Clock Reaction Revised: 9/28/15 CLOCK REACTIONS. Adapted from J. Chem. Ed. 2007, 84, ELETRONIC LABORATORY NOTEBOOK (ELN) INSTRUCTIONS

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1 CLOCK REACTIONS Adapted from J. Chem. Ed. 2007, 84, ELETRONIC LABORATORY NOTEBOOK (ELN) INSTRUCTIONS If you have not already done so, go to the eresources webpage and click on LabArchives (ELN). Read watch the podcast and thoroughly read the directions on how to use the ELN. All work for this experiment must be recorded, attached, or answered in the ELN. Create a pre & inlab page in the Experiment #1 s folder containing the following sections: 1) Equipment and Chemical Tables. Pictures of typical equipment, instrumentation, special reaction set-ups should be attached. Chemicals with constants needed in the experiment and NFPA Fire Diamonds should be listed in the table. (A special Widget is available for making the Fire Diamonds.) In order to have a complete table, you must look ahead at the calculations and think about constants that will be needed. You will also need to enter the information found on the reagent labels in lab, so leave space. 2) Procedures and Observations. PROCEDURAL STEPS MUST BE IN YOUR OWN WORDS DO NOT COPY THE LAB MANUAL. All quantitative data should be recorded with the correct number of significant figures and units. All qualitative observations should indicate the color, depth of color, and either the crystallinity of a solid or the clarity of a liquid. Any cues that indicate a reaction has occurred (such as heat evolution, color change, gas evolution, et cetera) should be noted. It is acceptable to write the procedures and observations together in one section as you perform the experiment. Text for observations should be made blue, while all other text should be left black. All Logger/Pro data files created should be recorded or attached to this ELN page Postlab questions at the end of this document should be answered on a new page in Experiment #1 s folder.

2 INTRODUCTION Clock reactions contain a complex mixture of chemicals that react to cause a physical change after a certain amount of time (induction period). They are called clock reactions because different physical changes occur at predictable time intervals. Many clock reactions change color at predictable times, controlled by the concentration of reactants added. In this experiment, the physical properties we will observe are color changes resulting from starch complexing with iodine species and fluorescence changes resulting from a whitening agent in laundry detergent complexing with iodine species. Many different types of chemicals are involved in this experiment: acids, oxidizing and reducing agents, chromophores, and fluorophores. The acids are easy to spot because acid is typically in the name. Another indication of an acid is it s chemical formula: if H precedes the rest of the formula, it s usually an acid (acetic acid, a.k.a. vinegar, HC 2 H 3 O 2 ). Acids are proton donors, but be careful, one of the acids in this experiment does double duty as a reducing agent (ascorbic acid, a.k.a. vitamin C, HC 6 H 7 O 6 ). A reducing agent is a chemical species that is oxidized (loses electrons) as it reduces another species (iodine, I -, from iodine tincture). Conversely, an oxidizing agent is reduced (gains electrons) as it oxidizes another species (hydrogen peroxide, H 2 O 2 ). Chromophores are chemical species that absorb visible radiation, and, therefore, appear colored. In this experiment starch complexes with iodine to create a deep purple species. Fluorophores are capable of fluorescence (light emission). The fluorescent dye used in this experiment comes is in a commercial laundry detergent containing fluorescence whitening agents (also called optical brighteners). Fun fact: Your clothes fluoresce (glow) under UV lights because they are washed with detergents that contain fluorescent dyes. Our observation of a chromophore s color is the result of the absorption of certain wavelengths (colors) of visible radiation. While we will simply be observing color (and not quantitatively measuring absorbance) in this experiment, you should understand that your qualitative color observations could ultimately lead to quantitative absorption measurements. We will however, quantitatively measure fluorescence. What is fluorescence? Absorption occurs when a wavelength of light is absorbed by a chemical (in our case: whitening agent) exciting an electron to a higher energy orbital (Figure 1). How does the electron release energy and get back to the

3 ground state? The most common pathway is the transformation of the energy into vibrational motions of the molecule (heat). However, some molecules are fluorophores, emitting light when the electron returns to the ground state. This phenomenon is called fluorescence (Figure 2). Excitation of the electron (absorbance) must occur before the electron can relax and emit a photon of light (fluorescence). Energy LUMO HOMO ΔE a = hν a Absorption Figure 1. Absorbance of a compound (electron excitation). Energy LUMO HOMO ΔE e = hν e Emission Figure 2. Fluorescence of a compound (Electron Relaxation/Radiative Emission) We will be using fluorometers to measure fluorescence. These instruments have a light source, a sample holder, and a detector. The light source exposes the sample to a certain wavelength of light and the detector measures the light coming out of the sample. For this experiment we will just be looking for the presence or absence of fluorescence; later in the course we will calculate sample concentrations based off the detector s measurement of light coming out of the sample.

