Edexcel A Level Chemistry:复习笔记6.1.4 Conventional Cell Representation

Conventional Cell Representation

 

Conventional Representation of Cells

  • As it is cumbersome and time-consuming to draw out every electrochemical cell in full, a system of notation is used which describes the cell in full, but does not require it to be drawn.
  • An electrochemical cell can be represented in a shorthand way by a cell diagram (sometimes called cell representations or cell notations)

19.1.1-Conventional-cell-representation

The conventional representation of voltaic cells

  • By convention, the half cell with the greatest negative potential is written on the left of the salt bridge, so Eθcell = Eθright – Eθleft
    • In this case, Eθcell = +0.34 – -0.76 = +1.10 V.

     

  • The left cell is being oxidized while the right is being reduced
  • If there is more than one species in solution, and the species are on different sides of the half-equation, the different species are separated by a comma
  • This method of representing electrochemical cells is known as the conventional representation of a cell, and it is widely used
  • If both species in a half reaction are aqueous then an inert platinum electrode is needed which is recorded on the outside of the half cell diagram

Some Examples

  • For the iron(II) and iron(III) half cell reaction a platinum electrode is needed as an electron carrier
  • The half equation is

Fe3+(aq) + e- ⇌ Fe2+(aq)

  • So the cell convention as a left hand electrode would be

Pt 丨Fe2+(aq), Fe3+(aq)

  • Notice the order must be Fe(II) then Fe(III) as the left side is an oxidation reaction, so Fe(II) is oxidised to Fe(III) by the loss of an electron
  • The platinum electrode is separated by the phase boundary (vertical solid line), but the iron(II) and iron(III) are separated by a comma since they are in the same phase
  • Non-metals will also require a platinum electrode
  • If chlorine is used as an electrode the reduction reaction is

Cl2(g) + 2e- ⇌ 2Cl-(aq)

  • The conventional representation of the half reaction would be

Cl2 (g), 2Cl- (aq) | Pt

  • Notice that the half cell reaction is balanced; however, it would be also correct to write it as

Cl2 (g), Cl- (aq) | Pt

  • This is because conventional cell diagrams are not quantitative- they are just representations of the materials and redox processes going on
    • Most chemists tend to show them balanced anyway

     

  • Combining these two half cells together gives

Pt  | Fe2+(aq), Fe3+(aq)  ∥  Cl2 (g), 2Cl- (aq) | Pt

  • As you can see the overall cell diagram is not quantitative as the left side is a one electron transfer and the right side is a two electron transfer

 

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