Rates of a Chemical Reaction --  The Iodine Clock Reaction

Discussion

The rate of a chemical reaction can be defined as the amount of product(s) that are formed from the reactants in a particular unit of time.  It could also be defined as  the amount of reactants consumed over a period time  Frequently the rate is expressed in moles of product formed per second, but other units are used as well.

All chemical reactions involve collisions of the reacting particles.   Any factor that will increase the rate at which effective collisions occur will increase the rate of the chemical reaction.  An effective collision is one that occurs with sufficient energy and with the correct orientation of the reacting particles. Increasing the concentration of the reactants increases the likelihood that these collisions will occur. Therefore it increases the rate of a reaction.   An increase in temperature raises the average kinetic energy of the reacting particles.  This in turn increases the likelihood that the particles will collide and that they will collide with sufficient energy .  Thus  increasing the temperature also increases the reaction rate in two different ways

 To illustrate the effects of concentration and temperature on the rate of a chemical reaction we will investigate a chemical reaction sometimes called the iodine clock reaction.

Two solutions, designated solution A and Solution B will be mixed together. The completion of the reaction is marked by a dramatic color change. Solution A contains iodate ion, IO3- and solution B contains hydrogen sulfite ion, HSO3- and soluble starch. The reaction takes place in two steps. The ionic equations for these two steps are as follows:

  1. IO3- (aq) + 3 HSO3- (aq)    I- (aq) + 3 SO42- (aq) + 3 H+ (aq)
  2. 5 I- (aq) + 6 H+ (aq) + IO3- (aq) 3 I2 (aq) + 3 H2O (l)

The presence of soluble starch (not shown in the above equations) and free iodine, I2, produces a characteristic blue color. The rate of the chemical reaction can be determined by timing the interval between the time the two solutions are mixed together and the time the blue color appears. By varying the concentration of the reactants at constant temperature and then by varying the temperature holding the concentration constant, it is possible to determine the effects of these two factors on the reaction rate.

Equipment and materials

 

Set of 18 x 150 mm test tubes  and rack

 

2  beaker  50 cm3

 

2 Graduated cylinders 10 cm3

 

250 cm3 beaker

 

Dropper s  marked solution A and Solution B

 

Plastic cup

 

Solution A :  Potassium Iodate  KIO3 (containing IO3- ion)

 

Clock with second hand

 

Solution B  Sodium metabisulfite in sulfuric acid.  (contains HSO3- ion)

 

Thermometer

 

 

 

 

Note    The Concentration of KIO3  in solution A is 4.3 g per  liter or 0.0200 M

            Concentration of Solution  B  =  0.200 g of Na2S2O5 4 grams of starch and 5 cm3 of 1.0 M H2SO4 per liter

Safety Precautions
Avoid spilling the solutions on your skin and clothing. Solution B contains some acid and may emit some sulfur dioxide or sulfur trioxide fumes. Avoid inhaling it. Put on your safety glasses and keep them on as long as you are in the lab.


 
 
 
Procedures
Part One: The effect of concentration on reaction rate

 

          Using a clean graduated cylinder designated A, measure out 10.0 cm3 of  the potassium iodate solution (solution A). Transfer this solution to a 18 x 150 mm test tube.

          Then using a clean graduated cylinder designated B, measure out 10.0 cm3 of sodium metabisulfite solution (Solution B) and transfer this solution to second 18 x 150 mm test tube. Place this test tube in row next to the test tube containing solution A.

          Make a series of solutions following the table below..  You will successively dilute solution A, using less solution A and replacing the difference with water.   For each solution A sample put a corresponding sample of exactly 10 cm3 of solution B in a second test tube. When you have finished you should have the following sets of test tubes prepared.

Set

Solution A (cm3)

Water(cm3)

Solution B (cm3)

1

10.00

0.0 0

10.00

2

9.00

1.00

10.00

3

8.00

2.00

10.00

4

7.00

3.00

10.00

5

6.00

4.00

10.00

6

5.00

5.00

10.00

 

          For each set take the solution A and the solution B and pour them together into a 50 cm3 beaker. Begin timing and swirl gently.   Record the time it takes for the blue color to appear and record your data  table.


Part Two: The effect of temperature on reaction rate

          Using a clean graduated cylinder designated A, measure out five separate samples of 10.0 cm3 of solution A. Transfer each of these solutions to a 18 x 150 mm test tube.

          Then using a clean graduated cylinder designated B, measure out five separate samples of 10.00 cm3 of solution B. Transfer each of these solutions to a  second  corresponding 18 mm  x 150 mm test tube. 

          Put each pair of test tubes in a different temperature environment using hot water and cold or ice water.  Try to get temperatures of about 45oC, 35oC, 25oC, 15oC, and 5oC. You do not have to have these temperatures exactly, but be sure that you do record your actual temperatures.  For example

o         Take one test tube of solution A and one test tube of solution B and store them in an ice bath.

o         Take a second set and place them in a hot water bath and allow them to come to equilibrium.

o         Take a third set determine their temperature and run the reaction as in step A. Record the reaction time.. Do the same for the set in the ice bath.

o         To get two other temperatures,  cool the hot water with some tap water and allow a set of test tubes to come to equilibrium. Remove the ice from the ice water bath and add some tap water to warm it up to an intermediate temperature.

          Take the set that is in hot water. Determine the temperature of one of the two test tubes and pour them together into one test tube and record both the temperature and reaction time.   Do the same for the other temperatures

Data

Record your data for part one and part two in a suitable data table

 
Calculations and Analysis

Plot your data from part one on a graph, putting cm3 of solution A on the x axis and reaction time on the y axis. Be sure to label your graph and make it large enough to avoid sacrificing precision.  Plot a second graph in which you put cm3 of solution A on the x axis and the reciprocal of reaction time (1/t) on the y axis.

Plot your data from part two on a graph, putting temperature on the x axis and reaction time on Plot a second graph in which you put temperature on the x axis and the reciprocal of reaction time (1/t) on the y axis. 
Make all graphs at  least a half page in size

Describe the shape of each graph.   Based on your experimental data, make a general statement about the effects of concentration and temperature on the rate of a chemical reaction. Explain your statements in terms of collision theory.