Crystal-violet-lab

Crystal-violet-lab - “a; DETERMINATION OF THE RATE LAW...

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Unformatted text preview: “a; DETERMINATION OF THE RATE LAW FOR THE CRYSTAL VIOLET REACTION The purposeof this laboratory study is to determine how the rate of disappearance of the crystal violet ion is related to its initial concentration and the initial concentration of hydroxide ion. As time elapses, the concentration of the crystal violet ion will be measured spectrophotometrically. The equation for the reaction is shown here: Mack" D (Han {HsclaN LQWM . 9H +O—MGH212 _0H_._ -—©—NICH312 g equ'valmt to Q Q ., (“30’3" {HaChN (racial A simplified version of the equation is: CV+ + OH' -—-* CVOH crystal violet The molar mass of crystal violet chloride (which is the salt we use)=408.00 The rate law for this reaction is in the form: rate = k[CV+}m[OH']n, where k is the rate constant for the reaction, mis the order with respect to crystal violet {CV+), and nis the orderwith respect to the hydroxide ion. Since the hydroxide ion concentration is much larger than the concentration of crystal violet, [OH'I will not chanea rectal: durin this ex riment. So rate = k[CV+]m[OH' ]n= k‘[CV+]m, where k' =k{OH‘]n and k’ is called a pseudo rate constant. As the reaction proceeds, the violet-colored reactant will be slowly changing to a colorless product. The crystal violet absorbs a broad band of colored light; the best wavelength to useis km 589 run. You will monitorthe absorbance of the crystal violet solution as a function of elapsed time. We will assume that absorbence is proportional to the concentration of crystal violet (Beer's law). Absorbance will be used in place of concentration in plotting the following three graphs: I Absorbancevs. time: A linear plot indicates a pseudozero order reaction (k‘ =—slope). 0 ln Absorbance vs. time: A linear plot indicates a pseudo first order reaction (k‘ =—slope). *- 1/Absorbance vs. time: A linear plot indicates a pseudo second order reaction (k' = slope). The order of the reaction is that to; which the graph of {(51 v. time has the most 1m ear plot for the logest period of time. hydroxide, and youwill find the rate constant, it. To find 11, you will do the experiment using a different [OI-1'], therefore having a different k', while holding [CV+] constant. [Recall that the order of a reaction with respect to a given reactant will bea rational number.] Find k (the actual rate constant), by usingthe equation, l<' = MOI-11“. Calculate k twice; once for the data from runone, once firom the data for runtwo. Your two values should be close to each other. MATERIALS FOR EACH GROUP FROM STOCKROOM 100 vaolumetric flask 1 or 2- 5 mLpipets 3- 10 mLpipets 1- 25 mLpipet pipet'bulb cuvette for Spectrophotometer timer 90-100 ml. 0.1 M NaOH, oonc. known and recorded to 4 decimal places 50ml.crystal violet solution, not to exceed .0160 g/L,conc. knownto 38F IN ROOM OR FROM LOCKER Spec 20 D+ spectrophotometer paper towels tissues to wipe cuvettes 6- 100 or 150 meeakers 1 or 2 stirring rods deionized water H28 04 phenol phthalein Read the entire lab study before writing anything in yon: notebook. Summ table for the two reaction mixtures “Run 1" and "Run 2": Reagent distilled water Run] 15.00 mL RunZ 10.00 mL PROCEDURE 1. Clean the pipets and the volumetric flask. Rinse thoroughly; the final rinse being deionized water. Wash and dry 4 100 ml. beakers. Turn on the spectrophotometer by turning the Power Switch clockwise. Allow the spectmphotometer to warm upfor at least 15 minutes before using. Set the desired wavelength (589 nm) with the Wavelength Control Knob. Set the Filter Lever to the “340-599 nm”position. 2. Make 100.0 mi. of 'a 0.05 M NaOH solution, the molarity knownto four decimal places: Place about 20 mL of the standard 0.1 M NaOH in one of the 100 mLclean, dry beakers from your locker. Use this NaOH solution to rinse one of the 25 ml. pipets by drawing several small portions of the solution into the pipe! and rolling it around to touch all of the surfaces. Discard all of the rinse solution. Now the pipet and the beaker are wet with the solution to be used in them. Pour about 65 mL of the standard 0.1M NaOI-I into the beaker. Pipet 50.00 mLof the standard 0.1 M NaOI-I into the 100.0 mi. volumetric flask, by usingthe 25.00 mL pipet twice. Carefully fill the volumetric flask to the mark with deionized water, and mix the solution by inverting the flask at least 20 times. crystal violet soln. 10.00mL 10.00mL 3. Prepare the spectrophotometer. A 15 minute warm-up is required. Use 589 nm for this experiment. Do the following before each run: a) Set the display mode to Transmittance by pressing the Mode Selection Key until the appropriate LED is lit. Adjust the display to 0.0%T with the Zero Control. Make sure that the sample compartment is empty and the cover is closed. b) Fill the clean cuvette about 3” full (up to the mark) with distilled (deionized) water. Wipe the cell with a tissue to remove droplets, dust and fingerprints. If there are air bubbles, tap the tube gently with a pen to dislodge them. c) Insert the cuvette into the sample compartment andalign the guide mark on the cell with the guide mark at the front of the sample compartment. Press the cell firmly into the sample compartment and close the lid. d) Adjust the display to read 100% using the 100%T control knob. Once you have begun a series of readings, do not change any of the Settings. 