Page 33 - Battery Reference Book

1/18 Introduction to battery technology

According to the equations derived above, the poten- It should be remembered that the standard potential
tial of any electrode is determined by the standard refers to the condition in which all the substances in
potential E:l, and by the activity or activities of the the cell are in their standard states of unit activity.
ions taking part in the electrode process. These activi- Gases such as hydrogen, oxygen and chlorine are thus
ties are variable, but the standard potential is a definite at 1 atm. pressure. With bromine and iodine, however,
property of the electrode system, having a constant the standard states are chosen as the pure liquid and
value at a given temperature. If these standard poten- solid, respectively; the solutions are therefore saturated
tials were known, it would be a simple matter to with these elements in the standard electrodes. For all
calculate the actual potential of any electrode, in a ions the standard state of unit activity is taken as the
solution of given concentration or activity, by using hypothetical ideal solution of unit molality or, in other
the appropriate form of Equation 1.34. The standard words, a solution for which the product my is unity,
potentials of many electrodes have been determined, where rn is the molality of the ion and y its activity
with varying degrees of accuracy, and the results coefficient.
have been tabulated. The principle of the method The standard reduction potentials, corresponding to
used to evaluate E:l for a given electrode system is the oxidation potentials in Table 1.2 but involving
the reverse electrode processes, would be obtained by
to measure the potential E of the electrode, on the reversing the sign in each case; thus, for example, for
hydrogen scale, in a solution of known activity; from the zinc electrode,
these two quantities the standard potential EZ1 can
be calculated at the experimental temperature, using Zn, Zn2+ EEl = 1-0.761 V Zn = Zn2+ + 2e
Equation 1.34. Actually the procedure is more com-
plicated than this, because the activities are uncer- Zn2+, Zn E:l = -0.761 V Zn2' + 2e = Zn
tain. The results obtained for the standard oxidation whereas, for the chlorine electrode,
potentials of some electrodes at 25°C are recorded in
Table 1.2; the appropriate electrode process is given in
each case.
Cl-, C12(g). Pt EZl = +1.358V $Clz(g) +e = C1-

Table 1.2 Standard oxidation potentials at 25°C on the hydrogen
scale
1.8 Activities of electrolyte solutions
Electrode Reaction E:, The use of activities instead of concentrations in the
types of thermodynamic calculations dealing with cells
K, K+ K+K++e +2.924 is of great significance. The extensive use of the activ-
Na, Na+ Na --f Na+ +e +2.714 ity term has been seen in the preceding equations.
zn, Zn2+ zn + Zn2+ + 2e f0.761 For an ideal solution, activity equals the concentra-
Fe, Fez+ Fe + Fez+ + 2e f0.441 tions of dissolved electrolytes. Very few solutions,
Cd, Cd2+ Cd + Cd2+ + 2e +0.402 in fact, behave ideally, although in some cases very
co, co2+ co + CO'+ + 2e +0.283 dilute solutions approach ideal behaviour. By defini-
Ni, Ni2+ Ni + Ni2+ + 2e +0.236 tion, however, cell electrolytes are not dilute and hence
Sn, Sn2+ Sn + Sn2+ + 2e +0.140 it is necessary when carrying out thermodynamic cal-
Pb, Pb2+ Pb + Pb" + 2e +0.126
culations to use activities rather than concentrations.
Pt, &Hz, H+ &Hz+H++e fO.OOO Most electrolytes consist of a solute dissolved in a
R, Sn2+, sn4+ Snz+ + sn4+ + 2e -0.15 solvent, commonly water, although, in some types of
Pt, cu+, cu2+ CU+ + CU*+ + e -0.16 cell, solutions of various substances in organic solvents
Ag, AgCl(s), C1- Ag + C1- + AgCl + e -0.2224 are used. When a solute is dissolved in a liquid, the
cu, cu2+ cu + CU*+ + 2e -0.340 vapour pressure of the latter is lowered. The quanti-
Pt, Fe(CN):-. Fe(CN)g- Fe(CN):- + Fe(CN)g- + e -0.356 tative connection between the lowering of the vapour
Pt, 02, OH- 20H-+ &Oz+HzO+2e -0.401 pressure and the composition of a solution was dis-
covered by F. M. Raoult. If po is the vapour pressure
Pt, MS), 1- 1- + 412 +e -0.536 of pure solvent at a particular temperature, and p is
Pt, Fez+, Fe3+ Fez+ + Fe3+ + e -0.771 the vapour pressure of the solution at the same tem-
Ag, Ag+ Ag + Ag' +e -0.799 perature, the difference po - p is the lowering of the
Hg, HgZf Hg --t $H& +e -0.799 vapour pressure. If this is divided by po the result,
Pt, Hg:+, Hg2+ Hg;+ + 2Hg2+ + 2e -0.906 (po - p)/po, is known as the relative lowering of the
Pt, Br3(1), Br Br- + $Brz + e vipouS pressure for the given solution. According to
one form of Raoult's law, the relative lowering of the
Pt, CMg), c1- C1- -+ &Clz + e -1.358 vapour pressure is equal to the mole fraction of the
pt, ce3+, ce4+ ce3+ + ce4+ + e -1.61
solute in the solution. If 1z1 and n2 are the numbers