CHAPTER 3. REACTION RATES 3.5
Even at a fixed temperature, the energy of the particles varies, meaning that only some
of them will have enough energy to be part of the chemical reaction, depending on the
activation energy for that reaction. This is shownin figure 3.4. Increasingthe reaction
temperature has the effect of increasing the number of particles with enough energy to
take part in the reaction, and so the reaction rate increases.
Probability of particle withthat KEKinetic Energy of Particle (KE)The distribution of particle kinetic en-
ergies at a fixed temperature.Average KEFigure 3.4: The distribution of particle kinetic energies at a fixed temperatureA catalyst functions slightly differently. The function of a catalyst is to lower the activa-
tion energy so that moreparticles now have enough energy to react. Thecatalyst itself
is not changed during the reaction, but simply provides an alternative pathway for the
reaction, so that it needs less energy. Some metals e.g. platinum, copper and iron can
act as catalysts in certain reactions. In our ownhuman bodies, enzymes are catalysts
that help to speed up biological reactions. Catalysts generally react withone or more
of the reactants to forma chemical intermediatewhich then reacts to form the final
product. The chemical intermediate is sometimes called the activated complex.
The following is an example of how a reaction that involves a catalyst might proceed.
C represents the catalyst, A and B are reactantsand D is the product ofthe reaction of
A and B.
Step 1: A + C→ AC
Step 2: B + AC→ ABC
Step 3: ABC→ CD
Step 4: CD→ C + D
In the above, ABC represents the intermediatechemical. Although thecatalyst (C) is
consumed by reaction 1, it is later produced again by reaction 4, so that the overall
reaction is as follows:
A + B + C→ D + C
You can see from this that the catalyst is releasedat the end of the reaction, completely
unchanged.