4 SAFETY PRECAUTIONS Safety goggles, aprons, and gloves must be worn at all times in the laboratory. Malonic acid (CH 2 (COOH) 2 ) is corrosive and an irritant. Manganese sulfate (MnSO 4 (H 2 O)) is toxic and sulfamic acid (H 3 NSO 3 ) is an irritant. Sodium iodate (NaIO 3 ) is a strong oxidant and is an irritant. Iodine tincture (I 2 /I - in a 1:1 solution of ethanol and water) is a reducing agent that can burn eyes and skin as well as stain clothes. Hydrogen peroxide (H 2 O 2 ) is a powerful oxidizier. It can cause skin and eye irritation. Wash affected areas thoroughly with cold water. The vitamin C (ascorbic acid, HC 6 H 7 O 6 ) may be contaminated through student activities and are NOT for internal use. Report all spills, accidents, or injuries to your TA. Before starting the experiment, the TA will randomly ask students to do a quick demonstration or talk-through of one of the following: 1) How to properly handle a cuvette (how to wipe it down, what to wipe it down with, how to hold it) 2) Where the waste in Part A and Part B are disposed Read the technique documents and watch the videos on the course website to prepare for these demonstrations every week. Everyone will have presented at least one topic by the end of the quarter. The demonstrations should be short (>1 min) and will be graded. PROCEDURE Work in pairs for all parts of this 2 week experiment. Part A and B will be competed in week 1. Part C and D will be competed in week 2 using the MORE (Model, Observe, Reflect, and Explain) approach where you take knowledge that you gained from week 1 in order to hypothesize what will happen in week 2.

5 WEEK 1 Part A. Traditional Clock Oscillation. In this section you will be performing the Briggs-Rauscher Oscillation. Cyclical reactions oscillate between two different colors (the result of two different chromophores being created) that can be visually observed. The mechanistic steps are complicated and will not be considered in any detail (feel free to look it up on the internet if you re interested). Observe the color changes occurring and hypothesize about the role of the chemicals used in this part are they acids, oxidizing agents, reducing agents, more than one role? You ll also need to think about what you seen in Part A and B when doing the prelab for Part D next week. But mostly, practice your solution making skills and enjoy the show! 1. Measure out 10 ml of DI water with a graduated cylinder and pour it into a 50 ml beaker. Add ~100 mg of sodium iodate and ~200 mg of sulfamic acid to the beaker. Record the exact mass of the sodium iodate and sulfamic acid added. This is solution A. 2. In a different 50 ml beaker, add 10 ml of 3% hydrogen peroxide, ~200 mg of malonic acid, and a small amount of manganese sulfate (~size of a grain of sand). Record the exact mass of the malonic acid added. Add 5 drops of starch solution; make sure to gently shake the starch solution to mix any starch that has settled to the bottom. This is solution B. 3. Swirl both beakers until all the solid dissolves. 4. Add solution B into solution A. Swirl. Record your observations. 5. Place waste in the appropriate waste bottle. DON T poured down the drain. Part B. Distinct Steps in the Oscillation Reaction. Vitamin C Solution Preparation 1. Use a mortar and pestle to crush a vitamin C tablet (contains 500 mg ascorbic acid). 2. Transfer the crushed tablet to a 50 ml beaker and add 1 tablespoon of DI water. What is the difference between DI water and normal tap water? 3. Stir the solution with a glass stir rod and let the undissolved solids settle to bottom of the glass as you work on the rest of the experiment.

