3.2.2 Kinetics - Catalysts
Students should:
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Catalysts
The presence of a catalyst increases the rate of a reaction
Catalysts provide an alternative mechanism with a lower activation energy
Heterogeneous catalysts
These are in a different phase to the reactants.
For example a solid catalyst used to increase the rate of reaction between two gases, such as iron in the Haber process, or vanadium(V) oxide in the Contact process.
In heterogeneous reactions (where the reactants are in different states) the size of the particles of a solid may change reaction rate, since the surface is where the reaction takes place, and the surface area is increased when the particles are more finely divided (therefore smaller solid particles in a heterogeneous reaction tend to produce a faster reaction).
Hetrogeneous catalysts provide a reactive site on which an activated complex forms, weakening the bonds and increasing the rate of collisions thus increasing the rate of reaction.
Homogeneous catalysts
These are in the same phase as the reactants.
For example, the use of sulfuric acid to catalyse the esterification of an alcohol and a carboxylic acid.
Homogeneous catalysts operate by reacting with the reactants and eventually producing a reaction pathway of lower activation energy (and also being regenerated at the end of this process).