Why Aren't Reaction Orders Always Integers?

[george_ong]

I have just done an experiment on the clock reaction between iodine and persulfate ions. Using my experiment result, I have determined that the reaction orders are about 1.2 with respect to both persulfate and iodine ions.

There is this question ' Explain why the reaction orders should be integers' in the exercise section of my lab manual which I am supposed to answer. Can anyone give me a reason why the reaction orders can be rounded off to the nearest integer?

Thanks.

 

[Borek]

It is not about rounding, in most cases reaction orders ARE integers. They reflect reaction mechanism.

 

[epenguin]

Yes, often the reaction orders are integral and observed deviation is due to experimental error.

However integral reaction orders reflecting mechanism are necessarily true only for elementary reactions. That is when A -> B or A + B -> C (or C + D) is the mechanism , not just the overall result of a more complicated mechanism. For elementary mechanisms moreover, the reaction order is 1 or for each participant (or 2 if the reactants are reacting with themselves A + A -> A₂).

But if the reaction mechanism is a series of steps (with or without back reaction in some of the steps) the overall result even though each step its first order may be a concentration dependence that is more complicated than simple first order. Which gives chemists one possibility or necessity for disentangling to resolve just what is the mechanism.

Then, guessing, what were the observations in this experiment? Was it just the time taken for the solution to suddenly change colour? If so the reciprocal of that time is not, or not self-evidently, the velocity of any reaction. It is the result of a moderately complex process , see e.g. http://en.wikipedia.org/wiki/Iodine_clock_reaction

In principle you'd have to derive the rate equations for that reaction, solve (integrate) them, and then find time for [I₂] to exceed a certain threshold level.

In principle. Maybe there are simplifications possible - look up the references in the link.