HETEROGENEOUS CATALYSIS

Explain what is meant by the term " heterogeneous catalysis ".
Give an example and explain how the catalyst works.

In heterogeneous catalysis, the physical state of the catalyst is different from that of the reactants.

Example of heterogeneous catalysis is the reaction between gaseous iodine and hydrogen to form hydrogen iodide catalysed by solid platinum.
Pt(s)
H2(g) + I2(g) ------------> 2HI(g)

Solid platinum provides a suitable surface for the adsorption of the hydrogen and iodine molecules. This is possible because platinum has many empty or half-filled d-orbitals that can be used to form temporary bonds with the reacting molecules.

The adsorption process not only increases the concentration of the reactants on the platinum surface, it also weakens the covalent bonds in the molecules which leads to a decrease in the activation energy. Furthermore, the reacting molecules are now in the correct orientation for new bonds to be formed.



----H----H----I----I----H----H----I----I----H----H----
temporary bonds
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Platinum surface

* the bonds between H-H and I-I are being broken.
* the bonds between H-I are being formed


After that, the HI molecules are released from platinum surface so that other reactant molecules can be adsorbed.