Introduction
I. The reaction between iodide and hydrogen peroxide produces triiodide. The triiodide ions are reduced back to iodide by thiosulfate at a much faster rate, until the thiosulfate is consumed. Then the triiodide ions form a blue starch-pentaiodide complex, causing the solution to turn from clear to dark. The initial concentration of iodide and thiosulfate are varied, and using the time it takes for the different solutions to change color, a rate order can be determined (see activity sheet developed by Amy Palmer).
EQUATIONS:
A. 3 I- (aq) + H2O2 (aq) + 2 H+ (aq) à I3- (aq) + 2 H2O (l)
B. I3- (aq) + 2 S2O32- (aq) à 3 I- (aq) + S4O62- (aq)
C. 2 I3- (aq) + starch Û blue I5--starch complex + I- (aq)
Reaction B is the fastest until all S2O32- is consumed. I3- is consumed as fast as it is produced, preventing the blue I5--starch complex from forming. When S2O32- is gone, reaction C takes over and I3- and starch immediately react to form the blue complex.
Supplies
Short List
- Goggles and gloves
- 12 x 100 mL beakers
- 1 L volumetric flask
- 2 x 250 mL volumetric flask
- 150 mL beaker
Chemicals
1.74 g sodium thiosulfate pentahydrate (Na2S2O3 · 5H2O)
1.0 g starch
100 mL 2.0 M sulfuric acid (H2SO4)
0.5 M potassium iodide (KI)
100 mL 3% H2O2
Prior to Lecture (prep time ~ 20 min.)
1. Prepare the following solutions:
- Solution A-1: 0.50 M KI (There is stock 0.5 M KI in demo room) – volume needed is 15 mL total
- Solution A-2: 0.028 M Na2S2O3 – 1.74 g Na2S2O3 · 5H2O in 250 mL
- Solution A-3: 1% starch solution, dissolve 1 g soluble starch in 100 mL of water (beaker is fine), solution will need stirring and some heat to dissolve the starch
- Solution B: In a 1 L volumetric flask, add 100 mL 2.0 M sulfuric acid (H2SO4) and 100 mL 3% H2O2 to get a solution that is 0.20 M in H2SO4 and 0.09 M in H2O2. This should be made shortly before the demonstration, within 24 hours has been sufficient.
2. Prepare 12 beakers for the reaction, six solution A and six solution B, to be combined in lecture, according to the chart. Total volume of each solution is 20 mL, total volume once combined is 40 mL (this is the volume used for concentration calculations)
|
|
|
Solution A |
Solution B |
|||||||
|
Beaker |
Volume of 0.50 M KI stock (mL) |
Final [KI] (M) |
Volume of 0.028 M S2O32- stock (mL) |
Final [S2O32-] (M) |
Volume of 1% starch solution (mL) |
Volume of DI H2O (mL) |
Volume of 0.09M M H2O2 and 0.20M H2SO4stock (mL) |
Final [H2O2] (M) |
Final [H+] (M) |
|
|
1 |
5 |
0.0625 |
8 |
0.0056 |
1 |
6 |
20 |
0.045 |
0.1 |
|
|
2 |
4 |
0.05 |
8 |
0.0056 |
1 |
7 |
20 |
0.045 |
0.1 |
|
|
3 |
3 |
0.0375 |
8 |
0.0056 |
1 |
8 |
20 |
0.045 |
0.1 |
|
|
4 |
2 |
0.025 |
8 |
0.0056 |
1 |
9 |
20 |
0.045 |
0.1 |
|
|
4b |
2 |
0.025 |
4 |
0.0028 |
1 |
13 |
20 |
0.045 |
0.1 |
|
|
5 |
1 |
0.0125 |
8 |
0.0056 |
1 |
10 |
20 |
0.045 |
0.1 |
|
To Conduct Demonstration:
- Solutions A and B are pre-measured into six beakers each. (The B beakers are all the same, but it is helpful to label things 1A, 1B, etc) Set up the beakers in two rows so they can be quickly combined.
- Have students ready to time the reactions. Begin combining solutions (start with solution 5) by pouring the B beakers into the A beakers and start a timer. Swirl them to agitate. The solutions will change colors in the order of: 4b, 1, 2, 3, 4, 5, taking approximately 30 seconds to 2 minutes. Note that the color change may not be as instantaneous as it’s supposed to be if the solutions aren’t agitated.
- Record the times and determine the rate from the limiting reagent, thiosulfate, as outlined in the activity document. Sample data set:
|
Beaker |
[KI] |
Time (s) |
|
1 |
0.0625 |
30.1 |
|
2 |
0.05 |
35 |
|
3 |
0.0375 |
38.3 |
|
4 |
0.025 |
55.5 |
|
5 |
0.0125 |
103.6 |
|
4b |
0.025 |
28 |
Safety and Disposal
Goggles should be worn. All solutions are sink disposable with plenty of water.
Reference:
B.Z. Shakhashiri, Chemical Demonstrations Vol. 4, p.39, 1992.
(This is adapted from Procedure B)
Acknowldegements
Kristin Boles, spring 2016, Amy Palmer and Laurel Hyde Boni, spring 2015