CH 112 Special Assignment #4 Chemistry to Dye for: Part C PRE-LAB ASSIGNMENT: Make sure that you read this handout and bring the essentials to lab with you. Review Light, energy and color (pp 17-18), Measuring amounts in solution (pp 102-103), and Kinetics (pp 136-141) in Adventures in Chemistry. Here are the pre-lab questions for this week. 1. The concentration of a stock solution of sodium hydroxide is 0.500 M. You add 2.00 ml of this stock solution to a 10.0 ml volumetric flask and bring the volume of the volumetric flask up to 10.0 ml with deionized water. What is the concentration of sodium hydroxide in the new solution you have just created? 2. You then take 2.0 ml of the new solution and add 1.0 ml crystal violet solution. What is the concentration of sodium hydroxide in this mixture? INTRODUCTION: We complete our work for Dyenomite this week with an investigation into their dye decolorization dilemma. They have been having problems with their newest shade of purple: crystal violet. This dye, as you witnessed last week, produces a vibrant color, but the tie-dye process normally requires that the color be set in highly basic washing soda. During this stage, crystal violet rapidly loses its color, suggesting that base may be involved in the decolorization of this dye. Reaction of interest Your goal is to characterize the presumed reaction (Figure 1) between crystal violet and hydroxide ion (produced by a base). That is, what factors affect this reaction? How can Dyenomite minimize the decolorization? You will test the effects of concentration (for both crystal violet and hydroxide ion) and temperature on the rate of the decolorization reaction.
C + + OH _ C OH PURPLE COLORLESS Figure 1. Presumed reaction between crystal violet and hydroxide ion that results in formation of a colorless molecule from a purple one. Rate of Crystal Violet Decolorization The most convenient way to determine the rate of crystal violet decolorization is to monitor the loss of purple color over time. We will use an instrument called an Ocean Optics spectrophotometer to monitor the absorbance of light by crystal violet. Any material that absorbs light in the visible spectrum appears colored. Such a material is called a chromophore, and can be useful as a pigment or dye. We will characterize the wavelength of light best absorbed by crystal violet and then use the loss of this absorbance in the presence of base to monitor the decolorization reaction. A cartoon of the spectrophotometer is provided below (Figure 2). White light illuminates the sample and can be absorbed. The remaining light enters an optical fiber and is efficiently transmitted to the spectrophotometer and then analyzed by a computer. The amount of light absorbed by the sample can be determined by measuring the light signal with the sample in the optical path relative to a colorless blank (water). The basic principle behind the analysis is that the more particles of a chromophore there are in a solution, the higher is the absorbance of light detected by the spectrophotometer. That is, the more concentrated (darker color) the sample is, the more light it will absorb compared to the blank. Figure 2. Schematic of the spectrophotometer. White light shines on the sample, which absorbs only certain wavelengths of light. The light transmitted through the sample is analyzed by the computer, which then determines what wavelengths have been absorbed. A more concentrated solution will have a greater absorbance and appear darker in color. 2
Chemists generally measure reaction rates in units of concentration per minute, much like you would measure the rate at which you drive in miles per hour. The rate of the crystal violet decolorization reaction is therefore equal to the change in concentration of crystal violet per minute. This rate can be conveniently measured as the change in absorbance of the solution per unit time (because crystal violet is the chromophore responsible for absorbing light). Because the software run with the spectrophotometer will plot absorbance versus time, you can determine the rate from the slope of the resulting line. This is an approximation because the rate changes over the course of the reaction and is not constant. However, early in the reaction, there should be a Figure 3. As the crystal violet loses its color during the reaction with base, its concentration, and thus its absorbance, decreases over time. The relationship between absorbance and time should be linear for the early part of the reaction (the first minute or so). linear relationship between concentration and time (Figure 3). The absolute value of the slope of this line is the rate (rates must be positive values by convention). More about concentration Chemists typically measure concentration in molarity, abbreviated upper-case M. Molarity is equivalent to the number of moles per liter of solution (mol/l). For example, if you purchase a chemical solution that is 10.0 M HCl, that means you have 10.0 moles of HCl per 1 liter of solution. If you start with the 10.0 M HCl as your starting ( stock ) solution, you can make a new solution of a smaller concentration from it. How? The molarity of your starting solution (in mol/l) times the volume of starting solution you use (in L) is equal to the molarity of the new solution (in mol/l) times the volume of new solution you make (in L). This is sometimes represented as M 1 V 1 = M 2 V 2, where M 1 equals the molarity of the stock solution, V 1 equals the volume of the stock solution, M 2 equals the molarity of the diluted solution, and V 2 equals the volume of the diluted solution. If three of the variables are known, you can solve for the fourth variable. For example: Heidi buys a 1-liter bottle of 5.0 M acetic acid. For her photographic fixer she needs 20.0 liters of a 0.1 M acetic acid solution. To make her new solution, how many liters of the 5.0 M acetic acid would she need to use? Here is how to set up the problem: 3
