Page 185 - Instant notes

F3
ENERGETICS AND MECHANISMS

Key Notes
The temperature dependence of the rate constant of the majority
of chemical reactions is described by the Arrhenius equation,
where E a (the activation energy) and A (the pre-
exponential factor) are characteristic parameters for the reaction.
They may be determined experimentally from a plot of lnk
against 1/T.
This simple model to describe the rate of a bimolecular reaction
assumes that reaction occurs when two reactant species collide
with an energy along their line of centers greater than the
activation energy for the reaction. The species are treated as hard,
structureless spheres that only interact when the distance between
their centers is less than the collision radius (the sum of the radii
of the colliding reactants). The derived rate constant also includes
a steric factor to account for the probability that molecules collide
with the correct relative orientation to permit reaction.
This theory interprets chemical reaction in terms of a loosely-
bound activated complex which acts as if it is in equilibrium with
the reactant species. The molecular configuration of the activated
complex corresponding to the maximum energy along the
reaction coordinate between breaking of old bonds and formation
of new bonds is known as the transition state. The derived rate
‡
‡ ‡
constant is given by K K where K is the equilibrium constant
‡
between reactants and activated complex and k is the first order
rate constant for decomposition of the activated complex into
products. These parameters can be calculated from statistical
mechanics given a postulated model of the activated complex.
A catalyst increases the rate of chemical reaction by providing an
alternative reaction pathway with lower activation energy than
the reaction pathway in its absence. A catalyst is not consumed
and therefore does not appear in the chemical equation for the
reaction. A homogeneous catalyst is in the same phase as the
reactants whilst a heterogeneous catalyst is in a different phase.
Related topics Molecular behavior in perfect Empirical approaches to
gases (A2) kinetics (F1)
Free energy (B6) Statistical thermodynamics (G8)
Fundamentals of equilibria
(C1)