6 To investigate the main steps that occur in the oscillation, we ll change a few of the chemicals so that they don t regenerate themselves to go through another cycle. The first step is the oxidation of iodide (I - ) to triiodide (I - 3 ) in the presence of hydrogen peroxide (H 2 O 2 ) and an acid: 3 Iodide (I - ) + Hydrogen peroxide (H 2 O 2 ) + 2H + à Triiodide (I - 3 ) + 2H 2 O (1) For the second step, if starch is present the newly formed triiodide (I 3 - ) reacts to form a purple starch-i 5 - chromophore: Triiodide (I 3 - ) + starch (colorless) à starch-i 5 - complex (purple) + Iodide (I - ) (2a) Alternatively if the second step contains a fluorophore (like the whitening agent in laundry detergent), triiodide (I - 3 ) acts as a fluorescence quencher. (A fluorescence quencher provides a nonradiative pathway for the electron to relax to the ground state. In other words, the solution no longer will fluoresce.) Triiodide (I - 3 ) + hν + fluorescent dye Triiodide (I - 3 ) + non-fluorescent dye (2b) In the third step, the addition of vitamin C (ascorbic acid, HC 6 H 7 O 6 ) results in the reduction of triiodide back to iodide: I ascorbic acid (HC 6 H 7 O 6 ) à 3 I - + 2H + + tetrahydroxy-diketo-hexanoic acid (3) What effect does the vitamin C addition have on the color and fluorescence of the solution? Aside: You ll notice the use of teaspoons and tablespoons. Why? You can try most of these steps (except for the fluorometer just use a black light) at home. Part Ba. Color Change Steps 4. In a 100 ml beaker, add 2 tablespoons of 3% hydrogen peroxide, 1/8 teaspoon of starch solution (make sure to gently shake the starch solution to mix any starch that has settled to the bottom), and 1 tablespoon of white vinegar. Record your observations. 5. Add ¼ teaspoon of 2% iodine tincture (iodide). Record your observations. 6. Add ¼ teaspoon of the vitamin C solution you made earlier in the experiment. Try your best to measure out the solution without disturbing the solids settled at the bottom. Record your

7 observations. (If nothing occurs, add another ¼ teaspoon of the vitamin C solution.) Part Bb. Fluorescence Change Steps 7. In a 100 ml beaker, add 2 tablespoons of 3% hydrogen peroxide, 1/8 teaspoon of liquid laundry detergent, and 1 tablespoon of white vinegar. Observe the mixture under long wavelength UV light and record your observations (color, glowing intensity). 8. Connect the SpectroVis Plus to the LabQuest2, and calibrate the spectrometer: In the screen, click sensors à calibrate. Allow the lamp to warm up. Prepare a blank by filling an empty cuvette with DI water. Wipe the cuvette off with a Kimwipe. Place the blank in the spectrometer. Once the warm-up period is complete, select Finish Calibration and OK. 9. Click on the USB: Abs box and from the dropdown menu select Change Units à Fluorescence (405 nm). 10. Obtain the fluorescence spectrum of the hydrogen peroxide/laundry detergent/vinegar solution. To do this, fill an empty (dry) cuvette with the solution. Wipe the cuvette off with a Kimwipe, place the cuvette in the spectrometer. Click in the screen. The fluorescence spectrum of the solution will be displayed. Click. 11. Pour the contents of the cuvette back in the beaker. Add ¼ teaspoon of 2% iodine tincture (iodide). Observe the mixture under the UV light (use the same wavelength as before) and record your observations. 12. Rinse the cuvette 2-3 times with the solution you made in the last step. Pouring the contents back into the beaker after each rinse. Fill the cuvette with the solution you made in the last step and obtain the fluorescence spectrum. The previous run by selecting the store option after you hit. The fluorescence spectrum of the solution will be displayed. Click. 13. To view all runs overlapped, click on the box that says Run 2 and select All Runs. Send this data to your ELN. 14. Pour the contents of the cuvette back in the beaker. Add ¼ teaspoon of the vitamin C solution you made earlier in the experiment. Try your best to measure out the solution without disturbing the solids settled at the bottom. Record your observations. (If nothing occurs, add another ¼ teaspoon of the vitamin C solution.) 15. All waste from part B can be poured down the sink.

