The Vitamin C Clock Reaction

Transcription

1 The Vitamin C Clock Reaction Abstract: Clock reactions are chemical curiosities because when reagents are mixed, nothing seems to happen for a period of time, then all of a sudden there is an abrupt change in color. The vitamin C clock reaction remains colorless for a while, then changes abruptly to blue-black. In this experiment, we will determine the kinetics that explains this strange behavior. Introduction The basis for the vitamin C clock reaction is the relatively slow reaction of hydrogen peroxide with iodide ion, and it the rate of this reaction that we ll actually be measuring: + I I (aq) + (l) In the presence of starch, the iodine (I ) which is produced forms the well known blueblack complex which is commonly used to test for the presence of starch. This test is used to detect counterfeit money, because US paper money is printed on linen paper which contains no starch, while most other kinds of paper contain starch as a filler. An iodine pen gives a yellow mark on US currency, but the ink turns blue-black on ordinary paper. If reaction () were the only reaction involved, mixing solutions of hydrogen peroxide and iodide ion would gradually change from colorless to blue-black. That s not what we observe, because the solution also contains a small amount of vitamin C. Vitamin C reacts rapidly with iodine, so as long as the vitamin C is present, the iodine which is produced by reaction () is converted back to I - by the vitamin C. When the vitamin C is finally used up, the iodine produced by reaction () reacts with starch to give the color change. The reaction of vitamin C with iodine is a well known reaction. As a matter of fact, titration with iodine is one method for determining the vitamin C content of foods. The equation for the reaction is: r I I - () I + C (aq) + + I - (aq) + C (aq) (a) Note that equation () contains abbreviated structural formulas, where carbon atoms are assumed to be located at each point where straight line bonds intersect. For example, the small arrow points to the position of a carbon atom in the vitamin C (ascorbic acid) molecule. Also, hydrogen 83

2 atoms are not always shown; two hydrogen atoms would complete the Lewis structure for the carbon that the small arrow points to in the product (dehydroascorbic acid), giving a total of 4 bonds and 8 valence electrons. The dehydroascorbic acid product of reaction () is unstable and reacts rapidly to give the final product,,3-diketogulonic acid, according to equation (3), but this causes no observable change: (3) + To summarize: To make Solution A, we ll mix a solution of vitamin C to tincture of iodine solution, which is red-brown, and the solution will immediately become colorless as the vitamin C rapidly reduces the I to I -. We ll then mix the solution with Solution B, which is aqueous hydrogen peroxide. The peroxide slowly oxidizes the I - to I, but excess vitamin C in Solution A reduces the I rapidly to I -, so no color appears until the vitamin C is consumed. Then (after several seconds or minutes) the solution turns blue-black. Starch Typically, about 0% of the starch in most species is amylose, which forms a colloidal dispersion in water. The remainder is amylopectin, which is completely insoluble. igh amylose cornstarch is unusual, containing 85% amylose, so it is used where a soluble form is needed. The amylose molecule is a polymer of glucose units joined by the alpha-,4- acetal linkages (Fig. ). Figure. alpha-,4- acetal linkages in amylose ydrogen bonding and interactions with the surrounding water pull the amylase into a helical structure, as shown in Fig.. The molecule may have many glucose units, so it resembles a helical spring with many coils. Iodine While your body contains only about 0.0 g of iodine, it is an essential mineral, and deficiency leads to a disease condition called goiter. Goiter is rare because most salt Figure. Amylose helix 84

