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CHEN 4860 Chemical Engineering Laboratory II REACTIONS EXPERIMENT BACKGROUND, EQUIPMENT & OBJECTIVES Fall 2008 Background o Phenolphthalein is a chemical that changes color when in an alkaline environment. When phenolphthalein (Ph) is added to an alkaline solution it rapidly & irreversibly changes to its quinoid form (Ph2-); this form is pink in color and has a peak absorbance at a wavelength of 550 nm. Ph2then slowly and reversibly changes to the colorless carbinol form PhOH3-. You will study the kinetics of this second reaction by monitoring Ph2concentrations via spectrophotometry. o Read & study the paper Alkaline Fading of Organic Dyes: An Ideal Reaction For Homogeneous Reactor Experiments , Andres & Hile, Chemical Engineering Education, Winter 1976. The Equipment o you will be provided with the appropriate chemicals, a jacketed reactor vessel (capacity 600 ml), a water bath connected to the vessel jacket and a spectrophotometry system that includes a probe and a means of data logging Operation o Turn on light source and let it warm up o Log on to the PC o Open Logger Pro software from the desktop o Add 500 ml of the 0.250 M NaOH solution. This solution will be premixed and in a plastic carboy on one of the counters. Make sure a magnetic stir bar is present in the bottom of the reactor. o Replace the cover on the reactor and position the thermometer and fiber optic Dip Probe into the reactor head. o Set stirrer to about 8 to give good mixing without the stir bar bouncing. Rattle the Dip Probe to remove any bubbles from the tip. The probe should be oriented such that flow can go through the tip (opening parallel to the circulation flow). o Position the fiber optic cable leading from the Light Source to the Dip Probe such that it bends as little as possible at the point where the cable inserts into the probe. It is important to minimize bends in the cable and to also not move the fiber optic cable too much during each run, as this can cause attenuation of the signal- also sharp bends or undue stress at the insertion point can permanently damage the cable. o Go to Set Up Sensors under the Experiment menu. Click on Spectrometer: 1 (If the Spectrometer is not listed, go to Experiment, Setup Sensors and click on Auto Detect Sensors. If the spectrometer is still not detected at this point, unplug the USB cable at the rear of the PC, wait a few seconds and plug back in). Check to make sure that Wavelength Smoothing is set to zero and that Samples to Average is set to 1 . The wavelength range should be set to 380 to 950 nm. There is no need to adjust integration time- it will automatically be set later during calibration. Close the dialog box. o On the menu bar, go to Experiment and open Calibrate, then Spectrometer: 1 from the drop down menu. You may skip the warm-up o o o o o o o o o o o step shown in the dialog box since the lamp has already been on for several minutes. Also, there is no need to insert a blank cuvette as instructed- the blank sample in our case is the NaOH solution as read by the dip probe. Calibrate by clicking on Finish Calibration. The software will automatically zero the spectrometer and scan wavelengths to set the appropriate integration period. Click on OK when finished. Click on the rainbow colored icon on the menu bar to configure data collection from the spectrometer. Check Abs vs. Time under Collection Mode. As stated above, Ph2- has a maximum absorbance centered on about 550 nm. You will take an average of the absorbance readings from 545-555 nm for during the experiment. Therefore, check the box that says Treat Contiguous Wavelengths as a Single Range. Check only the eleven wavelengths 545, 546, 547. . . . 555. Click OK when finished. Click on the stopwatch icon located on the menu bar. The collection mode should be set on time-based . Set sample rate to 1 sample per second. Check the Continuous Data Collection and the Oversampling boxes. Triggering should be disabled. Click on Done . Make sure you have saved any data from previous runs and then clear the data by going to the Data drop down menu and selecting the last option, Clear All Data. You are now ready to begin the first run at room temperature. Add approximately 10 ml 0.01 M phenolphthalein stock to the small (30 ml) plastic beaker in the drawer under your bench. Keep this solution to use for all runs. Use the Eppendorf pipette to carefully draw 0.750 ml (750 ul) of the 0.01 M phenolphthalein stock solution from the beaker. Note the temperature of the NaOH solution. Start data collection by clicking on the green Collect button on the menu bar. About 3 seconds later inject the contents of the pipette into the reaction vessel. Observe the change in concentration decreasing exponentially at first and then gradually approaching the equilibrium concentration. It will take about 30 minutes for the reaction to reach equilibrium, when the forward and reverse rates are equal. At this point there should be only random fluctuations in the absorbance readings. Stop the data collection. Note the final temperature of the reactor solution. Save your file to a folder on your network drive: go to the File menu and choose Export As and Text. You will be able to open this file for analysis as a tab delimited text file in MS Excel. Clear your data by going to the Experiment menu and choosing Clear Latest Run from the drop down menu. Repeat the above for temperatures of approximately 30, 40 & 50 C by heating the reaction solution with the circulating bath. Instructions on running the circulating bath are located in a binder in a drawer under your bench. After finishing all runs, thoroughly clean the reactor, all glassware, the Dip Probe and lab benches (it is especially important to remove any NaOH from the mirror on in the probe tip - failure to do so will damage the tip by etching the mirror)! Make sure there is no NaOH residue on any of the equipment or work surfaces. Turn off the light source. Place washed equipment in the drying rack by the sink. Generally clean up. Objectives - Generally, you are to determine various parameters associated with the Ph2- reaction with OH-. Specifically, you should: Show how equations 3 & 4 in the Andres & Hile article can be manipulated to form equation 5 determine k1 , k1, k2 & Keq for each temperature run via appropriate analysis of your data (express your data in concentration terms, not absorbances) use a van t Hoff plot to determine the heat of reaction use an Arrhenius plot to determine reaction activation energies and pre-exponential factors

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