edited by GEORGEL. GILBERT Denison University Granville, Ohio 43023
A Simple and Dramatic Demonstration of Overvoltage SUBMITTEDBY
John L. Bradford Alvie L. Davis Abilene Christian University Abilene. Texas 79699
as can be seen by substituting silver wire for copper wire in the demonstration. Bubbles will not form at a copper or silver wire that has been amalgamated. This demonitrates the well known high hydrogen overvoltaxe of mercury. An int&es&g 'ariation of the demonstration utilizes an inch-long piece of each of the three metals-copper, platinum, and zinc-arranged at 120-degreeangles and connected at the center by a spot of solder: When this three-metal connection is nlaced in the acid. bubbles of hvdroeen are observed mostlv " a t the copper and platinum; hut as before, the rate of gas evolution is much ereater a t the surface of the platinum wire. The demonstration as described is easily visualized only for small erouos. but can he dis~lavedfor a larae audience bv using overhead projector &d ;petri dish orshallow heake;. For nroiection. the three-metal connection described above . " is recommended. ~~
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CHECKED BY
David A. Franz Lycaming College Williamsport. PA Overvoltage, also called overpotential, activation overpotential, and kinetic polarization, is the voltage in excess of the reversible value which is required to cause a cell reaction to proceed at an appreciable rate. Its magnitude varies with temperature, current density, composition of the electrodes, and the products of the reaction. It is most pronounced for processes that yield gaseous products, but is observed to a variable degree with all types of electrode processes. The dependence of overvoltage on the composition of a metal surface a t which the discharge of a gas oocurs can he easily and dramatically demonstrated by the following procedure. A strip of reagent grade zinc ribbon (J. T. Baker product number 4290) is dipped into a solution of 1.0 M hydrochloric acid and observed. For this concentration of acid at room temnerature. the rate of hvdroaen " - evolution is verv slow-only a few bubbles are formed. Next, with the strip of zinc still dinned into the acid, a clean Copper wire is touched to the imm&ed surface of the zinc. o n e c a n now observe a copious supply of hydrogen being evolved. Close examination reveals that the hydrogen bubbles are forming at the surface of the conner -. wire and not on the zinc. Evidently, electrons are given up by the zinc to the copper and then transferred to the aqueous hydrogen ions. If the demonstration is repeated using a platinum wire in place of the copper, the rate of hydroaen evolution is noticeably increased above that observed when the copper wire was used. These observations show that the overvoltage for hydrogen discharge is largest for zinc, smaller for copper, and still smaller for platinum. This is in accord with the published values1 of 0.746 V for zinc, 0.584 V for copper, and 0.068 V for smooth platinum. These values are for 25°C and a current density of 0.01 A/cm2. The hydrogen overvoltage of silver is close to that of copper
674
Journal of Chemical Education
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National Academy of Sciences, "International Critical Tables." McGraw-Hill, New York, 1929, Vol. 6, pp. 339-340.
Hydrogen evolution from a copper wire in contact with zinc in acid solution.
