Concentration Dependence on the Rate of a Reaction

Brief Description: 

Ten iodine clock reactions are sequenced to the William Tell Overture.  The effect of concentration on reaction rates is demonstrated.


Demonstrate the dependence of reaction rates on concentration.

Explanation of Experiment: 

The reaction takes place in three steps:

  1. 3 H2SO3  +  HIO3  →  H2SO4  +  HI                    fast reaction
  2. 5 HI  +  HIO3  →  3 H2O  +  3 I2                           (blue complex)
  3. H2SO3  +  H2O  +  I2  →  H2SO4  +  2 HI             (clear) instantaneous

Reaction 3 is instantaneous, reducing iodine to iodide as soon as the iodine forms in reaction 2.  When the hydrogen sulfite ion (from sulfuric acid) is used up, iodine can no longer be reduced, and it then reacts with the starch to produce the blue complex.

Materials Preparation: 

Materials Needed:

  • Water, distilled or deionized
  • Sulfuric acid, H2SO4, 6 M, 5 mL
  • Potassium iodate, KIO3, 3 g
  • Sodium bisulfite, NaHSO3, 0.4 g
  • Soluble starch, 2 g or 30 mL
  • Small plastic bottles, 125 mL, 20
  • 250 mL beakers, 10
  • Stirring rods, 10
  • Internet access to play William Tell Overture

Prepare solutions A and B, followed by ten sets of dilutions.

Solution A:  2.0 g KIO3 per liter of water

Solution B:  2.0 g soluble starch in 500 mL water, bring to boil, cool, add 0.40 g NaHSO3 and 5.0 mL 6 M H2SO4.  Dilute to 1 L with water.  Starch solution can also be prepared by adding 30 mL liquid laundry starch to 500 mL water.

Divide the solutions using the following ratios (mL) using burets to accurately dispense the desired quantities:

Solution A dilution:

Cup #     A1     A2     A3     A4     A5     A6     A7     A8     A9     A10

Soln A    50     48      45     42     39      36     34      32      29      27

Water     00     02      05     08     11      14      16      18       21      23

Solution B Dilution:

Cup #      B1     B2     B3     B4     B5     B6     B7     B8     B9     B10

Soln B      50     48     45     42      39     36     34     32      29      27

Water       00     02     05     08     11      14      16     18       21      23

Ideally, the lab coordinator or presenter has access to the internet to play the YouTube video of the Milwaukee Symphony Orchestra rendition of the William Tell Overture Finale which runs approximately two minutes.


Ten student volunteers are recruited from the audience.  Each volunteer is asked to hold one pair of solutions (A and B).  The YouTube video of the Milwaukee Symphony rendition of the William Tell Overture Finale is started and the "conductor" or laboratory coordinator gives the cue at the 58 second mark of the video for all the student volunteers to pour solution A into solution B and mix with the stirring rod provided (no more than 5 seconds of mixing needed).  It is expected that the last solution pair will change color as the music comes to an end.

This demo requires practice.  Temperature has a drastic effect on the rate of reaction.  Use fresh solutions only.  Test with full strength A and B just prior to doing the dilutions.  Equal volumes of undiluted A and B should react in about ten seconds.  If too fast, dilute solution A slightly with water.  If too slow, add a few more drops of sulfuric acid to solution B.  It is imperative that the student volunteers understand that they are NOT to mix until given the cue.  Brief mixing of the solutions is all that is needed.  Sequentially, the lab coordinator taps the shoulder of the appropriate student to extend their right arm in presentation style toward their beaker to signal the color change to the audience, leading to the finale.


Sulfuric acid is severely corrosive to eyes, skin and other tissue.  There is considerable heat of dilution with water.  Dilute solutions harmful to skin and eyes.  Sodium bisulfite is a severe irritant to skin and tissue as an aqueous solution.  Potassium iodate is an oxidizer and a tissue irritant.  Wear chemical splash goggles, chemical resistant gloves and a chemical resistant lab coat.


Solutions can be flushed down the drain with excess water.


General Concept:

Type of Reaction:

Primary Reference: 
Bilash, B., Gross, G.R. and Koob, J.K. (2010), A Demo A Day - A Year of Chemical Demonstrations, Flinn Scientific Inc., pp. 190-191.
Secondary Reference(s): 
Brice, L.K., J. Chem. Ed., (1980), 57, 152.


This demo may take as long as five hours to set up.  The preparation time is substantial, but the impact of this demonstration is clear when performed properly.