Remember M 1 V 1 = M 2 V 2 where or (5.0 mol/l)*(?l) = (0.1 mol/l)*(20.0l) Solving for V 1 gives 0.4 L or 400-mL. M 1 = 5.0 M acetic acid V 1 =? L M 2 = 0.1 M acetic acid V 2 = 20.0 L Therefore, Heidi would need to pour 400-mL of her purchased acetic acid into her container and bring the volume of solution up to 20.0 liters. LEARNING GOALS: Be able to: Understand the concept of molarity and dilution of a solution. Operate a spectrophotometer. Understand the relationship between absorbance and concentration. Understand factors that affect reaction rates. PROCEDURE: Part 1. Observation of the decolorization reaction 1. Add 1.0 ml crystal violet solution (2.5 x 10-5 M) and 2.0 ml sodium hydroxide solution (0.5 M) to a small beaker. Record your observations. 2. Watch the mixture for a few minutes. Again, record your observations. Part 2. Characterization of crystal violet s absorbance 1. Calibrate the spectrophotometer with deionized water. Choose Calibrate Spectrometer from the Experiment menu. The calibration dialog box will display the message: Waiting 60 seconds for lamp to warm up. Following the instructions in the dialog box to complete the calibration, use a cuvette filled about ¾ full with deionized water. Check to make sure the non-frosted, clear sides are in the light path. The cuvette should be inserted all the way into the sample chamber. You should feel that the cuvette is gently, but firmly, held in place so that you cannot twist the cuvette. Click Finish Calibration and then click OK. This step blanks the spectrophotometer with water. 2. Using a plastic disposable pipette, add 4 drops of crystal violet to a cuvette and fill to the line with deionized water. Carefully mix the solution by covering the cuvette with a piece of Parafilm and inverting. 4
3. Place the cuvette containing dye into the sample chamber. Click on and then. 4. You can read the absorbance using the Examine tool, by clicking on. Then move the cursor along the spectrum. The wavelength and absorbance will be displayed in the new dialog box in the data window. Record the wavelength at which the absorbance value is the greatest. This is the wavelength of maximal absorbance, or λmax. What color of light is this? Record the corresponding absorbance at this wavelength. 5. Repeat steps 2-4, using first only 2 drops of crystal violet (plus water) and then 8 drops of crystal violet (plus water). The λmax should be the same in each case, as this value is dependent on the structure of the chromophore only. The absorbance value should vary, however. Record these values. Important Spectrophotometer Notes: Follow your supervisor s instructions for the proper use of the spectrophotometer. Wipe the outside walls of EACH cuvette with a Kimwipe to remove all smudges, dust, and liquid before inserting into the cuvette holder. Hold the cuvette by the two frosted sides. Make sure the clear sides of the cuvette are in the pathway of the light beam when placed in the spectrophotometer sample chamber. Part 2. The effect of hydroxide s concentration on the rate of decolorization 1. Using tape, label three 10.0-mL volumetric flasks as follows: OH - #1, OH - #2, and OH - #3. DO NOT cover the meniscus with the label. 2. Pipette the sodium hydroxide stock solution (0.5 M) into the labeled volumetric flasks as follows: OH - #1: add 1.0 ml NaOH OH - #2: add 2.0 ml NaOH OH - #3: add 4.0 ml NaOH 3. Carefully, bring the volume of liquid up to the etched white ring level (bottom of meniscus should sit on the line) with deionized water for each flask. You will find a plastic dropper a very useful tool for doing this. 4. Place a stopper into the top of each flask, hold firmly, and turn flask upside down several times to ensure complete mixing. 5. Set-up a cuvette rack of 3 cuvettes. Label the cuvette rack with tape (not the cuvettes) to identify the NaOH solutions as #1, #2, and #3. 6. Add 2.0 ml of each diluted hydroxide solution to the appropriate cuvette. 7. Prepare the spectrophotometer to acquire rate data as follows. Click on the Configure Spectrometer Data Collection icon,, located on the right-hand side of the toolbar to open the Configure Spectrometer Data Collection display. 8. Click Abs vs. Time (under Set Collection Mode). Click on Treat Contiguous Wavelengths as a Single Range. The wavelength of maximum absorbance should be automatically selected. We want to average over a range of wavelengths for better precision. To average over a range of wavelengths, scroll 5
