Chapter 14 - Chemical Kinetics

  1. Reaction Rates
    1. Change in property / change in time
      1. mi/hr
      2. ft/sec
      3. gal/min
    2. Chemical reaction rate
      1. M / s
    3. Average reaction rate
      1. varies with time interval
      2. varies with species monitored
        1. fix with multiplication of rate by the inverse of the stoichiometric factor.
    4. Instantaneous rate
    5. Initial rate
    6. Rates always < 0

Problems: 1,3,5

  1. Dependence of Rate on Concentration
    1. Rate Law
      1. order of reaction
      2. rate constant
        1. units
    2. Must calculate from experimental data
    3. Examples

Problems: 9-15 odd

  1. Changes of Concentration with Time
    1. First order reaction
      1. ln[A]t - ln[A]0 = -kt
        1. ln([A]t/[A]0) = -kt
        2. ln[A]t = ln[A]0 - kt
      2. ½ life
        1. constant wrt concentration for rxn
        2. t1/2 = 0.693 / k
    2. Second order reactions
      1. 1/[A]t - 1/[A]0 = kt
      2. t1/2 = 1/k[A]0
        1. NOTE - ½ life is dependent on initial concentration
        2. will change as reaction progresses!

Problems: 19-25 odd

  1. Temperature and Rate
    1. Rate increases as temperature increases
    2. collision model
      1. sufficient energy
        1. Activation energy (Ea)
        2. Varies with reaction
          1. many rxn Ea about 25 kJ/mol
          2. increase of 10C (at RT) doubles rate for these.
      2. proper orientation
        1. transition-state complex
    3. Arrhenius Equation


        1. A - frequency factor
          1. Frequency
          2. Probability of proper orientation


      2. Deriving:


Problems: 29,31,35,37

  1. Reaction Mechanisms
    1. Elementary steps
      1. Unimolecular
      2. Bimolecular
      3. Termolecular
    2. Mechanism for NO2 + CO CO2 + NO
      1. NO2 + NO2 NO3 + NO
      2. NO3 + CO NO2 + CO2
      3. Rate Laws
        1. Rate-determining step

Problems: 43-49 odd

  1. Catalysis
    1. Allows Rate of a reaction to increase
      1. Inhibitor - decreases the rate of a reaction
    2. Homogeneous Catalysts
      1. Same phase as reactants
      2. Br2 for catalysis of peroxide decomposition (2H2O2 2H2O + O2)
        1. Br2 + H2O2 2Br- + 2H+ + O2
          1. not all, since bromine is consumed at this point
        2. 2Br- + H2O2 + 2H+ Br2 + 2H2O
          1. Bromine regenerated, H+ consumed, water produced
    3. Increases rate of reaction by increasing A or decreasing Ea.
      1. Normally Ea is decreased
    4. Heterogeneous Catalysts
      1. catalytic converter
      2. usually solid in gas phase reaction
      3. Adsorption
        1. binds to surface
        2. Active sites
        3. Explain why no leaded gasoline w/catalyst
    5. Enzymes
      1. biochemical catalysts
      2. Lock & Key mechanism
      3. Enzyme inhibition
        1. Poisons

Problems: 51-57 odd, 61, 63

LEARNING GOALS:

Express the rate of a given reaction in terms of the variation in concentration of a reactant or product with time.

Calculate the average rate over an interval of time, given the concentrations of a reactant or product at the beginning and end of that interval.

Calculate instantaneous rates from a graph of reactant or product concentrations as a function of time.

Explain the meaning of the term rate constant and state the units associated with rate constants.

Determine the rate law from experimental results that show how concentration affects rate.

Calculate the rate, rate constants, or reactant concentration given two of these together with the rate law.

Use the equation ln[A]t - ln[A]0 = -kt and ln([A]t/[A]0) = -kt to determine (a) the concentration of a reactant or product at any time after the reaction has started, (b) the time required for a given fraction of the sample to react, or (c) the time required for the concentration of a reactant to reach a certain level.

Explain the concept of reaction ½ life, and describe the relationship between rate constant for a first order reaction.

Determine the rate law for a given higher-order reaction from the appropriate data.

Use the collision model of chemical reactions to explain how reactions occur at the molecular level.

Explain the concept of activation energy and how it relates to the variation of reaction rate with temperature.

Determine the activation energy for a reaction from a knowledge of how the rate constant varies with temperature.

Explain what is meant by the mechanism of a reaction using the terms elementary steps, rate-determining step, and intermediate.

Derive the rate law for a reaction that has a rate-determining step, given the elementary steps and their relative speeds; or, conversely, choose a plausible mechanism for a reaction given the rate law.

Describe the effect of a catalyst on the energy requirements for a reaction.

Relate the factors that are important in determining the activity of a heterogeneous catalyst.

Explain how enzymes act as biological catalysts using the lock-and-key model.