Mercury beating heart
Encyclopedia
The mercury beating heart is an electrochemical
redox reaction between the elements mercury
, iron and chromium. The reaction causes a blob of mercury in water to oscillate.
The observeable reaction demonstrates an effect of a non-homogeneous electrical double layer. It is often used as a classroom demonstration.
, immersed in an electrolyte
such as sulfuric acid
which contains an oxidizing agent
such as hydrogen peroxide
, potassium permanganate
, or potassium dichromate. The tip of an iron nail is positioned almost touching the mercury. If the position of the nail tip is just right, the mercury blob begins to oscillate, changing shape. A video clip of this is available at http://www.youtube.com/watch?v=3WF8zFieuKU
The net reaction is that the dichromate oxidizes the iron. This favorable reaction drives the mercury oxidations/reductions and the oscillations in shape. When the dichromate is all reduced, the reaction stops?
There may be other mechanisms involved, however. Lin et al. appear to report that the oscillations occur without the presence of the oxidizing agent, though the mercury does not appear to get an oxidizing layer on it and the oscillations are much weaker.
An electrical double layer forms between the surface of the mercury droplet and the electrolyte solution. At rest this layer is uniform. When the iron tip is introduced a redox reaction starts in which iron is oxidized to the ferric
ion. At the same time the oxidizing reagent is spent for instance when hydrogen peroxide together with hydronium
ions is reduced to water. Because the oxidation only takes place in the vicinity of the tip and the reduction process covers the whole droplet surface the surface tension
is no longer homogeneous resulting in oscillations.
Although this reaction is mediated by changes in surface tension, it is very similar in mechanism to other chemical oscillators such as the Belousov–Zhabotinsky reaction, which has several intermediate redox reactions driven by the oxidation of malate by bromine.
The mercury beating heart was first observed by Carl Adolf Paalzow in 1858. Jöns Jakob Berzelius
is reported to have used electrode
s.
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
redox reaction between the elements mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, iron and chromium. The reaction causes a blob of mercury in water to oscillate.
The observeable reaction demonstrates an effect of a non-homogeneous electrical double layer. It is often used as a classroom demonstration.
The experiment
In the experiment a droplet of mercury is placed in a watch glassWatch glass
A watch glass is a circular, slightly convex-concave piece of glass used in chemistry as a surface to evaporate a liquid, to hold solids while being weighed, or as a cover for a beaker...
, immersed in an electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
such as sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
which contains an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
such as hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
, potassium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
, or potassium dichromate. The tip of an iron nail is positioned almost touching the mercury. If the position of the nail tip is just right, the mercury blob begins to oscillate, changing shape. A video clip of this is available at http://www.youtube.com/watch?v=3WF8zFieuKU
The explanation
In one variation the mechanism is thought to be the following: The dichromate oxidizes the mercury, forming a layer of mercury sulfate. In the process the dichromate is reduced to the chromium (III) ion. The oxidized layer on the mercury reduces the surface tension of the blob and the blob flattens out coming in contact with the iron nail. Then the mercury sulfate oxidizes the iron to the iron (II) ion, and in the process is reduced back to metallic mercury. Once there is no oxide coating left on the mercury blob, the surface tension increases and the blob rounds up and loses contact with the nail, to start the process over again.The net reaction is that the dichromate oxidizes the iron. This favorable reaction drives the mercury oxidations/reductions and the oscillations in shape. When the dichromate is all reduced, the reaction stops?
There may be other mechanisms involved, however. Lin et al. appear to report that the oscillations occur without the presence of the oxidizing agent, though the mercury does not appear to get an oxidizing layer on it and the oscillations are much weaker.
An electrical double layer forms between the surface of the mercury droplet and the electrolyte solution. At rest this layer is uniform. When the iron tip is introduced a redox reaction starts in which iron is oxidized to the ferric
Ferric
Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron or Fe2+...
ion. At the same time the oxidizing reagent is spent for instance when hydrogen peroxide together with hydronium
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
ions is reduced to water. Because the oxidation only takes place in the vicinity of the tip and the reduction process covers the whole droplet surface the surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
is no longer homogeneous resulting in oscillations.
Although this reaction is mediated by changes in surface tension, it is very similar in mechanism to other chemical oscillators such as the Belousov–Zhabotinsky reaction, which has several intermediate redox reactions driven by the oxidation of malate by bromine.
The mercury beating heart was first observed by Carl Adolf Paalzow in 1858. Jöns Jakob Berzelius
Jöns Jakob Berzelius
Jöns Jacob Berzelius was a Swedish chemist. He worked out the modern technique of chemical formula notation, and is together with John Dalton, Antoine Lavoisier, and Robert Boyle considered a father of modern chemistry...
is reported to have used electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s.
See also
- Simple classroom demonstrations: Chemical gardenChemical GardenA chemical garden is an experiment in chemistry normally done by adding solid metal salts such as copper sulfate or cobalt chloride to an aqueous solution of sodium silicate . This results in growth of plant like forms in minutes to hours. The chemical garden was first observed and described by...
- Barking dog reactionBarking dog reactionThe "Barking Dog" is an exothermic chemical reaction that results from the ignition of a mixture of carbon disulfide and nitric oxide .It has been known for centuries; in 1853, Justus von Liebig was using the bright blue flash and the distinctive ‘woof’ sound of the demonstration to enthrall his...
- blue bottle reaction
External links
- Mercury beating heart video: video 1
- Mercury beating heart video: video 2
- Pulsating Electrochemical Reaction: Mercury Beating Heart: http://jchemed.chem.wisc.edu/JCESoft/CCA/pirelli/pages/cca3heart.html