Background Information

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In the 1830s, Michael Faraday published his experiments using the recently discovered voltaic column to decompose substances through the use of electric current. Electrolysis is an oxidation-reduction process involving a conversion of electrical energy to chemical energy. The electrolytic cell is a galvanic cell operating in reverse. The automobile battery is an example that acts as a collection of galvanic cells when delivering electric current, but acts as a collection of electrolytic cells when being recharged.

Faraday first described the quantitative relationships between the amount of electric charge (number of electrons) that has passed through an electrolytic cell and the amount of materials that has formed at the electrodes. These are summarized as Faraday’s Laws of Electrolysis: 

  1. The mass of substance reacting at an electrode is directly proportional to the total amount of electric charge that has passed through the cell.

     

  2. The masses of the substances reacting at the electrodes are in direct ratio to their equivalent masses. 
  • The equivalent mass of a reacting substance is defined as its mass that reacts
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    with one mole of electrons in the oxidation of reduction process. In the case of sodium and chlorine, the equivalent masses of the sodium and chlorine are equal to their molar masses; the equivalent mass of copper is equal to its molar mass divided by two. The second law is a consequence of the stoichiometry of the balanced half-reactions.

    Through exhaustive experimentation, the charge of a single electron has been determined to be 1.602 x 10-19 coulombs (C). (The coulomb charge unit – defined as useful for much larger charged objects – is inconvenient for expressing such a small charge, so other electrical charge units are commonly used.) One mole of electrons has a total charge calculated to be 96,485 C; this quality is defined as faraday (F):

     1 F = 96, 485 C/mol e-

     Electric currents (l) are measured in amperes (A), amps for short, and defined in terms

    I = Q/t

    1 A = 1 C/s

     For example, a constant current of .600 A (milliamperes) over a period of 2.00 x 102 seconds represents

    Q = I x 5 = 0.600 A x 200s = 0.600 C/s x 200s = 120 C

    a movement of 120 coulombs. The number of moles of electrons (n) transported during the time interval is

    = 1.24 x 10-2 mol e-1

    Time intervals measured in minutes and hours must be converted to seconds in such calculations.