http://www.ck12.org Chapter 18. Kinetics
Hydrogen peroxide is decomposing all the time, but the addition of solid potassium iodide catalyzes the decompo-
sition of hydrogen peroxide into water and oxygen gas. The effect of adding potassium iodide can be seen in the
following video: http://www.youtube.com/watch?v=mtjbg6ZF5K4 (2:06).
MEDIA
Click image to the left for more content.Potential Energy Diagrams
A potential energy diagram can illustrate the mechanism for a reaction by showing each elementary step of the
reaction with its own distinct activation energy (Figure18.9).
FIGURE 18.9
The potential energy diagram shows an
activation energy peak for each of the
elementary steps of the reaction. The
valley between these peaks represents
the intermediate for the reaction.The reaction, whose potential energy diagram is shown in the figure above, is a two-step reaction. The activation
energy for each step is labeled (Ea 1 and Ea 2 ). Additionally, each elementary step has its own activated complex (AC 1
and AC 2 ). In between the two activated complexes is a potential energy well that corresponds to the intermediates in
this reaction. Unlike activated complexes, intermediates can exist for long enough to be detected and characterized.
Note that the overall enthalpy change of the reaction (∆H) is unaffected by the individual steps, since it depends only
on the initial and final states.
Lesson Summary
- Chemical reactions often consist of a sequence of simpler reactions, called elementary steps. All of the
elementary steps of a reaction combine to produce the reaction mechanism.