THE BLUE ZIPPY!
Two Competing Reactions:
H2O2 + 3I- + 2H+ → I3- + 2H2O
I3- + 2S2O22- → 3I- + S4O62-
The triiodide ion is responsible for the color change.
The timine is conrolled by the initial amount of sodium thiosulfate added.
When the sodium thiosulfate is depleted the solution turns blue.
The four nonlimiting reagents are: hydrogen peroxide, sulfuric acid, potassium iodide, and (corn) starch. Each of these is mixed in a 1:1 ratio with the next, usually 1cc. With sodium thiosulfate as the limiting reagent, the amount added is varied to generate a different time of completion of the reaction.
In general, the more sodium thoisulfate present in the reaction, the greater the reaction time. This is because there is a linear relationship between the amount of sodium thiosulfate and the reaction time at a specific temperature. The usual amounts of thiosulfate added are from 100 to 600 mL. By generating a calibration curve in the competition venu, we are able to determine how much thiosulfate to add to the iodine clock to have the car stop at the time predicted by the velocities of the time trials.
The calibration curve (above) was conducted at a temperature of 298K. In the future, we expect to have a three dimensional correlation between time, the amount of sodium thiosulfate, and the temperature. This will eliminate the need to generate a calibration curve the day of competition.
Other factors that effect the performance of the iodine clock include the decomposition of the reagents, particularly the hydrogen peroxide and the starch. We solve this problem by mixing the appropriate molarities of the reagents the morning of competition.