HOME

TheInfoList



OR:

A chemical clock (or clock reaction) is a complex mixture of reacting
chemical compounds A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
in which the onset of an observable property (discoloration or coloration) occurs after a predictable induction time due to the presence of clock species at a detectable amount. In cases where one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change after a reproducible time lapse.


Types

Clock reactions may be classified into three or four types:


Substrate-depletive clock reaction

The simplest clock reaction featuring two reactions: :A → C (rate k1) :B + C → products (rate k2, fast) When substrate (B) is present, the clock species (C) is quickly consumed in the second reaction. Only when substrate B is all used up or depleted, species C can build up in amount causing the color to change. An example for this clock reaction is the sulfite/iodate reaction or iodine clock reaction, also known as Landolt's reaction. Sometimes, a clock reaction involves the production of intermediate species in three consecutive reactions. :P + Q → R :R + Q → C :P + C → 2R Given that Q is in excess, when substrate (P) is depleted, C builds up resulting in the change in color.


Autocatalysis-driven clock reaction

The basis of the reaction is similar to substrate-depletive clock reaction, except for the fact that rate k2 is very slow leading to the co-existing of substrates and clock species, so there is no need for substrate to be depleted to observe the change in color. The example for this clock is pentathionate/iodate reaction.


Pseudoclock behavior

The reactions in this category behave like a clock reaction, however they are irreproducible, unpredictable and hard to control. Examples are chlorite/thiosulfate and iodide/chlorite reactions.


Crazy clock reaction

The reaction is irreproducible in each run due to the initial inhomogeneity of the mixture which result from variation in stirring rate, overall volume as well as geometry of the reactors. Repeating the reaction in the statistically meaningful manners leads to the reproducible cumulative probability distribution curve. The example for this clock is iodate/arsenous acid reaction. One reaction may fall into more than one classification above depending on the circumstance. For example, iodate−arsenous acid reaction can be substrate-depletive clock reaction, autocatalysis-driven clock reaction and crazy clock reaction.


Examples

One class of example is the iodine clock reactions, in which an iodine species is mixed with
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
reagents in the presence of starch. After a delay, a dark blue color suddenly appears due to the formation of a triiodide-starch complex. Additional reagents can be added to some chemical clocks to build a
chemical oscillator A chemical oscillator is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes. They are a class of reactions that serve as an example of non-equilibrium thermodynamics wit ...
. For example, the
Briggs–Rauscher reaction The Briggs–Rauscher oscillating reaction is one of a small number of known oscillating chemical reactions. It is especially well suited for demonstration purposes because of its visually striking colour changes: the freshly prepared colourless s ...
is derived from an iodine clock reaction by adding
perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous s ...
,
malonic acid Malonic acid (IUPAC systematic name: propanedioic acid) is a dicarboxylic acid with structure CH2(COOH)2. The ionized form of malonic acid, as well as its esters and salts, are known as malonates. For example, diethyl malonate is malonic acid' ...
and manganese sulfate.


See also

* Circadian clock *
Chemical oscillator A chemical oscillator is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes. They are a class of reactions that serve as an example of non-equilibrium thermodynamics wit ...


References

{{DEFAULTSORT:Chemical Clock Chemical kinetics Clocks Non-equilibrium thermodynamics Oscillation Articles containing video clips