When a metal electrode is placed in a solution of its ions, e.g. Cu in a solution containing Cu2+ ions, a potential difference (voltage) is set upbetween the two. This can occur in two ways:
We cannot be quite certain which of these processes occurs in a particular case because we cannot measure the potential difference between electrode andsolution (known as electrode potential).
Measurement would require an electrical connection to be made between a voltmeter and the solution by inserting a metal wire or strip, which would inevitablyact as another electrode with its own electrode potential.
That is why we can only measure the sum of two electrode potentials, never a single electrode potential. To overcome this difficulty, wearbitrarily assign an electrode potential of zero to one particular half-cell and compare all other half-cells with this standard. By internationalagreement, the standard hydrogen electrode has been chosen as the standard reference electrode.
The diagram below represents a standard hydrogen electrode connected by a salt bridge to another electrode, or half-cell, to make a cell. This cellgenerates a potential difference, known as the e.m.f., which can be measured by a high resistance voltmeter (one which allows no current to pass).
The general cell can be represented by a short-hand diagram, where M represents any metal and its ions.
The e.m.f.: E = ER - EL
ER = standard electrode potential of right-hand electrode.
EL = standard electrode potential of left-hand electrode.
By convention, the sign of the e.m.f. is the polarity of the right hand electrode in the cell diagram, and the hydrogen electrode is always written on theleft.
The electrode potential developed in a half cell depends on:
Standard conditions are:
The diagram below shows two metals inserted into solutions of their ions. The two solutions are joined by a salt bridge, and the two metal electrodes areconnected by an external circuit. The cell has an e.m.f. which is equal to the difference between the standard electrode potentials of the two metals,and a current flows through the external circuit.
This cell can be represented as:
|reduced species||phase boundary||oxidised species||salt bridge||oxidised species||phase boundary||reducedspecies|
The convention is to always write the half-cell with the more positive electrode potential on the right-hand side.
|E.M.F.||= ER - EL|
|= +0.34 - (-0.76)|
|= +1.1 volts|
From a cell diagram it is possible to predict which way a chemical reaction will go.
In the above cell the overall chemical reaction is:
If the e.m.f. of the cell is positive as drawn, then in general, in a cell
the equation for the reaction is:
Calculate the e.m.f. of the cell:
Write the overall chemical equation for the reaction.
|e.m.f.||= EAg - ENi|
|= +0.80 - (-0.25)|
As the e.m.f. is positive the reactants are Ni(s) and Ag+(aq) and the products are Ni2+(aq) and Ag(s).
If the members of a half-cell are in the same phase they are separated only by a comma. An inert electrode (Pt) is shown next to the more reduced species in the half cell in which it is used.
Electrode potential charts provide a useful way of displaying and using data. They can be used to make predictions about the direction of a particularredox reaction. In constructing electrode potential charts:
Problem. What reaction will occur between the MnO4- / Mn2+ system and the Fe3+ / Fe2+system? Write an equation for the overall reaction.
Hence MnO4- reacts with Fe2+.
NB. The use of electrode potentials tells you what is feasible. In practice the reaction may not take place because the activation energy for thereaction is too high. Hence it tells you nothing about the rate of the reaction.
The electrode potentials refer to standard conditions. If a reaction is not feasible under standard conditions, it may be possible to change theconditions which will alter the values of the electrode potentials. The reaction may then take place.