Metals are often able to form several cations with different charges. Cerium, for example, forms Ce3+ and Ce4+ ions, and thallium forms Tl+ and Tl3+ ions. Cerium and thallium ions react with each other as follows
Elementary step: 2 Ce4+(aq) + Tl+(aq) ? 2 Ce3+(aq) + Tl3+(aq) (very slow)
The reaction rate is enhanced by the addition of Mn2+ (aq), which changes the reaction's mechanism to:
STEP 1: Ce4+(aq) + Mn2+(aq) ? Ce3+(aq) + Mn3+(aq)
STEP 2: Ce4+(aq) + Mn3+(aq) ? Ce3+(aq) + Mn4+(aq)
STEP 3: Mn4+(aq) + Tl+(aq) ? Mn2+(aq) + Tl3+(aq)
a) As noted above, this reaction is very slow. Provide a likely reason for this, and explain briefly.
b) In the presence of Mn2+(aq), the reaction is first order in [Ce4+] and first order in [Mn2+]. Based on this which of the three steps in this mechanism is the rate limiting step? Why?