HOMOGENEOUS CATALYSIS

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

Homogeneous catalysis is a reaction where the physical states of the catalyst is the same as that of the reactants.

Example of homogeneous catalysis is the reaction between aqueous iodide ion and aqueous peroxodisulphate ion catalysed by aqueous iron(III) ion.
Fe3+
S2O82-(aq) + 2I-(aq) ------------> I2(aq) + 2SO42-(aq)

In the presence of aqueous iron(III) ion, the reaction occurs in two steps.

Step 1 : 2Fe3+(aq) + 2I-(aq) ----------> 2Fe2+(aq) + I2(aq)
Step 2 : 2Fe2+(aq) + S2O82-(aq) ----------> 2Fe3+(aq) + 2SO42-(aq)

The catalysed reaction has lower activation energy because it involves the collision of oppositely charged particles while the uncatalysed reaction involves the collision of like charged particles.

Homogeneous catalyst works by forming an intermediate (Fe2+) and then being regenerated in the following step. Fe3+ ion manages to do so because it can vary its oxidation states between +3 and +2. By doing so, it helps to transfer the electron from I- to S2O82-.