26.2.3—Mode of action of a homogeneous catalyst

A dissolved catalyst participates in one elementary step to form a temporary intermediate or changed oxidation state, then is regenerated in a later step. It lowers the overall activation barrier without appearing in the net equation.

Trace the catalyst through both equations and cancel it only after adding the steps. Fe²⁺/Fe³⁺ and NO/NO₂ cycles are useful examples.

Fe²⁺ can reduce S₂O₈²⁻ to SO₄²⁻ while becoming Fe³⁺; Fe³⁺ then oxidises I⁻ and returns to Fe²⁺.

A catalyst being regenerated does not mean it is unchanged in every intermediate step; its temporary chemical form is part of the mechanism.