A reaction profile
for a one-step reaction:
Note that the relative relationship between
reactants and products determines whether the reaction is exo- or
endo-thermic.
The reaction profile changes with catalysis
If the activation energy (Ea) is too large the reaction will occur extremely slowly (if at all). An appropriate catalyst may allow the reaction to go more quickly. The catalyst reduces Ea causing the increase in speed. Compare the catalyzed reaction profile (black) with the catalyzed profile (red).
The change that occurs when a catalyst is added can be observed on the
reaction profile.
A reaction profile
for a catalyzed reaction:
The activation energy (barrier) has
been lowered by the addition of a catalyst. In this example, the
reaction still occurs in one step. Since the activation energy has
been lowered, the energy of the transition state is also lowered.
Often this process is described as a stabilization of the transition
state.
The catalyst may change the reaction mechanism
The addition of a catalyst may cause the reaction to proceed in one or
more steps. If the reaction takes two or more steps then reactive
intermediates will be present. They appear on the reaction profile as
wells or depressions at (usually) higher energies than reactants or
products. Any reaction with one reactive intermediate will have two
activated complexes, one between reactants and reactive intermediates
and the other between the reactive intermediates and the products.
A reaction profile
for a catalyzed, multi-step reaction.
Since the reaction occurs in two steps, a reactive intermediate is
observed, along with two activated complexes
(transition states).
An Example
The decompostion of hydrogen peroxide is an example of a reaction
which is greatly speeded by the addition of a catalyst. Note that
even though the bromine molecules become bromide ions in the first
step of the reaction, molecular bromine is regenerated in the second
step. Thus, the catalyst is neither used nor consumed during the
course of the reaction. 
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