Iodine Clock Kinetics
After a brief induction period, this clear solution turns suddenly blue. Changing the concentration of reactants in this clock reaction changes the induction period. The reaction is first run with a certain concentration of reactants and the induction period is timed. Then the same reaction is run with the concentration of potassium iodide halved and the induction period is timed again. Lastly the hydrogen peroxide concentration is halved and the the induction period is timed. These kinetics data can be used to derive the order of dependency of the rate on the concentration of iodide ion and hydrogen peroxide.
This demo is good for a unit on kinetics. This demo takes about twelve minutes to present.
For the entire explanation of this demo see Bassam Shakhashiri.Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 4, pp 42-43. U. of Wisconsin: (1992). The following is a excerpt from this volume.
" The sudden change from colorless to deep blue solutions in this demonstration can be explained with the following sequence of equations:
- 3 I-(aq) + H202 + 2 H+(aq) ==> I3-(aq) + 2 H2O(l)
- I3-(aq) + 2 S2O3 2-(aq) ==> 3 I-(aq) + S4O6 2-(aq)
- 2 I3-(aq) + starch ==> starch-I5- complex + I-(aq)
The first equation indicates that, in an acidic solution, iodide ions are oxidized by hydrogen peroxide to triiodide ions. These triiodide ions are reduced back to iodide ions by thiosulfate ions, as indicated in equation 2. This reaction is much faster than the reaction of equation 1; it consumes triiodide ions as fast as they are formed. This prevents any readily apparent reaction of equation 3. However, after all the thiosulfate ions have been consumed by the reaction of equation 2, triiodide ions react with starch to form the blue starch-pentaiodide complex ." The "A" beakers contain sodium thiosulfate, potassium iodide, and a little bit of starch. The "B" beakers contain hydrogen peroxide and sulfuric acid.
- Reaction 1: [H2O2] = 0.045 M; [KI] = 0.050 M
- Reaction 2: [H2O2] = 0.045 M; [KI] = 0.025 M
- Reaction 3: [H2O2] = 0.0225 M; [KI] = 0.050 M
The concentrations of the other reactants remain constant. ([H2SO4] = 0.10 M; [Na2S2O3] = 0.0028 M) By comparing the length of the induction periods, conclusions can be drawn regarding the depndence of the rate of reaction on the concentration of reactants.
- Three 600 mL beakers containing clear solutions labeled "1A", "2A", and "3A". (See "Discussion" section for contents of the beakers.)
- Three 400 mL beakers containing clear solutions labeled "1B", "2B", and "3B". (See "Discussion" section for contents of the beakers.)
- Three glass stirring rods.
- A clock or watch for timing the reactions.
- Pour the contents of beaker 1B into the beaker labeled 1A. Begin timing. Stir vigorously until the solution turns blue. Record the time that elapsed.
- Pour the contents of beaker 2B into the beaker labeled 2A. Begin timing. Stir vigorously until the solution turns blue. Record the time that elapsed.
- Pour the contents of beaker 3B into the beaker labeled 3A. Begin timing. Stir vigorously until the solution turns blue. Record the time that elapsed.
Avoid getting any of the solutions on you. If you do get chemicals on your skin, wash thoroughly with soap and water. If you get chemicals in your eyes, flush with water for 15 minutes and seek medical attention.
For preparation, refer to Bassam Shakhashiri. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 4, p. 39. U. of Wisconsin: (1992). Modify the instructions for Procedure B as follows:
- Prepare two of the "Beaker 2" solutions. These are our beakers "1A" and "3A".
- Prepare one of the "Beaker 4" solutions. This is our beaker "2A".
- Our beakers "1B" and "2B" contain Shakashiri's acidified peroxide solution, "B-3".
- Our beaker "2C" is like Shakashiri's "B-3", but with half of the concentration of hydrogen peroxide.