3 contains iodine, but it s supplied as NaI (sodium iodide), not as the element, I. NaI is a white crystalline ionic compound, while I is a purple (almost black) solid which sublimes at room temperature, as evidenced in the photograph by the (purple) gaseous iodine in the flask above the crystals. The name iodine comes from the Greek iodes, meaning purple. Iodine is not very soluble in water, but it reacts with dissolved iodide to give the yellow/brown, linear I 3 - and I 5 - complex ions. Tincture of iodine (%),) contains 0.08 M I (0 g or 0.08 mol per liter) and.4%, or 0.6 M NaI (4 g or 0.6 mol) in a solution of ethyl alcohol (ethanol, drinking alcohol) and water. Thus when it is reduced by vitamin C, it provides 0.3 M I - for the reaction in Eq.. Iodine is used in the well known test for starch 3, in which a blue-black color results from the inclusion of the triiodide ion I 3 - (or I 5 - ), in the helical coil of amylase 4 as shown in Figure 3 (neither I - or I alone gives a positive test). Figure 3. Starch/iodine complex Kinetics Data Analysis To determine the rate law for a process, we first need to measure rates, then we need to find out how the rates change as the concentration of reactants change. If the rate doubles when the concentration of a reactant doubles, we know the rate is first order with respect to that reactant; if the rate remains the same when a concentration is doubled, we know the reaction is zero order with respect to that reactant, and so forth. We will be measuring the rate of the reaction in equation () by knowing that the time it takes for the starch/iodine color to appear is the time it takes for all the vitamin C to disappear. By equations () and (), mole of vitamin C is consumed for every mole of that is [ consumed, so the rate of the reaction can be expressed as, which is the same as [ Vit C. Not all of the hydrogen peroxide is consumed in any reaction, and that s good, because it s better to measure the rate of a reaction at the onset, when the concentration is known and the product concentration is too low to cause significant interference. So, for example, if we had a reaction mixture containing M vitamin C, 0.50 M, and M I - which turned blue-black after 0 seconds, the rate of the reaction would [ [ Vit C 0.000M 5 be R = = = = 5.0 x0 M / s 0s While the hydrogen peroxide concentration is reduced by 0.00 M, the iodide concentration is not changed at all, because as it is consumed by to make I, vitamin C reduction of the I replaces the I - immediately by equation (). The rate law for equation () will be [ a b + c R = = k[ [ I [... (4) 85

4 where brackets indicate molar concentrations, superscripts are the orders with respect to each species, and the ellipsis ( ) indicates that other species that have not been included may have an effect on the rate as well. To determine the orders a, b, and c, we determine the rate R for a set of concentrations [, [I -, and [ +, then repeat the experiment with a different concentration for one of the reactants, say [, to see what the new rate, R will be. Since all concentrations but one were held constant, any effect on the rate must be due to the change in one reactant. [ a b + c R = = k[ [ I [... (5) [ a b + c R = = k[ [ I [... (6) Then dividing equals by equals [divide the left side of (5) by the left side of (6) and the right side of (5) by the right side of (6), gives R R k[ [ I [ a b + = a b + k[ [ I [ c c [ = [ a a = [ [ a and taking the log of both sides gives R (3) = [ ln a ln R [ since ln[ a = a ln [. This expression can always be used to calculate the order (a) of the reaction if two rates are measured at two different known concentrations of one reactant, while all other concentrations are held constant. Again, this equation states mathematically that doubling the concentration of will: (a) have no effect on a process which is zero order with respect to (a=0) (b) double the rate of a process which is first order with respect to (a=) R (c) quadruple the rate of a process which is second order with respect to (a=) We will measure the rates of the reaction at just on p, so the order with respect to hydrogen ion, +, will not be determined. Temperature Effects: Determination of Activation Energy: In order to determine the activation energy, E a, for the reaction, we will run the reaction a second time at a higher temperature, determine the new value of the rate, and substitute the two rates and two temperatures into the Arrhenius equation: (4) ln R R E a = R T T 86

