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Concept
Bouteille bleue
Résumé
The blue bottle experiment is a color-changing redox chemical reaction. An aqueous solution containing glucose, sodium hydroxide, methylene blue is prepared in a closed bottle containing some air. Upon standing, it spontaneously turns from blue to colorless due to reduction of methylene blue by the alkaline glucose solution. However, shaking the bottle oxidizes methylene blue back into its blue form. With further shaking, this color-change cycle can be repeated many times. This experiment is a classic chemistry demonstration that can be used in laboratory courses as a general chemistry experiment to study chemical kinetics and reaction mechanism. The reaction also works with other reducing agents besides glucose and other redox indicator dyes besides methylene blue.
The mechanism of the blue bottle experiment requires an understanding of rates and mechanisms of complex interacting chemical reactions. In complex chemical reactions, individual sub-reactions can occur simultaneously but at significantly different rates. These, in turn, can be affected by reagent concentration and temperature. In most cases, the overall reaction rate is determined by the fastest single component reaction. However, when some processes form intermediate molecules which then react in other processes to form the end product, the rate of the overall reaction is determined by the rate of the slowest reaction. In such circumstances the intermediate products are usually in a steady state at low concentrations because they are highly reactive. Equilibrium state requires that all reaction forward and backward mechanism happens at the same rate. Thus, the overall net reaction is determined by the sum of all the mechanism steps where the rate depends on the concentration and temperature. The blue bottle experiment illustrates this principle of interacting reactions with different rates.
The blue bottle experiment requires only three reagents: potassium hydroxide solution, dextrose solution, and dilute methylene blue solution.
Source officielle
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