrams

  38. Reaction Pathway diagrams

  39. Time, min % E. coli remaining 0 100 10 70 20 21 30 6.3 60 0.6 Example: The following are the data from an experiment to assess the disinfection of wastewater with a given dose of chlorine. Assuming first-order kinetics, determine the rate constant.

  40. Effect of temperature on biological reaction rate The effect of temperature on reaction rate is given by theArrhenius equation: EA= activated energy, J/mol R = Universal gas constant 8.31J/mol-K T = Temperature in Kelvin = (oC + 273) A = Constant (not significantly affected by small temp. change

  41. Arrhenius Equation • Arrhenius Equation Describes all variables • Describes the dependence of reaction rates on energy, frequency of collisions, temperature and collision geometry • rate constant = k = Ae-Ea/RT Where A represents the frequency factor and The rest of the equation represents the fraction of the molecules with minimum energy for collision R = 8.31 x 10-3 kJ/mol.K

  42. Reaction Rates: Arrhenius Equation • rate constant = k = Ae-Ea/RT R = 8.31 x 10-3 kJ/mol.K

  43. Arrhenius Equation and Activation Energy Graph the data above and calculate The slope. The slope = - Ea / R

  44. Arrhenius Equation and Activation Energy

  45. Effect of Catalysts on Reaction Rate • Substances that speed up the rate of a chemical reaction by lowering the reaction barrier (changing the mechanism) • Heterogeneous catalysts (solid in solution) • Homogeneous catalysts -same phase

  46. Temperature, Reaction Rate and Activation Energy A catalyst effects the rate of reaction by impacting collision geometry • http://www.800mainstreet.com/7/0007-005-rea-t-cat.html

  47. Reaction Mechanism • Most reactions are bimolecular • An intermediate molecule is produced and then used in the subsequent reaction(s) • Each elementary step has its own Ea and k which combine to give the overall reaction

  48. Reaction Mechanism • Mechanisms are postulated from experimental data. • Molecularity unimolecular, bimolecular and termolecular

  49. Reaction Mechanism, Molecularity, and Rxn Rate