Chemical clocks are reactions that cause an observable event after a delay, often indicated by a change in color or production of gas. The iodine clock reaction is a common example. Cyclical reactions involve a solution that alternates between two or more states, and the Briggs-Rauscher reaction is an example of a three-color oscillating chemical clock. The Belousov-Zhabotinsky reaction creates complex patterns of concentric spirals and circles. The beating heart reaction of mercury involves changes in shape. These reactions are of interest in the study of chemical kinetics and self-organizing systems, and may have played a role in the origin of life.
A chemical clock is a scenario in which reacting chemicals cause a sudden, observable event after a delay that can be set relatively precisely by adjusting the concentrations of the reactants. Often the event is indicated by a change in color but can take some other form, such as the production of gas causing fizz. In some cases the change is cyclical and involves a solution that periodically alternates two or more states, usually indicated by different colors.
One of the simplest chemical clocks is known as an “iodine clock” reaction. Two colorless solutions are mixed, and after a pause, the resulting solution suddenly turns dark blue. In the most common version of the experiment, one solution contains a dilute mixture of sulfuric acid and hydrogen peroxide and the other a mixture of potassium iodide, starch and sodium thiosulfate. As the solutions are stirred, elemental iodine is released from the potassium iodide, but a more rapid reaction between the iodine and sodium thiosulfate converts it back to colorless iodide ions. When all of the thiosulfate has been used up, the iodine is able to react with the starch to produce a dark blue compound.
The cyclical or oscillating chemical reactions of the clock are particularly fascinating. Normally, a chemical reaction proceeds in one direction until it reaches a point of equilibrium. After this, no further changes will take place without the intervention of some other factor, such as a change in temperature. The swing reactions were initially puzzling as they appeared to defy this rule by spontaneously drifting away from equilibrium and repeatedly returning to it. In reality, the overall reaction proceeds to and remains at equilibrium, but in the process the concentration of one or more reactants or intermediates varies cyclically.
In an idealized chemical clock, there is one reaction that creates a product and another reaction that uses this product, with the concentration of the product determining which reaction takes place. When the concentration is low, the first reaction occurs, producing more product. An increase in the concentration of the product, on the other hand, triggers the second reaction, reducing the concentration and causing the first reaction. This results in a cycle where the two competing reactions determine the concentration of a product, which in turn determines which reaction will take place. After a certain number of cycles, the mixture will reach equilibrium and the reactions will stop.
One of the first cyclic chemical clocks was observed by William C. Bray in 1921. It involved the reaction of hydrogen peroxide and an iodized salt. The investigation of Bray and his student Hermann Liebhafsky showed that the reduction of iodate to iodine, with production of oxygen, and the oxidation of iodine back to iodate occurred periodically with cyclical peaks in oxygen production and concentration of iodine. This became known as the Bray-Liebhafsky reaction.
In the 1950s and 1960s, biophysicists Boris P. Belousov and, later, Anatol M. Zhabotinsky studied another cyclic reaction involving the periodic oxidation and reduction of a cerium salt, resulting in a change in color. If the Belousov-Zhabotinsky, or BZ, reaction is performed using a thin layer of the chemical mixture, a dramatic effect is observed, with small local fluctuations in the concentrations of the reactants leading to the emergence of complex patterns of concentric spirals and circles. The chemical processes taking place are very complex, involving as many as 18 distinct reactions.
Science instructors Thomas S. Briggs and Warren C. Rauscsher, using the above reactions as a basis, created an interesting three-color oscillating chemical clock in 1972. The Briggs-Rauscher reaction features a solution that periodically changes from colorless to to light brown to dark blue. If set carefully, it may take 10-15 cycles before it settles into equilibrium at a dark blue color.
An unusual chemical clock that involves changes in shape rather than color is the beating heart reaction of mercury. A drop of mercury is added to a solution of potassium dichromate in sulfuric acid and then an iron nail is placed near the mercury. A film of mercury I sulphate forms on the drop, which reduces the surface tension and causes it to stretch until it touches the iron nail. When this happens, electrons from the nail reduce mercury I sulfate to mercury, restoring surface tension and causing the stain to contract again, losing contact with the nail. The process repeats itself many times, resulting in a cyclical change in shape.
The chemical reactions of the watch are an ongoing area of research. In particular, cyclic or oscillating reactions are of great interest in the study of chemical kinetics and self-organizing systems. It has been speculated that reactions of this type may have been involved in the origin of life.
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