5 The value of the ideal gas constant (R) is 8.34 J/mol K. The rates R and R may be used in this equation if they are determined with all parameters except the temperature held constant; otherwise, rate constants k and k are normally substituted in this equation for R and R. Procedure:. Prepare a M solution of vitamin C by adding 0 ml of distilled water to a 500 mg tablet in a clean evaporating dish, and crushing the tablet with a pestle for several minutes, until all soluble solid is dissolved. Rinse the solution from the pestle into the evaporating dish, then pour the liquid into a 50 ml volumetric flask, and rinse the residual solution from the dish into the flask with distilled water from a squirt bottle, then dilute to the mark. Pour the liquid into a clean, dry, 00 ml beaker, and place a clean 5 ml syringe in the beaker. The suspended solids do not interfere with the experiment.. With the 5 ml syringe, add 5.0 ml of the vitamin C solution to a 5 ml volumetric flask (take the solution from near the surface, avoiding precipitate as much as possible), add.0 ml of tincture of iodine, then dilute to the mark. Enter your observations and the species present on the spreadsheet. This will be Solution A. Add it to a 50 ml beaker, and place a clean 3 ml syringe in the beaker for measuring this solution. 3. Add 7mL of 3% (0.9 M) and 5 ml of % starch solution to a 50 ml volumetric flask and dilute to the mark to make Solution B. Add it to a 00 ml beaker and place a clean 3 ml syringe in the beaker for measuring this solution. 4. btain 5 ml of acetic acid solution (vinegar, to reduce changes in p) in a 50 ml beaker, and place a clean 3 ml syringe in the beaker for measuring this solution. 5. Calibrate the colorimeter. Any wavelength (color) will be strongly absorbed by the blueblack starch iodine complex. Set up data acquisition to make absorbance measurements every 4 seconds. 6. Add the volume of vinegar solution required for reaction mixture to a colorimeter cuvette. Try to hold both the end of the syringe and top of the cuvette to avoid spilling. Now add measured volumes of Solution A and Solution B rapidly enough to cause mixing, or quickly mix them with a plastic swizzle stick. Place the cuvette quickly in the colorimeter, and commence repeated measurements. Prepare an identical sample to observe outside the spectrometer. 7. As soon as the absorbance rises sharply to above or so, stop data acquisition. Measure the temperature of the sample as soon as possible with the temperature sensor, and record the temperature in the spreadsheet table. Copy the time and absorbance data into Excel. Use the time when the absorbance first rises above baseline as the time entry for that run in the data table. 8. Rinse the cuvette and dry the outside, and repeat steps 6 and 7 for reaction mixtures -5 at room temperature (but copy only the absorbance readings into the template). 87

6 9. Nest two Styrofoam coffee cups, one within the other, to make an insulated constant temperature bath, and add a temperature sensor. It may be a good idea to put the cups in a 500 ml beaker so that they don t tip. alf fill the cup with icewater to get a temperature below about 5 o C (with all ice melted) or add warm tap water at about 3 o -36 o C. 0. Use the syringes to add vinegar and hydrogen peroxide solution to a cuvette, then cap it and place it in the water bath. Draw the required volume of vitamin C solution for reaction mixture 6 into the syringe, cap it with a LuerLok cap, and place it in the constant temperature bath with the temperature probe. Allow time enough so that the temperature of the solutions is about the same as that of the water, then record the temperature. Quickly remove the cuvette and syringe, dry the outside of each quickly, add the Vitamin C solution to the cuvette, and quickly insert it in the colorimeter to measure the absorbance as a function of time. When the reaction is complete, have the temperature sensor dried and ready to re-measure the temperature of the reaction mixture. The average of the initial and final temperatures will be used to approximate the temperature of the run. In a research lab, a temperature controller is built into the colorimeter to keep the temperature constant during runs. Equipment and Supplies Coors size 00A (70 ml, 8 cm diameter) evaporating dish; 3 and 5 ml syringes; 5 and 50 ml volumetric flasks; -50 ml beakers; -00 ml beaker; plastic cuvettes and stirrers; pestle. Starch Solution: Linit brand laundry starch may be used for the % starch solution (the MSDS at shows -0% cornstarch in water). Austin s Classic Starch (the MSDS at shows that it contains 3.97% starch with several other ingredients which do not interfere) can be diluted :4 with water. The starch solution may also be prepared by stirring 5 g of cornstarch with 30 ml of cold water, then pouring the suspension into 500 ml of vigorously boiling water, with stirring. The solution is allowed to cool before use. Tincture of Iodine: (%) is 0.08 M I with 0.l6 M NaI in ethanol/water solution. The NaI is added to help dissolve the nonpolar I by forming I 3 -. It may be prepared by dissolving 0 g of I ( mol) and 4 g (0.6 mol) of NaI in 500 ml of 95% ethanol and diluting to L with distilled water. Commercial 3% hydrogen peroxide: this is approximately 0.9 M. Vitamin C tablets: Chewable Vitamin C tablets should not be used, because they contain higher levels of excipients, including sweeteners and flavors, which may interfere with the clock reaction. ealthy Ideas Dietary Supplement) lists cellulose as second ingredient, thus does not dissolve completely. Country Life Vegetarian Vitamin C Crystals are the pure vitamin. 5. M Acetic acid or commercial vinegar can be used interchangeably. Prelaboratory Questions. What is the chemical equation for the reaction whose rate is being investigated?. What is the chemical formula for the second compound in equation ()? a. C b. C c. C d. some other 3. What is the molar mass of vitamin C (ascorbic acid)? 88

