MINILAB: 6.3 Starch-Iodine Clock Reaction

 

In a clock reaction, an observable change indicating that a reaction has taken place occurs at some point in time after two or more different reactants are mixed. How soon this change occurs depends on the speed or rate of the reaction. Altering any factors that change reaction rate will change how quickly the observable change is seen. Starch is an organic compound that reacts immediately with iodine to form a dark blue compound. The starch-iodine clock reaction involves timing the rate of formation of this blue compound after mixing two solutions. One solution reacts to produce iodine, and the other solution contains starch.

PURPOSE: to discover that changing concentrations and temperature will affect the rate of a reaction.

HYPOTHESIS:

 

MATERIALS: permanent marker, 5 large test tubes, test tube rack, graduated cylinder, starch-containing solution, iodine-producing solution, ice, hot plate, stirring rod, stop watch

PROCEDURE:

1. Use a permanent marker to label five large test tubes with the numbers 1 through 5, and place them in a test-tube rack.

2. Place 10 mL of starch-containing solution in each of the five test tubes.

3. Place tube 4 in an ice bath and tube 5 in a water bath at 35°C. Leave them there for at least ten minutes.

4. Add the following amounts of iodine-producing solution to the indicated test tubes, stir carefully with a clean stirring rod or a wire stirrer, and record the time it takes for the blue color to appear.

a. 10 mL to tube 1

b. 5 mL to tube 2

c. 20 mL to tube 3

d. 10 mL to tube 4

e. 10 mL to tube 5

5. Summarize your results in a table.

OBSERVATIONS:

 

Initial description

Time to turn blue

Final description

tube 1

 

 

 

 

tube 2

 

 

 

 

tube 3

 

 

 

 

tube 4

 

 

 

 

tube 5

 

 

 

 

 

ANALYSIS:

1. What effect does changing the amount, and therefore the concentration, of a reactant have on the rate of a reaction?

 

Why?

 

2. What effect does lowering the temperature have on the rate of a reaction?

 

Why?

 

3. What effect does raising the temperature have on the rate of a reaction?

 

Why?