CHE134 Summer Lecture 4

CHE134 Summer Lecture 4 - d - [MG - + dt - ] Obj104...

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Click to edit Master subtitle style Kinetics of Dye Bleaching d [ MG+ ] ---------- dt Obj104 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
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Objective: Examine the 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 with OH-
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Our objective is to determine k , n and m We write the reaction as MG+ + OH- w MGOH Rate law for the reaction is d[ MG+ ] rate = ------- = k [ OH- ] n [ MG+ ] m dt k = specific rate of the reaction m = order of the reaction with respect to MG+ n = order of the reaction with respect to OH- 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 = d ε ) 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 con the wil as t
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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- ] n [ MG+ ] 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 “ Isolation Experiment where q = k [ OH- ] n MG+ + OH- ¸ MGOH Both [OH-] and [MG+] change with time
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CHE134 Summer Lecture 4 - d - [MG - + dt - ] Obj104...

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