SINGLE ELECTRODE POTENTIAL
An electrochemical cell consists of two half-cells. With an open-circuit, the metal electrode in each half-cell transfers its ions into solution. Thus an individual electrode develops a potential with respect to the solution. The potential of a single electrode in a half-cell is called the Single electrode potential. Thus in Daniel cell in which the electrodes are not connected externally, the anode Zn/Zn2+ develops a negative charge and the cathode Cu/Cu2+, a positive charge. The amount of the charge produced on an individual electrode determines its single electrode potential.
The single electrode potential of a half-cell depends on : (a) concentration of ions in solution ; (b) tendency to form ions ; and (c) temperature.
Standard emf of a cell
The emf generated by an electrochemical cell is given by the symbol E. It can be measured with the help of a potentiometer. The value of emf varies with the concentration of the reactants and products in the cell solutions and the temperature of the cell. When the emf of a cell is determined under standard conditions, it is called the standard emf. The standard conditions are : (a) 1 M solutions of reactants and products ; and (b) temperature of 25o C. Thus standard emf may be defined as the emf of a cell with 1 M solutions of reactants and products in solution measured at 25o C. Standard emf of a cell is represented by the symbol Eo. For gases 1 atm. pressure is a standard condition instead of concentration. For Zn-Cu voltaic cell, the standard emf, Eo is 1.10V.
Zn | Zn2+(aq, 1M) || Cu2+(aq, 1M) | Cu Eo = 1.10 V
Determination of emf of a half-cell
By a single electrode potential, we also mean the emf of an isolated half-cell or its half-reaction. The emf of a cell that is made of two half-cells can be determined by connecting them to a voltmeter. However, there is no way of measuring the emf of a single half-cell directly. The emf of the newly constructed cell, E is determined with a voltmeter. The emf of the unknown half-cell Eo can then be calculated from the expression
Emeasured = ER - E L
If the standard half-cell acts as anode, the equation becomes
ER = Emeasured (Q EL = 0)
On the other hand, if standard half-cell is cathode, the equation takes the form
EL = - E measured (Q ER = 0)
The standard hydrogen half-cell or Standard Hydrogen Electrode (SHE), is selected for coupling with the unknown half-cell. It consists of a platinum electrode immersed in a 1 M solution of H+ ions maintained at 25 oC. Hydrogen gas at one atmosphere enters the glass hood and bubbles over the platinum electrode. The hydrogen gas at the platinum electrode passes into solution, forming H+ ions and electrons.
The emf of the standard hydrogen electrode is arbitrarily assigned the value of zero volts. So, SHE can be used as a standard for other electrodes.
The half-cell whose potential is desired, is combined with the hydrogen electrode and the emf of the complete cell determined with a voltmeter. The emf of the cell is the emf of the half-cell.
For example, it is desired to determine the emf of the zinc electrode, Zn | Zn2+. It is connected with the SHE. The complete electrochemical cell may be represented as :
Zn | Zn2+ || H+ | H2 (1 atm), Pt
The emf of the cell has been found to be -0.76 V which is the emf the zinc half-cell. Similarly, the emf of the copper electrode, Cu2+ | Cu can be determined by pairing it with the SHE when the electrochemical cell can be represented as :
Pt, H2 (1 atm) | H+ || Cu2+ | Cu
The emf of this cell has been determined to be 0.34 V which is the emf of the copper half-cell.
Eocell = EoCu/Cu2+ - E0SHE
=0.34 - Zero
The two situations are explained as follows :
When it is placed on the right-hand side of the zinc electrode, the hydrogen electrode reaction is
2H+ + 2e- ---- > H2
The electrons flow to the SHE and it acts as the cathode.
When the SHE is placed on the left hand side, the electrode reaction is
H2 -- -- -- > 2H+ + 2e-
The electrons flow to the copper electrode and the hydrogen electrode
as the anode. Evidently, the SHE can act both as anode and cathode and, therefore can be used to determine the emf of any other half-cell electrode (or single electrode).
According to IUPAC convention, the standard reduction potentials alone are the standard potentials. The values of the standard potentials at 25oC (298 K) for some common Reduction Half-reactions are listed in Table below.
Standard Reduction Potentials at 25oC (298K)
Predicting Cell EMF
The standard emf Eo, of a cell is the standard reduction potential of right-hand electrode (cathode) minus the standard reduction potential of the left-hand electrode (anode). That is,
Eocell = Eoright - E oleft
= Cathode potential - Anode potential
Let us predict the emf of the cell
Zn(s) | Zn2+(aq) || Ag+(aq) | Ag
by using the Eo values from the table.
Eocell = EoR - E oL
0.80 - (- 0.763)
0.80 + 0.763 = 1.563 V
Predicting Feasibility of Reaction
The feasibility of a redox reaction can be predicted with the help of the electrochemical series. The net emf of the cell reaction, Ecell, can be calculated from the expression
Eo cell = Eo cathode - Eo anode
In general, if Eocell = + ve, the reaction is feasible
Eo cell = -ve, the reaction is not feasible