Iodide Clock

Topics: Chemistry, Potassium iodide, Chemical reaction Pages: 2 (268 words) Published: September 27, 2008
How does a catalyst in the form of a metal ion affect the rate of reaction in an iodide clock reaction with potassium peroxydisulphate ions?

When peroxydisulfate ions (S2O82-) react with iodide ions (I-) in the presence of a starch indicator, they produce a dark blue solution. Reaction (1)
S2O82- + 3I-  I3- + 2 SO42-
(peroxydisulfate ion) (iodide ion) (iodine ion) (sulfate ion) To determine the rate law for this reaction a series of changes in the concentration of peroxydisulfate, [S2O82-], and a series of changes in the catalyst while mean the corresponding changes in time, t, needs to be observed and measured. To measure these changes in concentration over time, a chemical "clock" is needed to change the dark blue color back to a colorless solution. This chemical clock is created by adding a series of measured volumes of thiosulfate ion, S2O32- , to Reaction (1). Thiosulfate ions quickly react with iodine ions, I3- , to produce iodide ions. Reaction (2)

I3- + 2 S2O32-  3I- + 2 S4O62-
(iodine ion) (thiosulfate ion) (iodide ion) (tetrathionate ion) This in effect rewinds the clock for another time interval by changing all of the iodine back to iodide. In my reaction I will be using different metal ions to see which the best catalyst for this reaction is. I will also change the concentration of the potassium peroxydisulphate used in the reaction to see whether the catalyst works quicker with higher or lower concentrations of potassium peroxydisulphate.


2 250ml beakers
Stop clock
Safety Goggles
Lab coat
Scales accurate to 2dp
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