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 — Fall 2008
November 19, 2008
Lecture 33: Pre-equilibrium, competing reactions, and unimolecular reactions
References
1. Levine, Physical Chemistry, Chapter 16
Key Concepts
•The steady state approximation (the concentration of an intermediate is constant or time independent)
allows us to:
1. remove intermediate concentrations from rate expressions for reactants or products of the overall
reaction
2. solve algebraic equations as opposed to differential equations
•The steady state approximation is valid in cases where intermediates form slowly but are used up
quickly.
•The alternative or reverse case, where the intermediate forms quickly but is used up slowly results in
the pre-equilibrium approximation.
–For the generic mechanism:
A
k1
GGGGGGB
FGGGGGG
k−1
B
k2
GGGA
C
the pre-equilibrium approximation is valid when k2k−1and k2.
–The kinetic equilibrium is quickly established between A and B. Cforms much more slowly.
(Kinetic equilibrium before thermodynamic equilibrium).
–We can arrive at the intermediate concentration, [B] by using the equilibrium between A and B:
Keq =k1
k−1
=[B]
[A]−→ [B] = k1
k−1
[A]
–Obtaining an expression for [B] in terms of [A] allows us to again to (1) eliminate [B] from any
rate expression and (2) avoid solving differential equations.
•A more complex kinetic mechanism would be having competing reactions with exchange between the
products:
A
k1
GGGGGGA
B
A
k2
GGGGGGA
C
B
k3
GGGGGGB
FGGGGGG
k−3
C
In this case we can have a kinetic and thermodynamic product.
•The relative values of k1and k2determines the kinetic products (which forms faster).
•The relative values of k3and k−3determine which product is thermodynamically more stable (favored
at equilibrium).
•The thermodynamic and kinetic product are not necessarily the same.
