Determining the order of a reaction
Preamble:
The rate at which a chemical reaction proceeds is typically influenced by the amount of each reactant present and the temperature of the reaction vessel. And, typically, this relationship between the reaction rate and reagent concentration takes a simple form known as the rate law: Rate = k[A]^n[B]^m, where A and B are generic reacting species, k is a reaction-specific proportionality constant known as the rate constant, and n and m are the reaction order. The rate law parameters k, n, and m must be determined experimentally.
Once the rate law has been established, the rate of the chemical reaction can be determined. However, this is usually not the information which is sought. Consider how it would be received if while traveling over hill and dale to grandma's house, one of the kids asks, "How long till we get there?", and the father answers, "We are traveling at 60mph!" Similarly, we usually want to know how long it will be until the reaction is complete, or, alternatively, how much of a given reagent remains after a specified period of time, not how fast the reaction is proceeding at a given moment. (This is a bit of an overstatement, but ...) The rate itself is not an answer to either of the above queries.
The "iodine clock" demonstration is a reaction mechanism that has a very visible endpoint, and as such, can be useful for determining the order of a reaction.
KIO3(aq) + NaHSO3(aq) -> I2(aq) + Na2SO4(aq) + H2SO4(aq) + K2SO4(aq) + H2O(l)
There are two elementary steps of interest in the iodine clock:
Step 1: 1O3^(-)(aq) + 3HSO3^(-)(aq) -> I^(-)(aq) + 3SO4^(2-)(aq) + 3H^+(aq)
Step 2: 5I^(-)(aq) + 6H^+(aq) + IO3^(-)(aq) -> 3I2(aq) + 3H2O(l)
The iodine, I2, produced in Step 2 will react with starch (not shown in the equations), producing a deep blue-black solution. The rate of the entire reaction can be measured by timing how long it takes before the blue color appears once the two solutions are mixed. By altering the concentration of one of the reactants, the overall order of reaction can ultimately be determined.
