OK, so the overall ionic equation does not represent the mechanism. So, what is the mechanism?
If the reaction does not occur in one step, it must happen in a series of steps. The whole reaction cannot go faster than the slowest of these steps. (Think of two cars going along a narrow lane, a slow one in front and a potentially fast one behind it the second car may be capable of going quickly but it cant do so because the first one is controlling the overall speed). There must be at least two steps. Lets guess its actually two, a slow one (rate-determining) and a faster one to follow. One peroxodisulphate ion and one iodide ion must collide in the slow step. This would explain the first-order kinetics with respect to each of the two reactants. The other iodide ion presumably takes part in the later, fast step. Thats as far as we can go by deduction from the existing evidence and even to get this far we have had to make a guess ( philosophers of science call this a hypothesis which is a polite word for a guess!). Any description of a possible mechanism involves further guesses. Pe
