A catalyst is a substance that will speed up a reaction by providing an alternate pathway for the reaction to occur. The alternate pathway will have a lower activation energy. A catalyst does not effect the energies of the products or the reactants (ΔH) nor is it consumed in the reaction. The catalyst is used in the reaction but it is regenerated; it can be used over and over again.
Consider the uncatalyzed reaction, A + B → D. If we add a catalyst, we might end up with the following mechanism.
A + catalyst → C
C + B → D + catalyst
We see reactant A reacts with the catalyst to form product C. Then C then reacts with B to form the product D and the catalyst. From the energy diagram below, the activation energy for the uncatalyzed reaction is larger than the sum of the two activation energies for the catalyzed reaction.
The two step reaction has smaller activation energy than the one step reaction, and the catalyst is regenerated at the end of the reaction.
Catalysts are classified as homogeneous or heterogeneous. A homogeneous catalyst is one that is in the solution as either a gas, liquid, or a soluble solid. One example is the iodized catalyzed reaction for the decomposition of hydrogen peroxide. Without a catalyst it would take much longer for the reaction to occur. The uncatalyzed reaction is:
\(\displaystyle 2\;H_2O_2\;→\;H_2O\;+\;O_2\;\;\;\;rate\;=\; k[H_2O_2][I^-]\)
The mechanism for the two step catalyzed reaction is:
\(\displaystyle H_2O_2\;+\;IO^-\;→\;H_2O\;+ O_2\;+\;I^-\;\;\;(fast)\)
with the rate law
\(\displaystyle rate\;=\; k[H_2O_2][I^-]\)
Note how the catalyst is regenerated in the second step.
A heterogeneous catalyst is in a different phase from the solution. They are usually solids that interact with a gas or a liquid reactant. One example of a heterogeneous catalyst is Pd. The reaction takes place on the surface of the catalyst. Hydrogenation is the addition of hydrogen to carbon-carbon double bonds in alkenes. The insoluble catalyst is in the form of palladium as Pd-C, platinum as PtO2, or nickel as Ra-Ni. The H2 bond breaks with each hydrogen attaching to the surface of the metal catalyst. The alkene is also absorbed by the catalyst and the reaction takes place on the catalyst surface forming a saturated alkane.
Another example is in the food industry where hydrogen is added to unsaturated fatty acids with carbon-carbon double bonds to produce shortening and other types of spreads. In biological reactions enzymes are used as catalysts.
Back to Kinetics
Back to Study Guides for General Chemistry 2