![]() ![]() The resulting data then allows you to quantify the reaction kinetics. This is true even though the starch complex is not directly generated from the oxidation.Īside from the qualitative observation of the increasingly blue reaction vessel, you can periodically measure the starch concentration through spectrophotometry. Thus, for the entirety of the experiment, oxidation determines the progress of the dark blue hue. Additionally, once the reduction ceases, the oxidation continues to serve as the rate-determining step, as both the formation of the triiodide and the starch complex occur relatively quickly. The first reaction, the oxidation, occurs much slower than the reduction, making it the rate-determining step during that first phase of the reaction. The structure involves the amylose helix of the starch wrapping around the chain of triiodide ions. As a side note, due to the striking dark blue of the complex, a mixture of iodine and iodide called Lugol’s iodine is used to test for trace amounts of starch. This complex is responsible for the increasing dark blue of the reaction vessel. This triiodide ion then forms a complex with the starch. They react with one another to form the triiodide ion: Afterward, significant quantities of iodide and iodine exist at the same time. This back and forth between iodide and iodine continues until all thiosulfate oxidizes away. Importantly, as the reaction produces diatomic iodine, the thiosulfate re-reduces the iodine back to iodide: Once the solutions mix, the hydrogen peroxide oxidizes the iodide into diatomic iodine: The Chemistry of the Iodine Clock Iodine Clock Redox and Kineticsīefore the three solutions mix into one, each ionic species dissociates into their respective ions:ĭuring the reaction, K +, Na +, and HSO 4 – do not participate, remaining as spectator ions. Aside from that, the reductant concentration tends to be kept low, as very little is required, while the starch tends to be in excess. Thus, most lab procedures studying reaction kinetics will vary the concentrations of one or more of these species. Potassium iodide (KI) serves as the salt, while sulfuric acid (H 2SO 4) provides the required acidity. Importantly, gloves, safety goggles, and caution should be observed when using sulfuric acid and hydrogen peroxide to prevent chemical burns.Īs we’ll find out in a later section, the kinetics of the reaction depends on the concentrations of acid, iodide, and oxidant. The most common variant of the Iodine Clock Reaction uses sodium thiosulfate (Na 2S 2O 3) as the reductant and hydrogen peroxide (H 2O 2) as the oxidant. Once the solutions mix, the reaction begins. Second Solution: Iodine Salt, Reductant, Water.As mentioned before, these components become allocated between three different solutions according to these specifications: To perform the Iodine Clock Reaction, you need an iodine salt, a reductant, an oxidant, an acid, starch, and water as a solvent. Early stage of the dark blue color transformation of the iodine clock Iodine Clock Procedure In Egypt, the darkness of nighttime often arrives rather suddenly, similar to rapid dark color change in this reaction. Interestingly, some chemists colloquially call this reaction the “Egyptian Night” experiment. This color change corresponds to the progress of the reaction, which allows you to visually witness the kinetics in a way that most reactions do not provide. Once the solutions combine, however, the mixture gradually turns from clear to dark blue to near-black. In this experiment, you prepare two simple, transparent solutions. For this, the reaction persists as a staple of general chemistry lab demonstrations. The Iodine Clock Reaction is a classic chemistry experiment that demonstrates many basic principles of kinetics and redox chemistry. In this lab tutorial, we learn about the iodine clock reaction, including its procedure, underlying chemistry, and data analysis.
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