8 Make sure to clear your address and password of the LabQuest2 so others can t access your account. Shutdown the LabQuest2 and not simply put it to sleep. To shutdown the LabQuest2: press the home key, select System à Shut Down à OK. Post Lab Questions 1. In Part A, the reaction oscillates between two different colors. Using information and observations from Part B, propose a chemical species responsible for each color. 2. What does H + represent in the equation 1 of Part B? What is the H + source? Provide the chemical formula and scientific name. 3. What type of reaction does equation 1 represent? (Acid-base, double displacement, or oxidation-reduction). Explain your answer by indicating the roles played by the chemicals in the reaction. 4. The reactions represented by equations 2a and 2b are different because one contains a starch and the other contains a detergent. How else are the reactions different? 5. What happened when vitamin C was added to both solutions? What do you think would happen if you changed the amount of vitamin C? 6. In this experiment, you took the fluorescence spectra of 2 different solutions. Compare and contrast these spectra. 7. Answer the Prelab questions for week 2. (On a new ELN page.) Week 2 uses the MORE (Model, Observe, Reflect, and Explain) approach where you take knowledge that you gained from week 1 in order to hypothesize what will happen in week 2. Model: Your prelab assignment for week 2 is to use your knowledge from week 1 to construct a nano- and/or macroscale understanding of the chemistry you are about to perform. Observe: While completing the procedures in week 2, make detailed observations thinking about the model you created in the prelab. Reflect & Explain: Do your observations prove or disprove your model? Construct a short written report based on your observations that supports or refutes your initial model.

9 WEEK 2 Part C. Altering the Third Step. Model (Prelab) In part C of the experiment, we will add different amounts of ascorbic acid to a solution of hydrogen peroxide, vinegar, iodine tincture (iodide), and laundry detergent. Use the observations made in Part B to help you answer some of the following questions. Predict your observations 1. BEFORE ascorbic acid is added. 2. Immediately AFTER ascorbic acid is added hour AFTER ascorbic acid is added. (Hint: After the addition of ascorbic acid, we have in the mixture: Iodide, H +, H 2 O 2, along with a bunch of chemicals. What does a combination of I -, H +, and H 2 O 2 produce?) 4. After adding different amounts of ascorbic acid. Procedure/Observation Once everyone has finished the first 3 steps of part C, ask the TA to dim the lights. Monitor the rest of Part C under UV light. Which wavelength (long or short) should we use? 1. Make an ascorbic acid solution by dissolving ~300 mg of ascorbic acid in 9 ml of water. Stir until the acid is dissolved. Record the exact mass of acid you added. 2. Obtain three labeled 50 ml beakers. a. Into each beaker add 30 ml of 3% H 2 O 2 and 15 ml of acetic acid. Measure these volumes with a graduated cylinder. b. Into each beaker add 1.5 ml iodine tincture and 0.5 ml of liquid laundry detergent. Measure these volumes with graduated pipets. 3. In beaker #1, add 1 ml of the ascorbic acid solution you made earlier. Stir the mixture with a stir rod. Monitor under UV light. Record the time (in seconds) for the fluorescence to turn off. 4. Repeat the above step: In beaker #2, add 2 ml of the ascorbic acid solution. And in beaker #3, add 3 ml of the ascorbic acid solution.

10 5. All waste can be disposed of in the sink. Part D: Clock Reaction with Laundry Detergent. Model (Prelab) 1. In part D, we will be doing the exact same procedures as we did in part A except we will be adding laundry detergent instead of the starch solution. No observable color change will occur. What kind of change would you expect to observe instead? Explain why? Procedure/Observe Once everyone has finished the first 2 steps of part D, ask the TA to dim the lights. 1. Measure out 10 ml of DI water with a graduated cylinder and pour it into a 50 ml beaker. Add ~100 mg of sodium iodate and ~200 mg of sulfamic acid to the beaker. Record the exact mass of the sodium iodate and sulfamic acid added. This is solution A. 2. In a different 50 ml beaker, add 10 ml of 3% hydrogen peroxide, ~200 mg of malonic acid, and a small amount of manganese sulfate (~size of a grain of sand). Record the exact mass of the malonic acid added. Add 5 drops of laundry detergent. This is solution B. 3. Swirl both beakers until all the solid dissolves. 4. Add solution B into solution A. Swirl. Observe the reaction under a UV light. Which wavelength (long or short) should we use? Record your observations. 5. Place waste in the appropriate waste bottle and DON T poured down the drain. Reflect & Explain Do your results match your initial model for Part C and D? Write a brief paragraph explaining your observations and supporting your conclusions. If your results differ from your model, be sure to explain why you believe this is the case. Be sure to answer all points brought up in the modeling section questions.