Untitled - Kelly Walsh High School

Kinetics 197
The decomposition of hydrogen peroxide is a slow, one-step reaction, especially
if the solution is kept cool and in a dark bottle:
2 H 2O 2 l 2 H 2O O 2
However, adding ferric ion speeds up the reaction tremendously.
2 Fe 3 H 2O 2 l 2 Fe 2 O 2 2 H
2 Fe 2 H 2O 2 + 2 H l 2 Fe 3 2 H 2O
Notice that in the reaction, the catalyst, Fe 3 , reduced to Fe 2 , in the first step
of the mechanism. In the second step, oxidation of Fe 2 back to Fe 3 occurred.
Overall then, the catalyst remains unchanged. Notice also that although the catalyzed
reaction is a two-step reaction, it is significantly faster than the original
uncatalyzed one-step reaction.
A heterogeneous catalyst is in a different phase or state of matter than the reactants.
Most commonly, the catalyst is a solid and the reactants are liquids or
gases. These catalysts provide a surface for the reaction. The reactant on the
surface is more reactive than the free molecule. Many times these homogeneous
catalysts are finely divided metals. Chemists use an iron catalyst in the
Haber process, which converts nitrogen and hydrogen gases into ammonia. The
automobile catalytic converter is another example.
13-6 Mechanisms
Many reactions proceed from reactants to products through a sequence of reactions.
This sequence of reactions is the reaction mechanism. For example, consider
the reaction:
A 2B l E F
Most likely, E and F do not form from the simple collision of an A and two B
molecules. This reaction might follow this reaction sequence:
A B l C
C B l D
D l E F
If you add together the three equations above, you will get the overall equation
A 2B l E F. C and D are reaction intermediates, chemical species that are
produced and consumed during the reaction, but that do not appear in the overall
reaction.