Pulsating Electrochemical Reaction: Mercury Beating Heart
A drop of mercury in a watch glass is covered with a solution of potassium chromate in concentrated sulfuric acid. (In acidic solutions chromate, CrO42-, is converted to dichromate, Cr2O72-.) An iron nail is positioned so that it nearly touches the mercury. Eventually, the mercury drop starts to beat rhythmically, like a beating heart.
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Keywords
electric charge, electrochemical (galvanic) cells, evidence of chemical reaction, liquids, redox reaction, strong oxidizing agent, surface phenomena
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A solution of potassium dichromate in dilute sulfuric acid covers a drop of mercury in a watch glass. An iron nail is positioned so that it nearly touches the mercury. Eventually, the mercury drop starts to beat rhythmically, like a beating heart.
The dichromate oxidizes the mercury to mercury(I) ions, which combine with sulfate ions at the surface of the mercury drop to form a film of insoluble mercury(I) sulfate. This film decreases the surface tension of the mercury, allowing the drop to flatten. Eventually the mercury drop expands to touch the iron nail, at which time electrons flow from the nail to the mercury. The electrons reduce the mercury(I) ions to mercury, destroying the surface film. The surface tension increases and the mercury drop becomes more spherical, pulling back from the nail. When the mercury and the iron nail no longer touch, mercury(I) sulfate again builds up on the surface, and the process repeats.
 Here there is no electrical connection between the mercury and the iron. |
 Here electrons are transfered from the iron to the mercury.. |
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Discussion
The mercury drop, iron nail, and aqueous solution of acidic dichromate constitute a galvanic cell in which the mercury serves as the cathode and the iron serves as the anode. There is a voltage difference between the iron and mercury electrodes. During the time when there is no electrical connection between the mercury and the iron, dichromate oxidizes Hg(l ) to Hg2SO4(s ), which forms a layer on the mercury surface. This surface layer reduces the surface tension of the liquid mercury, allowing the mercury drop to flatten. Eventually the mercury contacts the iron nail and the cell is short circuited. Transfer of electrons from the iron to the mercury and thence to the Hg2SO4(s ) surface film reduces the Hg2SO4(s ) back to metallic mercury. This increases the surface tension of the mercury and the drop becomes rounder, causing it to recede from the nail and break the circuit. Then the surface film builds up again as a result of oxidation of mercury by the dichromate.
The equations for these processes are
| film | 2 Hg(l ) + SO42-(aq ) --> Hg2SO4(film ) + 2 e- |
| formation | Cr2O72-(aq ) + 14 H+(aq ) + 6 e- --> 2 Cr3+(aq ) + 7 H2O(l ) |
| film | Hg2SO4(film ) + 2 e- --> 2 Hg(l ) + SO42-(aq ) |
| reduction | Fe(s ) --> Fe2+(aq ) + 2 e- |
The overall reaction is oxidation of iron by dichromate:
Cr2O72-(aq ) + 14 H+(aq ) + 3 Fe(s ) --> 2 Cr3+(aq ) + 7 H2O(l ) + 3 Fe2+(aq )
This reaction can occur more rapidly when mediated by the oxidation and reduction of mercury, presumably because a surface coating of hydroxide or oxide on the iron slows the metal's reaction with dichromate ions and hydrogen ions in the aqueous solution. Some of the Gibbs free energy released by this overall reaction serves to drive the physical movement of the mercury drop.
A more detailed discussion of the mechanism of the mercury beating heart is available: Keizer, J.; Rock, P. A.; Lin, Shu-Wai J. Amer. Chem. Soc. 1979, 101, 5637-5649.
Demonstration Notes, Warnings, Safety Information, etc.
Exam and Quiz Questions
1. What is the purpose of the nail in this demonstration?
2. Write balanced chemical equations for the oxidation of mercury by chromate, and of iron by Hg22+.
3. Why does oxidation of mercury cause the droplet to flatten? Why does contact with the iron nail reverse the flattening of the droplet?