EMF: Assigning Potential to Change


A great many chemical reactions involve transfer of electrons.  But the transfer of electrons is generally not apparent.

For example, in

 2H2 + O2 -> 2H2 O
there typically is noise and other forms of energy release, but electron transfer is not obvious.  Even inspecting the equation does not reveal electrons.

It has long been known that rubbing a few substances creates what the ancient Greeks called electrics.    But the connection between electrics and matter suspected by some was hard to find.  The connection was not made by a direct study, but as often happens, by tangential experiments. Ben Franklin and others suspected that lightning might also be electric.  (Franklin’s kite efforts at verification lead to the lightning rod.)  Luigi Galvani’s verification attempt in Italy involved hooking frog legs to a clothes line during a thunderstorm.  He was disappointed when the legs didn’t twitch as they did when excited by electric sparks in his laboratory.  He was chagrined when he hung frog legs on a metal rod for storage and they commenced twitching.  Galvani interpreted the cause as animal electricity, but Alessandro Volta suggested and verified in 1800 that a reaction in piles of two metals (the hook and the rod) creates an electric spark.  Volta found that a key requirement was the alternating separation of the metals by something like wet cardboard.

We now realize that the materials giving up and gaining the electrons must be separated to make the transfer observable, measurable, or useful for generating power.

To clarify the transfer, chemists separate the balanced equation into two half reactions, one describing the loss of electrons (called OXIDATION), the other describing gain in electrons (called REDUCTION).

2H2 -> 4H+ + 4e-
4e- + O2 -> 2O2-
(the H+ and O2- bond together by electrical attraction to make 2H2O.)
2H2 + O2 -> 2H2 O
By using half reactions, the transfer of electrons becomes clear.

The “electrical motivation pushing” the transfer of electrons is referred to by chemists as EMF, electromotive force.  Physicists prefer the term ELECTRICAL POTENTIAL.  Consideration of the Volt, the unit of measurements, clarifies whether this is actually a force or the energy stored in the force.  The Volt is defined as potential energy per unit electric charge.  So while EMF paints a nice word picture, this is really a measure of electrical potential energy per unit charge.  As was true with reaction rates, chemists would like to know the forces of individual chemical bonds, but we settle for the much more convenient energy concept as a related substitute.

There is no longer any confusion about the chosen unit of energy: nearly everyone uses the Joule (and the calorie has become archaic).  But the unit of electric charge requires our consideration.  On first impression it seems most natural that the unit should be the “elementary charge,” that of the electron.  But except for nuclear chemistry and physics where the “electron volt” (eV) is universally used to measure energy for individual particles, a much larger unit of charge is needed.  The standard charge unit is the Coulomb (6.241,506,344•1018 electrons = one Coulomb).  This may seem strange to chemists who generally prefer to count large numbers in units of moles (6.022,136,736•1023 = one mole).  So it will take 96,500 Coulombs to make one mole.  (This factor is called a Faraday of charge and is needed when we want to relate amounts of matter to the charge transferred.)  The existence of TWO distinct numbers was caused because neither fundamental number was very easily determined.  So the Coulomb was defined in terms of the measured force between two wires carrying one Ampere of current, and the mole was defined in terms of however many atoms would be in an atomic weight in grams of any element.  I.e., both numbers were pragmatically defined in terms of easily measurable properties before the count of either number was known.

We measure the Voltage between half cells, each containing the reactants needed for half the reaction.  If we compare the Voltages of several half reactions to a common “standard.”  Traditionally the hydrogen half reaction is arbitrarily chosen, so compared to an identical hydrogen half cell, the hydrogen half cell’s voltage is zero.  By combining cell potentials using the common standard, the Voltage of other reactions can be predicted.  This is then used to predict whether a reaction will occur or not.

This page is still in development.  Additions and revisions are planned.


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© by D.Trapp