down the Select Wavelengths list to the wavelengths near your absorbance maximum and then click on about 5 wavelengths on both sides of the maximum. Click OK. 9. Click Data Collection.. Choose 1 minute for Length and 30 samples/minute for Sampling Rate. Click Done. 10. Place the special stir bar into cuvette #1, and then add 1.0 ml stock crystal violet solution (2.5 x 10-5 M). Immediately place the cuvette into the sample chamber, allow the solution to mix briefly, and then click on the button. Make sure that the stir plate is adjusted to allow for quick mixing, but not for splashing. 11. When 1 minute has elapsed, collection of data is automatically stopped. 12. Click on the Linear Fit icon,. This will display the linear equation and the correlation coefficient of the line. The absolute value of the slope is the rate of the reaction. Record this value. 13. Repeat steps 10-12 for cuvettes #2 and #3. When you click on the collect button, choose erase and continue. Part 3. The effect of temperature on the rate of decolorization 1. Set up a 250-mL beaker to use as a warm-water bath, filling it about halfway with warm tap water. Place a 10-mL graduated cylinder containing ~2.5 ml of the NaOH solution from flask #2 (prepared in Part 2) into the beaker. Heat the bath on a hot plate until the temperature is about 60 C. 2. Set up a cold-water bath in a 250-mL beaker by filling it about halfway with cold tap water and adding ice. Place a 10-mL graduated cylinder containing ~2.5 ml of the NaOH solution from flask #2 (prepared in Part 2) into the beaker. Allow the sodium hydroxide to sit in the cold-water bath for at least 5 minutes. 3. Set-up a cuvette rack of 3 cuvettes. Label the cuvette rack with tape (note: you cannot mark the cuvettes) to identify the different temperatures: room temperature (RT), warm, and cold. 4. Add 2.0 ml of room-temperature NaOH solution from flask #2 (prepared in Part 2) to the cuvette marked RT. 5. Add the special stir bar and then add 1.0 ml stock crystal violet solution (2.5 x 10-5 M). Immediately place the cuvette into the sample chamber, allow the solution to mix briefly, and then click on the collect button to acquire kinetic data for 1 minute at the λmax, choosing erase and continue. 6. Obtain the rate as described in Part 2 (step 12) and record this value in your notebook as corresponding to the RT sample. 7. Record the exact temperature of the warm-water bath. Quickly, add 2.0 ml of the warm NaOH solution to the cuvette marked warm. 8. Quickly add the special stir bar and then add 1.0 ml stock crystal violet solution (2.5 x 10-5 M). Immediately place the cuvette into the sample chamber, allow the solution to mix briefly, and then click on the collect button to acquire kinetic data for 1 minute at the λmax, choosing erase and continue. Obtain the rate and record it in your notebook. 9. Record the exact temperature of the cold-water bath. Quickly, add 2.0 ml of the cool NaOH solution to the cuvette marked cool. 10. Quickly add the special stir bar and then add 1.0 ml stock crystal violet solution 6
(2.5 x 10-5 M). Immediately place the cuvette into the sample chamber, allow the solution to mix briefly, and then click on the collect button to acquire kinetic data for 1 minute at the λmax, choosing erase and continue. Obtain the rate and record it in your notebook. Part 4. Data analysis 1. To determine the effect of hydroxide ion concentration on the rate of decolorization you will plot rate versus concentration. In order to do this, you must first calculate the concentration of sodium hydroxide in each of your diluted samples. The calculation for determining these concentrations is the same type of calculation as in the Pre-Lab Assignment. 2. Construct a graph with Excel as follows. The values for x (the independent variable) are the concentrations of the diluted solutions and should be listed in the first column. The values for y (the dependent variable) are the corresponding rates and should be listed in the second column. 3. Choose Insert Chart with XY (Scatter): Marked as the graph type. 4. Click on the data points within the graph and select Chart: Add trendline. 5. Click the Options tab, and then click on the box next to Display Equation on Chart. You should also click on the box next to Display R-squared Value on Chart. 6. Print your graph. What does it tell you about the relationship between hydroxide ion concentration and rate of decolorization? 7. To determine the effect of temperature on the rate of decolorization you will plot rate (y) versus temperature of sodium hydroxide (x). Construct an XY scatter plot as described above. 8. Print your graph. What does it tell you about the relationship between temperature and rate of decolorization? References: Adapted from Chemistry The Central Science, Laboratory Experiments, 6 th Edition, by J.H. Nelson and K.C. Kemp; Laboratory Inquiry in Chemistry, by R. C. Bauer, J. P. Birk, and D. J. Sawyer; and the CH 142 lab manual, Colby College, originally prepared by Julie T. Millard; thanks also to Prof. Rebecca Rowe at the University of Maine Chemistry Department. 7
CraC Special Assignment #4C REPORT SHEET NAME PARTNER DATE Crystal violet λmax DATA SUMMARY Also attach your two Excel graphs of the rate dependence on concentration and temperature. POST-LAB QUESTIONS 1. What color of light is absorbed by crystal violet? What relationship does this color have to violet? (Hint: see pp 608-609 of your textbook) 2. What is the relationship between the concentration of crystal violet and absorbance? 3. What is the relationship between the concentration of sodium hydroxide and rate of decolorization? Explain on a molecular level why this is expected. 4. What is the relationship between the temperature and rate? Explain on a molecular level why this is expected. 8
5. Address two possible sources of error in this experiment and how they could have influenced the observed results. CONCLUSIONS Use the space below to report on your experiment to Dyenomite, making sure that you address the original goals of this work. 9
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