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Chemical Kinetics

Chemical Kinetics. Questions of rate. Questions of path (Chemical Mechanisms) Carol Brown Saint Mary’s Hall. Factors which affect rate. Nature of reactants Temperature State of subdivision Catalysts Concentration. Nature of Reactants. Each chemical system is unique.

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Chemical Kinetics

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  1. Chemical Kinetics • Questions of rate. • Questions of path (Chemical Mechanisms) • Carol Brown • Saint Mary’s Hall

  2. Factors which affect rate • Nature of reactants • Temperature • State of subdivision • Catalysts • Concentration

  3. Nature of Reactants • Each chemical system is unique. • That is why the term “specific” is used so many times in kinetics.

  4. Temperature • Raising the temperature always increases the rate of the reaction. • The exact effect must be determined experimentally. • The “rule of thumb” is that raising the temperature 10oC will double the rate.

  5. Boltzman Distribution

  6. State of Subdivision • Only affects heterogeneous systems. • Examples include • Grain dust explosions • Gasoline fires • Gunpowder

  7. Catalysts • Homogeneous Catalysts • Heterogeneous Catalysts

  8. Concentration • Rate Laws • Each rate law must be determined experimentally!!!

  9. Rate • Rate is defined as the disappearance of a reactant or an appearance of a product over time. Eg.

  10. Form of a rate law • 2A + B --> 2C Rate = k[A]m[B]n order of reaction = m + n + ... “m” and “n” must be determined experimentally. Once the exponents of the reaction are known, the value for k (the specific rate constant) may be found.

  11. 2A + 2B --> C + D • The following data about the reaction above were obtained from three experiments.

  12. The following data were obtained for the reaction (CH3)3CBr with hydroxide ion at 55oC. (CH3)3CBr + OH- --> (CH3)3COH + Br-Write the rate law for this reaction. What is the specific rate constant, with units, for the reaction at this temperature?

  13. The following table gives reaction rate data for NO and H2 at 800oC. Write the rate law. Calculate the specific rate constant with units.

  14. Half-Life • The amount of time necessary for 1/2 of the reaction to occur.

  15. Summary of the kinetics for Reactions of the Type A --> Products that are zero, 1st, or 2nd order in [A].

  16. Requirements for reaction to take place. • 1. The collision must involve enough energy to produce the reaction. The collision energy must equal or exceed the activation energy. • 2. The geometric orientation of the reacting particles must be correct.

  17. Arrhenius Equationk=specific rate constant at a given temperature.A=Frequency factor which includes z (collision frequency) and p (steric factor). Ea is the activation energy; R=8.314 J/mol L K; T is Kelvin temperature.

  18. A Linear Transform

  19. Reaction Mechanisms • A reaction mechanism refers to the individual steps which a chemical reaction takes on its path to a final product. • Each step in the pathway is referred to as an elementary reaction. • The sum of all of the elementary reactions must add up to give the final balanced equation.

  20. The slowest step in the reaction pathway is the rate determining step. It determines the rate of the overall reaction.The orders of reaction for an elementary step are always equal to the coefficients for that step. However, the orders of the reaction for an overall reaction may not be equal to the stoichiometric coefficients of the balanced equation.The orders of reaction for the overall reaction are related to the stoichiometric coefficients of the rate determining step and any steps which come before it.

  21. If an order of reaction is less than the corresponding coefficient of a reactant, some of that reactant must be used up after the rate determining step.Intermediates are formed on one elementary step, and are consumed in a subsequent elementary step. Often a mechanism is confirmed by trapping and identifying a suspected intermediate. Intermediates do not appear in the final overall balanced equation.

  22. A mechanism problem • Step 1 N2H2O2 <===> N2HO2- + H+ (fast equilibrium) • Step 2 N2HO2- ----> N2O + OH- (slow) • Step 3 H+ + OH- ---> H2O (fast) • Nitramide, N2H2O2, decomposes slowly in aqueous solution. This decomposition is believed to occur according to the reaction mechanism above. The rate law for the decomposition of nitramide that is consistent with this mechanism is given by which of the following? • Rate=k[N2H2O2]/[H+]

  23. Another problem • 2 HA + B --> H2 + A2 + B The following mechanism has been proposed. HA + B <==> A- + BH+ (fast equilibrium) BH+ + A- + HA --> A2 + H2 + B (slow) 1. Write a rate law for the reaction. 2. In a certain reaction, the initial rate of disappearance of HA is 2.0 x 10-3 M/s. What is the initial rate of appearance of A2? 3. Identify the intermediates. 4. What is the role of “B”?

  24. Chain Reaction • H2 (g) + Br2 2 HBr • Br2 ---> Br . + Br . • Br . H2 ---> HBr + H . • H . Br2 ---> HBr + Br . • Etc. • Termination: • Br . + Br . ---> Br2

  25. Catalysts • Catalysts speed the rate of the reaction by changing the reaction mechanism and lowering the activation energy. • Homogeneous catalysts • Heterogeneous catalysts

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