K910: Effect of Concentration and Temperature on Rates – Iodine Clock

Note:  This demo has two parts, I & II.  When ordering, please specify whether you want Part I, Part II, or both at the same time.

 Introduction

I.  The reaction between iodate and bisulfite in acid medium produces iodine.  Solutions A (iodate) and B (bisulfite) are mixed at room temperature in differing concentrations, producing a blue-black starch/iodine complex after a length of time that depends on the concentrations of reactants.

II.  Identical concentrations of solutions A and B at three different temperatures (hot, room, and cold) are mixed.  The different lengths of time needed to achieve the starch/iodine complex shows the effect of temperature on reaction rates.

Equations:

A1.                   IO3- + 3HSO3- → I- + 3SO42- + 3H+

A2.                   IO3- + 8I- + 6H+ → 3I3- + 3H2O

A3.                   I3- + HSO3- + H2O → 3I- + SO42- + 3H+

 _____________________________________________________

A overall            9IO3- + 27HSO3- → 9I- + 27SO42- + 27H+

B.                     2I3- + starch → blue I5--starch complex + I-

 Reactions A1-A3 dominate (A3 is very fast) until all HSO3- is consumed.  I3- is consumed as fast as it is produced, preventing the blue I5--starch complex from forming.  When HSO3- is gone, reaction B takes over and I3- and starch immediately react to form the blue complex.

  To Conduct Demonstration:

I. Effect of Concentration

1.         Solution A is pre-measured into 500mL flasks and solution B is pre-measured into 250mL flasks in the following relative concentrations:     

2.         Have students prepare to time each reaction from the instant of mixing to the appearance of the dark blue color.

3.         Mix the two solutions by pouring solution B from the small flask into its identical number of A in the large flask; i.e. 1B into 1A, 2B into 2A, etc.  Swirl vigorously to mix the solutions.

4.         Compare times taken for each reaction.  Times should correspond proportionally to relative concentrations.

II. Effect of Temperature

1.         Solutions A and B are pre-measured as above in flasks 3A and 3B (1X concentrations) at the following temperatures:

2.         Have students time the reactions as in part I.  Mix as above by pouring flask 4B into flask 4A, 5B into 5A, and 6B into 6A.  The times of reaction show the effect of increased temperature on rate. 

  Safety 

Goggles should be worn. 

Reference:

B.Z. Shakhashiri, Chemical Demonstrations Vol. 4, p.16, 1992.