More specific catalysis question?

Question

Overall reaction: Tl+ + 2Ce4+ --> Tl3+ + 2Ce3+

For the catalysis of Thallium oxidized from Ti+ to Ti3+ by the mechanism:

Ce4+ + Mn2+ --> Ce3+ + Mn3+
Ce4+ + Mn3+ --> Ce3+ + Mn4+
Tl+ + Mn4+ --> Tl3+ + Mn2+
Why is the reaction slow without a catalyst? At first I thought it was because it is a termolecular reaction. But then I thought, why couldn't the reaction do this without a catalyst:

Tl+ + Ce4+ --> Tl2+ + Ce3+
Tl2+ + Ce4+ --> Tl3+ + Ce3+ ?

Why exactly is this reaction slow without the catalyst? What makes the top set happen any faster than the lower set?

Answer 1

Perhaps you should consider the stability of the Tl2+ ion. Perhaps it has a tendency, for example, to disproportionate.