In another well known "beating mercury heart" reaction, a 1 ml drop of mercury is covered by a dilute solution of a strong acid (H2SO4, HNO3) and crystals of an oxidizing agent (K2Cr2O7, KMnO4) are added to it. If an iron wire just touches the mercury globule, the "heart" (mercury globule) pulsates 2 to 3 times per second. The electrochemical cell formed is shown below.
Figure 30.4 The beating mercury heart
Fe (s) | H+ (aq), Cr2O72- (aq)|Hg (l)
(30.30)
In the cell, Fe is the negative electrode. When there is Fe...Hg contact, Fe is oxidized to Fe2+ + 2e -; the solvated Fe2+ diffuses into solution and the electrons are adsorbed on the Hg surface giving it a negative charge(due to adsorption of Hg2- on the surface). This negative charge on the drop repels the negative electrode and the electrical contact breaks. When the adsorbed negative ions desorb into the solution, the liquid drop assumes a spherical shape. The electrical contact to Fe is reestablished and the cycle repeats, exhibiting a beating mercury heart. Of course, watching this heart beat is far more dramatic than reading about it!
