With half-cell reaction: ½ Br2(aq) + e– → Br– (aq) The standard electrode potentials are very important and we can extract a lot of useful information from them. The values of standard electrode potentials for some selected half-cell reduction reactions are given in Table 3.1. If the standard electrode potential of an electrode is greater than zero then its reduced form is more stable compared to hydrogen gas. Similarly, if the standard electrode potential is negative then hydrogen gas is more stable than the reduced form of the species. It can be seen that the standard electrode potential for fluorine is the highest in the Table indicating that fluorine gas (F2) has the maximum tendency to get reduced to fluoride ions (F–) and therefore fluorine gas is the strongest oxidising agent and fluoride ion is the weakest reducing agent. Lithium has the lowest electrode potential indicating that lithium ion is the weakest oxidising agent while lithium metal is the most powerful reducing agent in an aqueous solution. It may be seen that as we go from top to bottom in Table 3.1 the standard electrode potential decreases and with this, decreases the oxidising power of the species on the left and increases the reducing power of the species on the right hand side of the reaction. Electrochemical cells are extensively used for determining the pH of solutions, solubility product, equilibrium constant and other thermodynamic properties and for potentiometric titrations.
Intext Questions 3.1 How would you determine the standard electrode potential of the system Mg 2+|Mg? 3.2 Can you store copper sulphate solutions in a zinc pot? 3.3 Consult the table of standard electrode potentials and suggest three substances that can oxidise ferrous ions under suitable conditions.
3 . 3 Nernst Equation
We have assumed in the previous section that the concentration of all the species involved in the electrode reaction is unity. This need not be always true. Nernst showed that for the electrode reaction: Mn+(aq) + ne– → M(s) the electrode potential at any concentration measured with respect to standard hydrogen electrode can be represented by: E(
Mn+ / M)
= E V( n + ) – M /M
RT [M] ln n+ nF [M ]
but concentration of solid M is taken as unity and we have
1 V RT E( n+ ) = E ( n + ) – ln n+ M /M M /M nF [M ]
(3.8)
V
E( n+ ) has already been defined, R is gas constant (8.314 JK–1 mol–1 ), M /M F is Faraday constant (96487 C mol–1), T is temperature in kelvin and [Mn+] is the concentration of the species, Mn+. Chemistry
68
2015-16(20/01/2015)