homogeneous catalysis

The principle of homogeneous catalysis:

The homogeneous catalyst provides an alternative pathway which has a lower activation energy than the original pathway. In the alternative pathway, an unstable intermediate is formed with lower activation energy, and the unstable intermediate is then rapidly changed to the products. The overall reaction rate speeds up in this way.

For example, the reaction between peroxodisulphate(VI) ion and iodide ion is slow.
S2O8^2-(aq) + 2I^-(aq) → 2SO42-(aq) + I2(aq) .. (slow)

Fe^2+(aq) can be used as a catalyst of the above reaction. When Fe^2+(aq) is added to the reaction mixture, Fe^2+(aq) reduces rapidly S2O8^2-(aq) to SO42-(aq), while Fe^2+(aq) is oxidized to an unstable intermediate Fe^3+(aq). The unstable intermediate Fe^3+(aq) then oxidizes rapidly I^-(aq) to I2(aq), while Fe^3+(aq) is reduced back to Fe^2+(aq). The overall reaction is fast.
S2O8^2-(aq) + 2Fe^2+(aq) → 2SO4^2-(aq) + 2Fe^3+(aq) .. (fast)
2Fe^3+(aq) + 2I^-(aq) → 2Fe^2+(aq) + I2(aq) .. (fast)