Page 198 - Instant notes

F5
RATE LAWS IN ACTION

Key Notes
A system approaching equilibrium consists of a forward reaction
from reactants to products and an opposing back reaction from
products to reactants. The rate at which the system approaches
equilibrium is equal to the sum of the forward and backward
rates.
The Lindemann mechanism is the sequence of underlying
elementary reaction steps that combine to yield an overall first
order rate law for observed unimolecular reactions. The
mechanism postulates that bimolecular collisions between
molecules A produce activated intermediates A* which either
deactivate through further collision or proceed along the reaction
path to products.
For a gas of partial pressure p above a surface with fractional
coverage θ (the ratio of the number of surface sites occupied to
the number available), rate of adsorption of gas to the surface is
k 1 p(1−θ) and rate of desorption of gas from the surface is k −1 θ,
where k 1 and k −1 are the adsorption and desorption rate constants,
respectively. If the adsorbed gas undergoes unimolecular reaction
with rate constant k 2 the observed overall rate of reaction is
k 1 k 2 p/(k −1 +k 1 p).
A photochemical reaction is initiated by absorption of one or
more photons. The corresponding rate law is the product of the
concentration of the absorbing species and a photochemical rate
constant J and is first order. The value of the rate constant
incorporates terms for the intensity of incident light and the
absorption coefficient of the molecule integrated over all
appropriate wavelengths.
Related topics Empirical approaches to kinetics Formulation of rate laws (F4)
(F1)
Rate law determination (F2) The kinetics of real systems
(F6)

Opposing reactions

All chemical reactions are potentially reversible but usually the reverse reaction is so
slow that it can be neglected. However, for reactions approaching an equilibrium that is