Experiment 8

Experiment 8 - EXPERIMENT 9 Oscillating Reactions...

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EXPERIMENT 9 Oscillating Reactions 1/6 E XPERIMENT 9 Oscillating Reactions 9.1 Purpose In experiment 9, the oscillatory Briggs-Rauscher reaction is investigated in terms of different initial conditions and different organic substrates. 9.2 Background Most reactions proceed smoothly, at varying rates, to a final state of equilibrium. Some, however, do not. They oscillate in time: the concentrations of reactant, product, or intermediate species fluctuate wildly, often leading to easily observable oscillations in time of these concentrations. Other reactions are known that produce oscillations in space—waves of reactants, products, or intermediates show up. Such oscillatory reaction systems are of great interest. Many natural phenomena, from firefly flashes to the heartbeat of mammals, are oscillatory chemical systems, and spatial oscillations, for example tiger stripes or zebra stripes, are also familiar. While oscillating biological reactions are incompletely understood, there are a number of oscillating reactions known among simple inorganic or organic molecules. Examples of oscillating chemical reactions are often clock reactions. We recall that a chemical clock is a complex mixture of reacting chemical compounds in which the concentration of one component shows an abrupt change accompanied by a visible color effect. The Briggs-Rauscher reaction is a prominent example of an oscillatory reaction, and the concentration of products and reactants can be approximated in terms of damped oscillations. A typical Briggs-Rauscher system consists of i) an aqueous solution of hydrogen peroxide H 2 O 2 ii) an aqueous solution of an iodate, such as potassium iodate KIO 3 . iii) a strong, chemically unreactive acid, such as sulfuric acid H 2 SO 4 .
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EXPERIMENT 9 Oscillating Reactions 2/6 iv) divalent manganese cations Mn 2+ as catalyst, which can be added as manganese sulfate MnSO 4 . v) An organic compound with an active, enolic hydrogen atom attached to carbon, R-H. The organic compound R-H will slowly reduce free iodine I 2 to iodide I - . Malonic acid HOOC-CH 2 -COOH is an excellent organic compound R-H. Another organic substrate that has been suggested is acetone H 3 C-CO-CH 3 . Starch is optionally added as an indicator to show the abrupt increase in iodide ion concentration [I - ] as a sudden change from the orange or amber color of free iodine I 2 to dark blue of the iodine-starch complex. The iodine-starch complex consists of an incorporation of triiodide I
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This note was uploaded on 04/29/2008 for the course CHEM 118 taught by Professor Jacobsen during the Spring '08 term at Tulane.

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Experiment 8 - EXPERIMENT 9 Oscillating Reactions...

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