Kinetics: Initial rates method

[linyen416]

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?

 

[Nino MMP]

The method of initial rates is referring to measuring the rate of reaction at the very start of the reaction. This is usually done by plotting the concentration of the reactants and products versus time and then taking the gradient of the curve at t = 0. The calculation you provided is the average rate, which is obtained by measuring the amount of product formed over a period of time and dividing it by the total reaction volume and time taken for the reaction. In terms of the reaction mechanism, the reaction between KI and S2O8 is a redox reaction that involves the oxidation of iodide ions to iodine molecules by persulfate ions. This can be further broken down into more specific steps, such as the formation of an intermediate complex and the transfer of electrons. However, this is not necessarily related to the rate law, which is determined primarily by experiments.

 

[Blueshift5]

The experimental method of determining rate law using initial rates is called the "method of initial rates" because it involves measuring the initial rate of reaction at various initial concentrations of reactants. This allows us to determine the order of reaction with respect to each reactant, as well as the overall rate law. The term "initial" refers to the beginning of the reaction, when the concentrations of reactants are at their highest and the rate is at its fastest.

The calculation you have provided appears to be for the average rate of the reaction, not the initial rate. The initial rate is typically measured by taking the slope of the tangent to the curve at t=0, as you mentioned, or by plotting a concentration vs. time graph and determining the initial rate from the initial concentration change. This is because the initial rate is often the most accurate representation of the reaction rate, as it is not affected by any secondary reactions or product formation that may occur later in the reaction.

The reaction mechanism for the iodine clock reaction may involve multiple steps, but the reaction mechanism often refers to the overall process of the reaction. In this case, the reaction mechanism may involve the initial reaction between KI and S2O8, as well as any subsequent reactions that may occur to form the final products. Determining the reaction mechanism from the rate law can be challenging and may require further experimentation or mathematical modeling.