Kinetic_144 - Kinetics of Dye Bleaching d CONCENTRATION vs...

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Kinetics of Dye Bleaching
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Kinetics of Dye Bleaching d [ MG ] ---------- dt CONCENTRATION vs TIME 0 0.2 0.4 0.6 0.8 1 1.2 TIME CONCENTRATION k [ m [OH - n C N H 3 C CH 3 N CH 3 CH 3 N CH 3 H 3 C + Cl - Chemistry including time Chemistry including time
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Objective: Examine the kinetics kinetics of the reactions by which a dye is bleached Concepts: Reaction Rate Rate Law Order of reaction Specific Rate Beer’s Law Linear Graphs from non-linear relationships Techniques: Using Spectrophotometer Isolation Experiment Apparatus: Spectronic 20 Constant Temperature Bath Timer
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This exercise explores the time dependence of the reactions of the dye: C N H 3 C CH 3 N CH 3 CH 3 + Cl - Malachite Green, MG To form the corresponding alcohol, MGOH which is colorless OH N H 3 C H 3 C N
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Our objective is to determine k , n and m We write the reaction as MG + OH - MGOH Rate law for the reaction is d[ MG ] rate = ------- = k [ OH - ] [ MG ] m dt k = specific rate of the reaction m = order of the reaction with respect to MG k is generally sensitive to temperature but not to concentrations
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Since the reactant, MG is colored but neither OH - nor the product, MGOH , is (i.e., OH - bleaches the dye) We can follow the concentration of MG as the reaction proceeds using the Absorbance of light and Beer’s Law A = e c Where, as usual, e will be the slope of the Beer’s Law plot for the dye. ( e = ε d ) Absorbance vs Concentration Fe-SA Complex at 530 nm 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 1 2 3 4 5 6 Concentration (mol/L X 10 ) Absorbance The dye concentration, and therefore its color, will become lighter as time progresses.
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Beer’s Law says that % transmittance is proportional to concentration A = True B = False
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Beer’s Law says that ABSORBANCE is proportional to concentration A = ε d c How is absorbance related to percent transmittance? A = 2 – log %T B = False
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d[ MG ] rate = - --------- = k [ OH - ] m dt If [ OH - ] didn’t change, the rate law would simplify to one involving only one changing concentration, [ MG ] d[ MG ] rate = - -------- = q [ MG ] m dt We can accomplish this experimentally by Using a large (excess), known [ OH - ] Then , [ OH - ] will not change significantly during the course of the reaction -- an “
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Kinetic_144 - Kinetics of Dye Bleaching d CONCENTRATION vs...

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