Editing Voltaic pile (section)

== Electromotive force ==

The strength of the pile is expressed in terms of its [[electromotive force]], or emf, given in volts. Alessandro Volta's theory of [[contact tension]] considered that the emf, which drives the electric current through a circuit containing a voltaic cell, occurs at the contact between the two metals. Volta did not consider the electrolyte, which was typically [[brine]] in his experiments, to be significant. However, chemists soon realized that water in the electrolyte was involved in the pile's chemical reactions, and led to the evolution of [[hydrogen]] gas from the copper or silver electrode.<ref name=Decker>{{cite encyclopedia |last=Decker |first=Franco |encyclopedia=Electrochemistry Encyclopedia |title=Volta and the 'Pile' |date=January 2005 |publisher=Case Western Reserve University |url=http://electrochem.cwru.edu/encycl/art-v01-volta.htm |url-status=dead |archive-url=https://web.archive.org/web/20120716205546/http://electrochem.cwru.edu/encycl/art-v01-volta.htm |archive-date=2012-07-16 }}</ref><ref name=Turner>{{cite book |title=Elements of chemistry: including the actual state and prevalent doctrines of the science |first1=Edward |last1=Turner |editor1-first=Justus |editor1-last=Liebig |editor2-first=William |editor2-last=Gregory |edition=7|publisher=Taylor and Walton |location=London |year=1841 |page=102 |url=https://books.google.com/books?id=pRPnAAAAMAAJ&pg=PA102 |quote=During the action of a simple circle, as of zinc and copper, excited by dilute sulfuric acid, all of the hydrogen developed in the voltaic action is evolved at the surface of the copper.}}</ref><ref name=Goodisman>{{cite journal |last=Goodisman |first=Jerry |title=Observations on Lemon Cells |journal=Journal of Chemical Education |year=2001 |volume=78 |issue=4 |pages=516 |url=http://surface.syr.edu/cgi/viewcontent.cgi?article=1001&context=che |doi=10.1021/ed078p516|bibcode=2001JChEd..78..516G }} Goodisman notes that many chemistry textbooks use an incorrect model for a cell with zinc and copper electrodes in an acidic electrolyte.</ref><ref name=Graham>{{cite book |title=The Geek Atlas: 128 Places Where Science and Technology Come Alive |first=John |last=Graham-Cumming |chapter=Tempio Voltiano |publisher=O'Reilly Media |year=2009 |isbn=9780596523206 |page=97 |chapter-url=https://books.google.com/books?id=HhEC0q-O1ewC&pg=PA97}}</ref>

The modern, atomistic understanding of a cell with zinc and copper electrodes separated by an electrolyte is the following. When the cell is providing an electrical current through an external circuit, the metallic zinc at the surface of the zinc anode is oxidized and dissolves into the electrolyte as electrically charged [[ions]] (Zn<sup>2+</sup>), leaving two negatively charged [[electron]]s ({{SubatomicParticle|electron}}) behind in the metal:

::[[anode]] (oxidation): Zn{{space|2}}{{arrow2|right|11px}}{{space|2}}Zn<sup>2+</sup> + 2 {{SubatomicParticle|electron}}

This reaction is called [[redox|oxidation]]. While zinc is entering the electrolyte, two positively charged [[hydrogen]] ions (H<sup>+</sup>) from the electrolyte accept two electrons at the copper cathode surface, become reduced and form an uncharged hydrogen molecule (H<sub>2</sub>):

::[[cathode]] (reduction): 2 H<sup>+</sup> + 2 {{SubatomicParticle|electron}}{{space|2}}{{arrow2|right|11px}}{{space|2}}H<sub>2</sub>

This reaction is called [[redox|reduction]]. The electrons used from the copper to form the molecules of hydrogen are made up by an external wire or circuit that connects it to the zinc. The hydrogen molecules formed on the surface of the copper by the reduction reaction ultimately bubble away as hydrogen gas.

One will observe that the global electro-chemical reaction does not immediately involve the electrochemical couple Cu<sup>2+</sup>/Cu (Ox/Red) corresponding to the copper cathode. The copper metal disk thus only serves here as a "chemically inert" noble metallic conductor for the transport of electrons in the circuit and does not chemically participate in the reaction in the aqueous phase. Copper does act as a catalyst for the hydrogen-evolution reaction, which otherwise could occur equally well directly at the zinc electrode without current flow through the external circuit. The copper electrode could be replaced in the system by any sufficiently noble/inert and catalytically active metallic conductor (Ag, Pt, stainless steel, graphite, ...). The global reaction can be written as follows:

::Zn + 2H<sup>+</sup>{{space|2}}{{arrow2|right|11px}}{{space|2}}Zn<sup>2+</sup> + H<sub>2</sub>

This is usefully stylized by means of the electro-chemical chain notation:

::(anode: oxidation) Zn | Zn<sup>2+</sup> || 2H<sup>+</sup> | H<sub>2</sub> | Cu (cathode: reduction)

in which a vertical bar each time represents an interface. The double vertical bar represents the interfaces corresponding to the electrolyte impregnating the porous cardboard disk.

When no current is drawn from the pile, each cell, consisting of zinc/electrolyte/copper, generates 0.76 V with a brine electrolyte. The voltages from the cells in the pile add, so the six cells in the diagram above generate 4.56 V of electromotive force.