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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!

kennedy-howe
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Look Ma! No Hands! A Chemical Clock Labettini

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  1. 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.

  2. 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.

  3. 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.

  4. 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

  5. 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.

  6. 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!

  7. Look Ma! No Hands!A Chemical Clock Labettini • CALCULATIONS

  8. Look Ma! No Hands!A Chemical Clock Labettini • EXAMPLE RESULTS:

  9. Look Ma! No Hands!A Chemical Clock Labettini • EXAMPLE RESULTS (5/20/10):

  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.

  11. 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 IO3- + 2 H2O2 + CH2(CO2H)2 + H+ ICH(CO2H)2 + 2 O2 + 3 H2O Malonic Acid Bubbles ! Sulfuric Acid Hydrogen Peroxide Potassium Iodate

  12. 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

  13. 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!

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