4. HUN ONE Pipet 15.00 1111. of deionized water into one of the clean, dry beakers (10 mL pipet + 5 ml. pipet). This beaker is now called the "reaction beaker”, and should besolabeled to avoid confusion. Place about 15 mLof your 0.05 MNaOl-I in another of the 100 mL clean, dry beakers. Use this NaOH solution to rinse one of the 5 mL pipets by drawing several small portions of the solution into the pipet and rolling it around to touch all of the surfaces. Discard all of the rinse solution. Now the pipet and the beaker are wet with the solution to be used in them. Pourabout 15 mLof the 0.05 MNaOI-I into the beaker. Pipet 5.00 mL of the 0.05 M NaOH into the reaction beaker. Place about 15 mLof the crystal violet solution in another of the 100 ml. clean, dry beekers. Use the CV solution to rinse one of the 10 ml. pipets by drawing several small portions of the solution into the pipet and rolling it around to touch all of the surfaces. Discard all of the rinse solution. Now the pipet and the beaker are wet with the solution to be used in them. Pour about 20 ml. of the CV solution into the beaker. Pipet 10.00 mLof the crystal violet solution into the reaction beaker. START THE TIMER WHEN THE CRYSTAL VIOLET IN THE PIPET REACHES THE TOP OF THE WlDE PART OF THE PIPET. Gently stir the mixture with a clean, dry stirring rod. Poura little of the mixture into the cuvette and roll the solution around the sides of the cuvette to touch all surfaces to replace the water with reaction mixture. Repeat this rinsing twice, discarding the rinsing, then fill the cuvette with reaction mixture up to the mark. Wipe the cuvette with tissue, remove air bubbles, and insert into the sample compartment, aligning the guide marks. Close the lid. Set the display mode to Absorbanceby pressing the Mode Selection Key until the appropriate LED is lit. No more than 3 minutes should have elapsed since the timer was started. Now, approximately every 30—40 seconds, one person reads the timer and one person reads Absorbance from the display and reoordsall on the data page. (Min, sec., Absorbance) The most reliable data is betweenA z .3 and .5. Stop when A < .15. (This will take about 20-25 minutes for Runl.) Discard the reaction mixturein the cuvette into your waste container, and completely clean the cuvette by shaking soapy water in it. Do not we a test mm W” a faint purple stain remains, remove it by soaking with some of your dilute NaOH. Be sure to completely remove any NaOH that you use by washing several times. 5. RUN TWO Readjust the spectrophotometer by performing Steps 3 a-d again. In a different "reaction beaker”, repeat the steps of Run 1, but this time use 10.00 rnLof water and 10.00 mL of the 0.05 MNaOH. Take readings of the timer and the spectrophotometer every 20-30 seconds. Again, stop when A < .15. (This will be faster for Run2.) When ail measurements are completed, turnoff the spectrophotometer by turning the Power Switch counterclockwise until it clicks. [f you should spill anything onthe instruflrent, be sure to clean and dry it immediately. it is always a good idea to write down the identification number of the instrument you used. Put the protective cover on the spectrophotometer before youleave. 6. Disposal of reagents. Your used solutions are slightly basic. Neutralize with dilute sulfuric acid (phenolphthalein is no longer pink), then you may pourthe solutioninthe sink. When you are sure you have finished, neutralize your standard N aOI-l solution and pourin the sink. The crystal violet may also bepoured downthe sink. 7. The graphs: Duringanother lab period we will use Microsoft Excel to plot the various functions, f(A), v. time. oero there ' 'thres toc v'o ' hat forw ' t of {A v. ti e has the most linear plot for the 1523th most of 8. The report: Need: date, title, purpose (may be brief), method (may be brief), procedure (may refer to the handout by title), , data pages (signed by instructor), calculations, conclusion (would be the complete rate law). Youwill turnin separate computer graphs. Each runwill take about one data page. Column headings for data and calculations could be: utLand concentrations of each reactant, followed by a large table for: Elapsed time, minutes seconds Absorbanoe 9. Assigned Problem: To show that you are justified in usingIOH'] as a constant: using the concentrations from run 1, show that the concentration of the NaOI-l has changed insignificantly when half of the crystal violet chloride has been consumed. JAE am ...
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Crystal-violet-lab - “a; DETERMINATION OF THE RATE LAW...

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