1. Chemical Clock Labettini
The purpose of this labettini is to illustrate that chemical reactions don’t happen instantly, but proceed at a measurable rate. REACTIONS OCCUR WHEN MOLECULES COLLIDE and THAT TAKES TIME!
You will measure and graph how long it takes for a “clock reaction” to cycle through its color changes: from blue to yellow and back to blue.

2. Chemical Clock Labettini
EQUIPMENT: Timer, 50 ml beaker, 10 ml graduated cylinder, stir rod, placemat (scrap paper)
Cycle
Time
Time (s)
Duration (s) 1
0:00.0 2 3 4 5 6 7 8 9 10 11 12 13 14 15

3. Chemical Clock Labettini
PROCEDURE SUMMARY:
Add 8 ml of Solution 1 to beaker. Rinse grad cylinder.
Repeat for Solution 2.
Get 8 ml of Solution 3 from front table but DO NOT add to beaker until your team is absolutely ready!
Add Solution 3 to the beaker. Stir once. Time the cycles as practiced starting with the first change to blue.
Record times for 15 cycles in notebook. Make a table as shown.

4. Chemical Clock Labettini
TIMING:
Once mixed, the “clock” will cycle from yellow to blue.
You must START timing when the solution first turns blue.
Every time the solution changes from yellow back to blue, you will press the LAP button (green button on left) and record the time. You will press the LAP button again to continue timing.
You must have a timer, a person who says “blue” when the solution changes, and a recorder. A fourth person who watches for changes in the reaction will be helpful.
Continue to time at least 15 cycles.
Practice your timing based on verbal cues from Dr. V
Practice your timing based on color transitions simulated in the following slide.

5. Chemical Clock Labettini
Use this simulated clock reaction to practice your timing skills. We will not proceed until all teams have successfully completed this simulation.
The End

6. Chemical Clock Labettini
PROCEDURE (complete after timing practice):
Get 8 ml of Solution 1 from front table using the labeled graduated cylinder. Use the dropper to top add or remove small amounts if necessary.
Pour Solution 1 into your beaker. Rinse cylinder thoroughly.
Repeat for Solution 2.
Get 8 ml of Solution 3 from front table but DO NOT add to beaker until your team is absolutely ready!
Add Solution 3 to the beaker. Stir once. Time the cycles as practiced starting with the first change to blue.
Record 15 times in notebook.

7. Chemical Clock Labettini
ANALYSIS
Calculate duration of each cycle by subtracting its start time from its stop time. You may use times to the nearest second.
Plot the cycle duration as a function of the cycle number (cycle 1, cycle 2, etc.). Be sure to plot neatly!

8. Chemical Clock Labettini
109.
- 93.
16.
CALCULATIONS
Cycle
Time
Time (s)
Duration (s) • 4
1:33. 93 16 5
1:49.
109 6
2:05.
125 17 7
2:22.
142
125.
- 109.
16.
142.
- 125.
17.

9. Chemical Clock Labettini
EXAMPLE RESULTS:

10. Chemical Clock Labettini
EXAMPLE RESULTS (5/20/10):

11. Chemical Clock Labettini
QUESTIONS: (Answer in notebook)
Did the cycles speed up or slow down as the reaction progressed? Why do you think that happened?
What else changed as the reaction progressed?
List three factors that might have affected the speed of the reaction.

12. Briggs-Rauscher Clock
Overall reaction acts like weights on pendulum which “drives” clock. But:
Reaction contains two stages.
First stage can take two paths, which change behavior of second stage.
Reaction oscillates (switches) between the two paths.
Iodusmalonic Acid Oxygen Water
IO H2O2 + CH2(CO2H)2 + H+  ICH(CO2H)2 + 2 O2 + 3 H2O
Malonic Acid
Bubbles !
Sulfuric Acid
Hydrogen Peroxide
Potassium Iodate

13. Briggs-Rauscher Clock
Stage 1
Stage 2
Low [ I- ] Path
HOI accumulates:
• Produces I2 and then I-
• Color turns yellow, then blue
• Switches to high [ I- ] path
HOI depleted:
• Consumes I2 and I-
• Color turns clear
• Switches to low [ I- ] path
High [ I- ] Path

14. Briggs-Rauscher Clock
Low [ I- ] path consists of 3 sub-reactions.
High [ I- ] path consists of 5 sub-reactions including catalysis by manganese hydroxide
Stage 2 consists of 2 sub-reactions
Reaction oscillates between two paths until overall reaction “runs down” — all reactants consumed and no new products formed. You know its over when the bubbles stop!