ChemBytes - Chapter 14

Chemical Kinetics answers

CHEMBytes: Additional Problems on Chemical Kinetics
  1. Articles found in the Lascaux Caves in France have a 14C disintegrqation rate of 2.25 disintegrations per minute per gram of carbon. How old are these articles?
    1.57 X 104 years
  2. The decomposition of gaseous nitrogen pentoxide occurs according to the following reaction:
    N2O5(g) 2 NO2(g) + 1/2 O2(g)
    The experimental rate law is
    - d[N2O5] / dt = kexp[N2O5] And the reaction mechanism is believed to be
    N2O5 <=> NO2 + NO3       (Kequil, fast equilibrium)
    NO2 + NO3 NO2 + O2 + NO       (k2, slow step)
    NO + NO3 2 NO2      (k3, fast step)
    (a) Show that this mechanism is consistent with the rate law, and express the experimental rate constant in terms of the rate constant for the elementary processes.
    kexp = 2k2Kequil
    (b) If k = 5 X 10-4s, how long does it take before the concentration of N2O falls to one-tenth its original value?
    It will take 4.6 X 103s, or approximately an hour and 15 min.
  3. Consider the set of reactions
    A + B <=> C + D        (k1 = k-1)
    C + E F        (k2)
    proceeding in a situation where all concentrations are approximately equal. What relationships between the magnitudes of k1, k-1, and k2 will lead to the following rate laws?
    (a) dF / dt = k[A][B][C] / [D](b) dF / dt = k'[A]{B]
    If k-1[D] >> k2[E], then relationship (a) holds. If k-1[D] << k2[E], then relationship (b) holds.