Cell Representation and sign convention
Cell Representation and sign convention:
The above Daniell cell may be conveniently represented in a compact form as Zn(s)| Zn2 (C1 ) || Cu2 (C2 ) | Cu(s).....................(1)
Here, the vertical line denotes a boundary separating two phases. The double vertical lines indicate a salt bridge. On either side of the salt bridge are two half cells, or electrodes. On the left hand side (L), oxidation (O) occurs. This electrode is referred to as the anode (A) and it is the negative (N) terminal. A simple way to remember this arrangement is to remember the word “LOAN” (left, oxidation, anode, negative). Automatically this implies that at the other electrode on the right (R), reduction (R) occurs; the electrode is called the cathode (C) and it is the positive terminal(P), (RRCP).
The reactions at the anode and the cathode are respectively,
Zn(s) = Zn2 2e
Cu2 2e = Cu(s)............................................(2)
The emf (electromotive force) of this cell is 1.103V. The potential (emf) of a single electrode or a half cell can not be measured; we always need to electrodes to obtain an emf.
Instead of listing the potentials of all “pairs” of cells, it is far more convenient to take any one half cell as the standard and list all the other electrode potentials (actually reduction potentials, as recommended by the international Union of Pure and Applied Chemists, IUPAC) with reference to this standard. This reference electrode is taken as the standard hydrogen electrode (SHE) whose emf is assigned a reference value of 0 V .
H e 1 / 2 H2 (g), E o H / H2 = 0 V................................(3)
Table 22.1 lists the standard electrode potentials of a number of half cells. In the standard electrodes, species in all forms (solids, liquids or gases) are present with unit activities, i.e., in their standard states. It is to be noted that emf is an intensive property and that the half cell 2H 2e H2 (g) will have the same emf as eq (3) but the free energy change for this reaction will be twice the value for eq. (3), because rGo = - nFo and rGo is an extensive property depending on the number of moles of electrons transferred.
Table 1 Standard electrode potentials at 298.15 K