The Foundations of Chemistry

CATALYSTS

Catalystsare substances that can be added to reacting systems to increase the rate

of reaction. They allow reactions to occur via alternative pathways that increase

reaction rates by lowering activation energies.

The activation energy is lowered in all catalyzed reactions, as depicted in Figures 16-15

and 16-16. A catalyst does take part in the reaction, but all of it is re-formed in later steps.

Thus, a catalyst does not appear in the balanced equation for the reaction.

For constant Tand the same concentrations,



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16-9

Figure 16-15 Potential energy
diagrams showing the effect of a
catalyst. The catalyst provides a
different mechanism, corresponding
to a lower-energy pathway, for the
formation of the products. A
catalyzed reaction typically occurs
in several steps, each with its own
barrier, but the overall energy
barrier for the net reaction, Ea , is
lower than that for the uncatalyzed
reaction, Ea. The value of Erxn
depends only on the states of the
reactants and products, so it is the
same for either path.

688 CHAPTER 16: Chemical Kinetics

Progress of

uncatalyzed reaction

Progress of

catalyzed reaction

Energy Energy

Ea

forward Ea

reverse

∆Erxn ∆Erxn

Ea′

forward

Ea′

reverse

Ea′ < Ea

Figure 16-16 When a catalyst is present, the energy barrier is lowered. Thus, more

molecules possess the minimum kinetic energy necessary for reaction. This is analogous

to allowing more students to pass a course by lowering the requirements.

Fraction of molecules with

a given kinetic energy

Kinetic energy

Minimum kinetic energy

for catalyzed reaction

Minimum kinetic energy

for uncatalyzed reaction

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 15.14, Catalysis and Reaction
Rate.

:F:F:F:F:F