Exp10_9-07 - Experiment 10 Physical Chemistry Laboratory...

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1 Experiment 10 Physical Chemistry Laboratory Analysis of an Oscillating Reaction revised (6/01/2007) Introduction This experiment will introduce you to a well-known oscillating reaction, to a model mechanism which describes the important features of its kinetic behavior, and to techniques for studying complex chemical kinetics. You will investigate the Belousov-Zhabotinskii (BZ) oscillating reaction, the oxidation of malonic acid by bromate ion in acid solution, catalyzed by Ce 3+ ion. The main overall reaction is 23 2 2 3 6 32 2 3 22 2 BrO CH COOH H O BrCH COOH CO H O −+ ++ + + () The concentration of Br - intermediate and the scattering/attenuation of laser light are monitored for the BZ reaction mixture to determine the critical Br - concentration and the period of the
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2 reaction. You will also be introduced to the basic principles of the Fast Fourier Transform (FFT) and Fourier smoothing using the time dependent optical absorption/scattering data. Oscillations of different frequencies are separated using Fourier transformation of the data from the time to the frequency domain. The transformed data is truncated and back Fourier transformed for the purpose of noise reduction. The kinetics of the reaction are first investigated with a kinetics simulation program which is used to explore the effect on the BZ reaction of variations in the initial conditions. You will develop a hypothesis about the effect on the reaction of certain variations in the initial conditions and then perform experiments to test the hypothesis. Theory Mechanism of Oscillations The mechanism for the Belousov-Zhabotinski reaction is fairly complicated, but was first worked out in 1972 and is now well understood. The reaction mixture has two main processes, and only one of them is working at any one time. Process A is the bromination of malonic acid by bromide and bromate, via a series of two-electron reductions: 23 3 3 6 33 2 2 2 2 Br BrO H O CH COOH CHBr COOH H O + ++ + + () Process B is again the bromination of malonic acid, but this time via a free radical process involving no bromide but Ce 3+ : BrO H O CH COOH Ce CHBr COOH Ce H O 2 2 3 2 4 2 54 4 8 + + + + At any time, the concentration of bromide ion determines whether Process A or Process B is operating. At high Br - concentrations, Process A works; at low [Br - ], Process B works. However, Process A consumes Br - , so that it eventually dies and Process B appears. The Ce 4+ produced by Process B regenerates bromide ion again, so that Process A returns to dominate. These two processes will continue to alternate until the supply of major reactants, BrO 3 - and malonic acid, have been exhausted. One of the tasks in this experiment is to determine the “critical” concentration of bromide at which switching occurs.
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This note was uploaded on 07/17/2008 for the course CHEM 541/42 taught by Professor Allen during the Spring '07 term at Ohio State.

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Exp10_9-07 - Experiment 10 Physical Chemistry Laboratory...

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