Look Ma! No Hands! A Chemical Clock Labettini
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Look Ma! No Hands! A 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.
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Look Ma! No Hands! A 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! - PowerPoint PPT Presentation
Transcript of Look Ma! No Hands! A Chemical Clock Labettini
Look Ma! No Hands!A 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.
Look Ma! No Hands!A Chemical Clock Labettini
• EQUIPMENT:Timer, 50 ml beaker, 10 ml graduated cylinder, stir rod
• 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.
Cycle Time Durat ion (s) 1 0:00.00
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Look Ma! No Hands!A 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.
Look Ma! No Hands!A 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
Look Ma! No Hands!A 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.
Look Ma! No Hands!A Chemical Clock Labettini
• ANALYSIS– Calculate duration of each cycle by subtracting its
start time from its stop time.– Plot the cycle duration as a function of the cycle
number (cycle 1, cycle 2, etc.). Be sure to plot neatly!
Look Ma! No Hands!A Chemical Clock Labettini
• CALCULATIONS
Cycle Time Duration (s)
• • •
4 1:33.02 16.25
5 1:49.27 16.12
6 2:05.39 16.15
7 2:21.54
• • •
109.27
- 93.02
16.25
125.39
- 109.27
16.12
141.54
- 125.39
16.15
0
5
10
15
20
25
30
0 5 10 15
Cycle Number
Cycle Duration (s)
Look Ma! No Hands!A Chemical Clock Labettini
• EXAMPLE RESULTS:
0
5
10
15
20
25
30
35
40
0 10 20 30
Cycle Number
Cycle Time (s)
Look Ma! No Hands!A Chemical Clock Labettini
• EXAMPLE RESULTS (5/20/10):
Look Ma! No Hands!A 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.
Briggs-Rauscher ClockOverall reaction acts like weights on pendulum which “drives” clock. But:
IO3- + 2 H2O2 + CH2(CO2H)2 + H+ ICH(CO2H)2 + 2 O2 + 3 H2O
Potassium Iodate
Hydrogen Peroxide
Malonic Acid
Sulfuric Acid
Iodusmalonic Acid Oxygen Water
• Reaction contains two stages.
• First stage can take two paths, which change behavior of second stage.
• Reaction oscillates (switches) between the two paths.
Bubbles !
Briggs-Rauscher Clock
HOI accumulates:• Produces I2 and then I-
• Color turns yellow, then blue• Switches to high [ I- ] path
Stage 1
Low [ I- ] Path
HOI depleted:• Consumes I2 and I-
• Color turns clear• Switches to low [ I- ] path
High [ I- ] Path
Stage 2
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!
