1B,EXP 0,Kinetics,F19.pdf - CHEMISTRY 1B Experiment 0...

CHEMISTRY 1B
Experiment 0 Kinetics
Introduction . Kinetics is the study of the rates of chemical reactions. In this experiment you will be
studying the rate of the redox reaction between the peroxidisulfate ion and the iodide ion in aqueous solution.
You will investigate how the rate changes when you vary the concentration of the reactants and also when
you vary the temperature. You will use these experimental measurements to determine the rate law and to
calculate the activation energy for the reaction. You can read about the basic ideas of kinetics- including rate
laws, rate constants, mechanisms, and activation energy- in the Kinetics chapter of your textbook.
The Reaction . The reaction that you will measure the rate of is the reaction between peroxydisulfate ion
(often called persulfate for short) and iodide ion : (rxn #1)
In order to time the progress of this reaction, you will run a "clock" reaction simultaneously in the same
solution. The clock reaction is a rapid reaction between the thiosulfate ion and iodine:
(rxn #2). Note that iodine is produced in reaction #1 and
consumed in reaction #2.
Here's how the reaction timing works. The initial reaction mixture will contain relatively large amounts of
persulfate and iodide; a small, precisely known amount of thiosulfate; plus a few drops of starch indicator
(which will turn blue in the presence of iodine). When all the reagents are mixed together at time zero,
reaction #1 will begin and I
2
will be created as a product. But all of the I
2
created will quickly be consumed
in reaction #2 and it will not have time to interact with the starch. This will be the situation until all the
thiosulfate (S
2
O
3
2- ) is used up. Once the thiosulfate is gone, the I
2
created in reaction #1 will begin to
accumulate in the solution and the iodine and starch will stick together to make a blue-colored complex. So,
when your reaction solution turns blue, you are really measuring the time it takes for all the thiosulfate to be
consumed. Because you know precisely the moles of thiosulfate you started with, you should be able to use
the stoichiometric relationship between thiosulfate and persulfate to calculate how many moles of persulfate
were consumed in that same interval of time, which will allow you to calculate the initial rate of the reaction.
(Note that during this short time interval, the [S
2
O
8
2– ] and the [I – ] change very little compared to their initial
concentrations.)
You will then repeat the experiment using different initial concentrations of persulfate and iodide to see how
that affects the rate of the reaction. This information will allow you to determine the experimental rate law
for the reaction. Finally, you will repeat the experiment at different temperatures to see how that affects the
rate of reaction and the rate constant, k. By plotting a graph of ln k as a function of 1/T, you will be able to
calculate the activation energy for the reaction.