In the iodine clock reaction involving persulfate and potassium iodide: why is the experimental method of determining rate law called 'method of initial rates' when acutally the 'average' rate is measured? How does 'intial' come into play? From what I've read it is because you're 'supposed' to be measuring the initial rate, but I thought the intitial rates must be found by plotting a concentration vs. time graph and using the gradient of the curve at t =0? Also, is this calculation correct: rate = (volume of thiosulfate added* concentration of thiosulfate added) (2*total volume of reaction mixture*time for blue colour) Or is this the 'initial' rate? I think it is the average rate. Regarding rection mechanism, I have found that the reaction mechanism of this iodine clock is only referring to the molecular steps in the first equation: the reaction between KI and S2O8. IS that right and how do I go about postulating a mechanism for this reaction based on the rate law I know: 1st order wrt both KI and persulfate? Or is the reaction mechanism harder to determine?