Kinetics of A Reaction

[jwilson1006]

I'm having trouble answering a question on my pre-lab that is: Write the rate law for this version of the iodine clock reaction.  Could the rate law have been predicted using coefficients in the balanced chemical equation? Explain.  
I have already determined that the rate law is q=k[I-]1[S2O8 2-]-1, but don't know if this could have been predicted with the equation which is: 2I-(aq) + S2O8 2-(aq) ---> I2(aq) + 2SO4 2-(aq). Please help.

[valdorod]

Rate laws can only be determined experimentally.  The coefficients of the balanced equation are not related to the exponents in the rate law.

However, this is not the case if the reaction is an elementary reaction.

[jwilson1006]

Thx I completely forgot about learning that in class.  I have another question, though.  It says: "Using the equation M1V1=M2V2, determine the initial concentrations for each reactant in each experiment.  This will be 21 different contentrations." We did seven different experiments mixing 1-6 drops of 0.010M KI, distilled H2O, 0.10M HCl, 2% starch, 0.0010M Na2S2O3, and 0.040M KBrO3, and then adding 0.040M KBrO3 and measuring the amount of time it took for the iodine in each experiment to react and, because of the starch, turn dark blue. I don't know which concentrations I'm supposed to use because wouldn't there be 49 different concentrations because there are seven reactants and seven experiments.

[valdorod]

You are only interested in the reactants that are part of your rate law. [I-] and [S2O8 2-]

the other reactants are not part of your rate law.  startch is just an indicator, the nitric acid is just acting to keep the solution acidic, the Na2S2O3 is just to keep the iodine reaction constant in terms of calculations, etc, etc, etc.

thus [I-] = initial volume*initial concentration/total volume of all components.
same for [S2O8]

basically you are diluting the two components as you place them into a new volume.

[jwilson1006]

Thx that helps a lot, but wouldn't that give me 14 concentrations, not 21?