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)
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 Eꝋcell = Eꝋright – Eꝋleft
- In this case, Eꝋcell = +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