IODINE CLOCK REACTION

AP Chemistry – Kinetics’ Lab

The rate of chemical reaction is determined by a number of factors including temperature, concentration of the

reactants, surface area of the reactants, and the presence of a catalyst.

In this experiment, you quantitatively examine the effect of concentration. You will also examine the effect of a

catalyst on the rate of a reaction.

The completion of the iodine clock reaction will be signaled by a change from a clear solution to a “purple”

(deep blue) solution.

The solutions (reactants) will be as follows:

Solution A: 0.20 M potassium iodide, KI (aq)

Solution B: 0.0050 M sodium thosulfate, Na2S2O3 (aq)

(Starch was added to this solution to act as an indicator to detect the presence of iodine ions.)

Solution C: 0.10 M ammonium peroxydisulfate, (NH4)2 S2O8 (aq)

In this reaction, solution B will be the limiting reactant. The reaction mechanisms is as follows:

2 I-1

(aq) + S2O8-2

(aq) I2 + 2 SO4-2

(aq)

I2 + 2 S2O3-2

(aq) 2 I-1

(aq) + S4O8-2

(aq)

As the iodine, I2 , is formed, it is used in the next step. The limiting reactant (solution B) is a source of the

thiosulfate ions, S2O3-2

(aq). When solution B is used up, the excess iodine formed will react with the starch to

form a deep blue (purple colored) solution.

In this experiment, you will vary the concentration of solutions A and C. You will then repeat part of the

experiment using a catalyst. The catalyst is a 0.01 molar copper(II) sulfate solution. The temperature will

remain constant throughout the experiments.

MATERIALS:

Apparatus:

3 – 125 mL Erlenmeyer flasks labeled A, B, and C containing the solutions A, B, and C

3 – 10 mL graduated cylinders labeled A, B, and C

50 mL beaker

stopwatch or a clock with a seconds hand

stirring rod

wash bottle with distilled water

safety goggles

The solution mixtures that you will use are listed below. The procedure for each of the experiments is the same.

WARNING: Contamination is a SERIOUS concern with this lab. Be sure that the beaker is cleaned after each

experiment with distilled water, ALSO be aware of the solutions and the graduated cylinders you pour solutions

into.

Place 5.0 mL of solution B in a small beaker. Prepare the other two solutions in each of their RESPECTIVE

graduated cylinder. When the solutions A and C are ready (and after solution B has been placed in the beaker),

note the time (or get your STOPWATCH ready) POUR solutions A and C CAREFULLY into the beaker and

begin timing the reaction. Stir the mixture with a CLEAN stirring rod. At the first sign of color change, STOP

the timing and record the time for the reaction.

Mixture Solution A ( mL ) Solution B ( mL ) Solution C ( mL )

1 10.0 5.0 10.0

2 7.5 + 2.5 water 5.0 10.0

3 5.0 + 5.0 water 5.0 10.0

4 2.5 + 7.5 water 5.0 10.0

5 10.0 5.0 7.5 + 2.5 water

6 10.0 5.0 5.0 + 5.0 water

7 10.0 5.0 2.5 + 7.5 water

Instructions for using the Catalyst:

In this part of the lab, you will be using the same procedure as above with an additional step. Add 5 drops of

the copper(II) sulfate (catalyst) solution to the graduated cylinder containing Solution C before you begin each

reaction. The reaction mixtures for this part are given in the table below:

Mixture Solution A ( mL ) Solution B ( mL ) Solution C ( mL )

8 10.0 5.0 10.0 + 5 drops CuSO4

9 7.5 + 2.5 water 5.0 10.0 + 5 drops CuSO4

10 5.0 + 5.0 water 5.0 10.0 + 5 drops CuSO4

11 2.5 + 7.5 water 5.0 10.0 + 5 drops CuSO4

Record your results on the data table on the next page. Follow your instructor’s instructions on disposing of the

solutions.

Before leaving the lab, clean the lab station and WASH YOUR HANDS thoroughly.

IODINE CLOCK REACTION

AP Chemistry – Kinetics’ Lab

Date Table:

Mixture Reaction Time in Seconds

1

2

3

4

5

6

7

8

9

10

11

Post-Lab:

1) Calculate the concentration of solution A ( FOR mixtures 1 through 4 ONLY ). Show the calculation for each.

2) Calculate the concentration of solution C ( FOR mixtures 1, 5, 6 and 7 ONLY ). Show the calculation for each.

3) Create a table that would “neatly” organize the concentrations of solutions A and C with respect to the time of

each trial. For example: heading of the table:

Mixture Solution A Solution C Time (in seconds)

4) After the concentrations have been correctly calculated (from post-lab questions 1 and 2):

a) Create three graphs for solution A ( FOR mixtures 1 through 4 ONLY for each of the graphs ):

i) plot the concentration versus time (seconds)

ii) plot the natural log of the concentration versus time (seconds)

iii) plot the inverse of the concentration versus time (seconds)

b) Create three graphs for solution C ( FOR mixtures 1, 5, 6 and 7 ONLY for each of the graphs ):

i) plot the concentration versus time (seconds)

ii) plot the natural log of the concentration versus time (seconds)

iii) plot the inverse of the concentration versus time (seconds)

5) Determine the Rate Law for this reaction.

6) Determine Rate Constant, k.

Special Notes to consider when answering these post-lab questions:

When determining the concentrations, consider the TOTAL volume after all solutions were mixed together.

For each graph, time (in seconds) is on the x-axis.

When plotting the graphs, you may use a graphing program or draw the graphs by hand (you will need to provide your

own graph paper), also, if you draw the graph by hand BE AWARE of the scale of the graph !!!

Also, each of you is responsible for your OWN work, calculations and graphs that are duplicated (copied) from either

your lab partner or ANYONE else, will result in a zero grade for the lab. In other words, do your own work !