Kinetics of the Persulfate-iodide

Kinetics of the Persulfate-iodide
Clock Reaction (4 points)
In this experiment you will investigate the kinetics of the reaction between persulfate
and iodide ions
S2O8
2- + 2I- ? 2SO4
2- + I2 (R1)
The rate of reaction may be measured by adding a small, known quantity of thiosulfate.
The iodine produced in this reaction (R1) is, as it is formed, reduced back to iodide by the
thiosulfate:
2S2O3
2- + I2 ? 2I- + S4O6
2- (R2)
thiosulfate tetrathionate
This continues until all the thiosulfate has been consumed, whereupon any further iodine
formed in reaction (R1) has nothing to react with, so begins to appear in the solution.
Iodine of course is coloured, so by determining when iodine first becomes visible, it is
possible to measure the speed of reaction (R1). To make the measurement more precise,
the colour of the iodine is enhanced by the addition of starch solution.
Self-indicating reactions of this type, in which nothing seems to happen for while and
then a change suddenly becomes visible, are known as clock reactions. Clock reactions
often appear in chemistry shows, and many involve iodine, since the colour change
resulting from the generation of iodine is particularly marked. However, other chemical
systems, such as the ceric/cerous/bromide reactions involved in the
Belousov-Zhabotinskii reaction [1] show similar behaviour.
A few clock reactions are cyclic, so that the abrupt change of colour repeats; these are
known as “oscillating clocks”, for obvious reasons. In an oscillating clock reaction multiple
stationary states exist; the system is repeatedly switched between those states. For this
to happen, chemical feedback is essential - this is typically the result of autocatalysis.
Limitation of the reaction rate by diffusion through the solution is also required. Further
details of oscillating reactions are given in reference [2].