7 4. What is the concentration of I - (from both I and NaI) that would result if the % tincture of iodine solution (see Supplies list for description) were completely reduced to I -? 5. Suppose a color change occurs in 30 seconds in a solution that contains 0.0 M I -, M [ vitamin C, and 0. M. What is the rate of the reaction in M/s, expressed as? 5. Show a calculation of the concentration of hydrogen peroxide in molar units for the 3% used in this experiment. Assume the density is.0 g/ml. 6. What is the concentration of a solution made by dissolving on 000 mg vitamin C tablet in 00 ml of water? WebCT Prelaboratory Questions The Supplemenal Materials include a native WebCT file with these questions so that they can be uploaded to create a WebCT quiz. Which is the reaction whose rate is being measured? a. + I - --> I (aq) + b. I + --> + + I - + c. I + C > + + I - + C d. + C > C e. I + starch --> blue/black I /starch complex Which reactions must go fast for this experiment to work? a. + I - --> I (aq) + b. I + --> + + I - + c. I + C > + + I - + C d. + C > C e. I + starch --> blue-black I /starch complex Which reaction must go more slowly than the others for the goals of this experiment to be achieved? a. + I - --> I (aq) + b. I + --> + + I

8 c. I + C > + + I - + C d. + C > C e. I + starch --> blue-black I /starch complex For this experiment to work, the reaction I + starch --> blue-black I /starch complex must be slower than the reaction a. + I - --> I (aq) + b. I + --> + + I - + c. I + C > + + I - + C d. + C > C e. I + I - --> I 3 I - If a reaction mixture initially containing vitamin C at M, at 0.8 M, and iodide ion at 0.03 M, and turns from colorless to dark after 0.8 seconds, what is the rate of the reaction, [ /? Enter the answer in the form.e- for ". x 0 - Answer What substances are mixed to make tincture of iodine? a. I, ethanol, and water b. NaI, ethanol, and water c. sodium triiodide and water d. NaI, I, ethanol, and water e., I, ethanol and water 90

9 What is the concentration of I - in a solution prepared by mixing 3.09 ml of tincture of iodine with 5. ml of 0.3 M vitamin C solution? Assume that the tincture of iodine solution contains M I and 0.5 M I -. (int: look up the equation for the reaction of I with vitamin C) Answer Solution A should be a. purple due to I in solution b. black due to the I starch complex c. colorless because it contains only I -, no I d. light brown because it contains both I - and I e. Colorless because it contains only starch. What is the molar mass of Vitamin C? a b c d e What is the concentration of Vitamin C in a solution prepared by mixing.5 ml of M Vitamin C with.9 ml of 0.0 M iodine? int: See the equation for the reaction in the Introduction. Answer Footnotes: a. Wright, S.W., J. Chem. Educ. 79, 4-43 (00). b. Gettys, N.S. and Jacobsen, E.K. Eds., J. Chem. Educ. 79, 40A-40B (00). c. 9

10 3 Charles phart, 4 Wolfram Saenger, The structure of the blue starch-iodine complex, Naturwissenschaften, [ISSN: (Paper DI: 0.007/BF , () 3 36(984). 9