The Richard Stockton College of New Jersey
Chemistry Program, School of Natural Sciences and Mathematics
PO Box 195, Pomoma, NJ
CHEM 3410: Physical Chemistry I โ Fall 2008
Kinetics practice problems
November 21, 2008
1. The formation of HI from H2and I2can be described by reversible second order kinetics.
H2+I2*
)2HI
However, direct bimolecular reaction between H2and I2is not observed to occur. A proposed
mechanism for the forward reaction is:
I2*
)2I(1)
H2+ 2 Iโ2HI (2)
(a) Derive a rate law assuming that step (1) is reversible, step (2) is irreversible and step (2) is
the slowest process.
(b) Derive a rate law assuming that step (1) is reversible, step (2) is irreversible and step (2) is
the fastest process. You can use the steady-state approximation for the concentration of the
intermediate [I].
Hint: For both parts, be sure to pay careful attention to the stoichiometry of the proposed elementary
reactions.
2. Consider the reaction:
2A+BโD
Suppose that the mechanism of this reaction is found to consist of the following elementary reac-
tions:
A+B
k1
EGGGGGG
GGGGGGC
kโ1
C
A+C
k2
GGGGGGA
D
(a) Show that this mechanism is consistent with the overall reaction.
(b) Write expressions for d[C]
dt and d[D]
dt in terms of [A], [B], [C], [D], k1,kโ1, and k2.
(c) Impose the steady-state approximation and solve for [C]ss.
(d) Use the steady-state approximation to derive the rate law for d[D]
dt in terms of [A], [B], k1,
kโ1, and k2.
(e) If k2[A]๎kโ1what are the apparent orders of the reaction with respect to [A] and [B]?
(f) If k2[A]๎kโ1what are the apparent orders of the reaction with respect to [A] and [B]?