Sign of a Cell Potential
The anode is defined as the electrode at which oxidation takes place, and the cathode is where reduction takes place. An easy way to remember this is that both anode and oxidation start with vowels, and cathode and reduction start with consonants.
Let's take the cell
which we can represent by
Zn | ZnSO4 aq || KCl aq || CuSO4 aq | Cu
In this cell, Zn is oxidized and Cu2+ is reduced. The overall reaction is
Zn + Cu2+ = Zn2+ + Cu.
On the left side we have the oxidation of Zn
Zn = Zn 2+ + e-
The electrons released result in a negatively charged Zn electrode. On the right we have
Cu2+ + e- = Cu
The electrons are drawn from the electrode to reduce the cupric ions, thus leaving a positive charge on the Cu electrode. If the two electrodes are connected by an external circuit we have a galvanic cell, and the flow of electrons will be from the Zn electrode to the Cu electrode.
The Zn electrode is the anode, as oxidation is taking place there, and the anode is negatively charged. The Cu electrode is the cathode, where we have reduction taking place. The cathode is positively charged. This is the reverse of an electrolytic cell where an applied potential causes the anode to be positive and the cathode to be negative.
This distinction can be clarified by picturing the process in this manner. Looking at the Cu electrode, if the external circuit were closed and current were passed, the result would be that Cu ions migrated toward the electrode where they would be reduced to Cu metal as electrons flowed through the circuit from the zinc to the copper electrode. The copper electrode is an electron source and is thus negatively charged. The cell is not in a state of equilibrium in this case. If the circuit is opened so that no current can flow the cell can then reach a state of equilibrium. Now if a cupric ion arrives at the surface of the Cu electrode and is reduced to copper metal by the transfer of two electrons to the ion, since there is no flow of electrons into the copper electrode from the external circuit, the only source of the electrons is from the copper metal in the electrode. Such a process would result in the formation of a new copper ion from one or two of the copper atoms in the electrode. So if we simply imagine the copper electrode as having some adsorbed cupric ions on the surface, this is consistent with the spontaneous tendency of the cupric ions to move toward the copper electrode and the result is a positively charged electrode. The opposite process would be happening at the zinc electrode where the spontaneous tendency is for zinc ions to migrate away from the surface of the electrode, leaving the excess electrons on the zinc metal.
IUPAC Conventions
1) nFE=-DG
2) Represented so that emf of cell is R with respect to L.
3) Electrode potential = cell potential with electrode on R vs standard hydrogen electrode on L.
Example:
1/2 H2 + AgCl = Ag + HCl(m)
H2, Pt | aq. HCl (m) | AgCl | Ag /p
If molality of HCl is < 9.1 mole kg-1, DG is negative and emf is positive (Ag is positive of Pt).
Example:
H2, Pt | H+ || Fe3+, Fe2+ | Pt
1/2 H2 + Fe3+ = H+ + Fe2+
Example:
For an electrode directly reversible to its cations the reduced species if the metal itself and its activity is one. Thus for a Zn electrode at equilibrium with Zn2+ ions in solution,
.