Reaction Kinetics

In this experiment we will investigate the reaction kinetics for the oxidation of iodine ion by hydrogen peroxide:

H2O2 + 2I- + H+ -> I2 + 2H2O

The rate of this reaction and its dependence on the concentration of iodine ion and hydrogen peroxide will be determined.

Reaction rates can be affected by the concentration of reactants, temperature and the presence of a catalyst.

The reaction rate is expressed as the rate of change of concentration of the participating species. In our case reaction rate, in terms of change of hydrogen peroxide concentration, may be expressed as:

rate = d[H2O2]/dt = k[H2O2]m[I-]n

where K = rate constant, (units are M/s)
m = order of the reaction with respects to the concentration of hydrogen peroxide
n = order of the reaction with respects to the concentration of iodine ion

It is given that in this reaction, rate is independent of the concentration of H+.

In order to determine the rate of oxidation reaction we will use a second simultaneous reaction, sometimes called a clock reaction. By reacting thiosulfate ions with the iodine product from our oxidation.

I2 + 2S2O3 2- -> 2I- + S4O6 2-

This reaction will consume all I2 as soon as it is produce from our oxidation reaction. This will prevent our solution from turning blue from the I2 product. Once all the thiosulfate is consumed the I2 will appear and our solution will turn blue. Since the stoichmetric ratio of thiosulfate ion to hydrogen peroxide is known (2:1) and the initial quantity of thiosulfate ion is also known, we can determine the amount of hydrogen peroxide consumed by our oxidation over the time the thiosulfate reacted with Iodine.

In our experiment we will perform several runs. Half holding the [H2O2] constant while varying [I-] and then holding [I-] constant while varying [H2O2]. This will allow us to determine the rate constant. Plotting the log[rate] vs log [H2O2] (with [I-] constant) will allows to determine m, order of reaction wrt to [H2O2]. The slope of this line will equal m, when log[rate] is plotted on the y-axis. Again, plotting the log[rate] vs log [I-] (with [H2O2] constant) will allows to determine n, order of reaction wrt to [I-].