Catalysts and Activation Energy
1. A catalyst speeds up a reaction by providing an alternative reaction pathway that has a lower
activation energy. At the end of the reaction, the catalyst remains chemically and
quantitatively unchanged.
2. Catalysts can be described as being homogeneous or heterogeneous.

Homogeneous catalyst - reaction takes place within a single phase, e.g. in the
decomposition of aqueous hydrogen peroxide, aqueous sodium iodide is used as a
catalyst.

Heterogeneous catalyst - reaction involves two different phases, e.g. nitrogen gas
and hydrogen gas react to form ammonia, using iron (solid) as a catalyst.
3. Consider the decomposition of the hydrogen peroxide.
2H2O2 (aq)  2H2O (l) + O2 (g)
This reaction is ordinarily very slow. When aqueous sodium iodide is added as a catalyst
to the hydrogen peroxide, the reaction becomes very fast. (Homogeneous catalyst)
The decomposition appears to take place in a 2-step process:
H2O2 (aq) + I- (aq)  H2O (l) + IO- (aq)
H2O2 (aq) + IO- (aq)  H2O (l) + O2 (g) + I- (aq)
Sum of the two reactions: 2H2O2 (aq)  2H2O (l) + O2 (g)
The iodide ions are not consumed in the reaction.
For every iodide ion (I-) used in the first step, an iodide ion is produced in the second.
Refer to ‘Elephant Toothpaste Experiment’.
Secktanbc/S4/catalysts
4. Consider the decomposition of nitrous oxide (nitrogen oxide).
2N2O  2N2 + O2
This reaction is usually slow, but if the reactants are bought into contact with a metal such
as gold, the reaction speeds up considerably. (Heterogeneous catalyst)
A catalyst lowers the activation energy required for the reaction.
The Ea required for the decomposition of N2O to nitrogen and oxygen is 250 kJ.
However, using the gold catalyst lowers the Ea to 120 kJ.
The reduction in activation energy occurs because the catalyst provides an alternate
pathway of lower energy for the reaction.
The N2O is chemically adsorbed on the metal surface.
A bond is formed between the O of the N2O and an Au atom. This weakens the bond
joining the O to the N, thus making it easier for the molecule to break apart.
Explanation (for HETEROGENEOUS catalysts)
A solid catalyst provides active sites on their surface for the reaction to occur.
The active sites serve as meeting points for the reacting molecules to come
together and form product molecules.
Activation energy is lowered as bonds are weakened at the active sites and the
reaction occurs faster.
Summary
A catalyst has no effect on the relative energies of the reactants and products, It does not affect
the equilibrium constant, Kc. It merely speeds up the reaction, thus allowing it to reach equilibrium
more quickly.
References:
http://as-bio-and-chem.blogspot.sg/2010/09/recapping-rates-of-reaction-kinetics.html
http://www.files.chem.vt.edu/RVGS/ACT/notes/catalysts.html
http://www.chem4kids.com/files/react_catalyst.html
http://www.dynamicscience.com.au/tester/solutions/chemistry/rates/catalyst.htm
Secktanbc/S4/catalysts