Iodide Clock
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?
Background
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.
Apparatus
• Burette
• Pipettes
• 2 250ml beakers
• Stop clock
• Safety Goggles
• Lab coat
• Scales accurate to 2dp
• Thermometer
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