electrode potential, or simply the potential of the reaction.
If we could measure the potential of two separate half-cell reactions:
we could determine the energy gain/loss in the transfer of an electron from an individual element. Unfortunately, such measurements are not possible (nor would these reactions occur in the natural environment: electrons are not given up except to another element or species§). This requires the establishment of an arbitrary reference value. Once such a reference value is established, the potential involved in reactions such as 3.102 can be established.
3.11.1.1 Hydrogen scale potential, EH
The established convention is to measure potentials in a standard hydrogen electrode cell (at standard temperature and pressure). The cell consists on one side of a platinum plate coated with fine Pt powder that is surrounded by H2 gas maintained at a partial pressure of 1 atm and immersed in a solution of unit H+ activity. The other side consists of the electrode and solution under investigation. A potential of 0 is assigned to the half-cell reaction:
(3.104)
where the subscript g denotes the gas phase. The potential measured for the entire reaction is then assigned to the half-cell reaction of interest. Thus, for example, the potential of the reaction:
is –0.763 V. This value is assigned to the reaction:
(3.105)
Table 3.3 EH° and pε° for some half-cell reactions.
| Half-cell reaction | EH° (V) | pε° |
| Li+ + e– ⇌ Li | −3.05 | −51.58 |
| Ca2+ + 2 e– ⇌ Ca | −2.93 | −49.55 |
| Th4+ + 4e– ⇌ Th | −1.83 | −30.95 |
| U4+ + 4e– ⇌ U | −1.38 | −23.34 |
| Mn2+ +2e– ⇌ Mn | −1.18 | −19.95 |
| Zn2+ + 2e– ⇌ Zn | −0.76 | −12.85 |
| Cr3+ +3e– ⇌ Cr | −0.74 | −12.51 |
| CO2(g) + 4H+ + 4e– ⇌ CH2O*+2H2O | −0.71 | −12.01 |
| Fe2+ + 2e– ⇌ Fe | −0.44 | −7.44 |
| Eu3+ + e– ⇌ Eu2+ | −0.36 | −6.08 |
| Ni2+ + 2e– ⇌ Ni | −0.26 | −4.34 |
| Pb2+ + 2e– ⇌ Pb | −0.13 | −2.2 |
| CrO42− + 4H2O +3e– ⇌ Cr(OH)3 + H2O | −0.13 | −2.2 |
| 2H+ + 2e– ⇌ H2(g) | 0 | 0 |
| N2(g) + 6H+ + 6e– ⇌ 2NH3 | 0.093 | 1.58 |
| Cu2+ + 2e– ⇌ Cu | 0.34 | 5.75 |
| UO22+ + 2e– ⇌ UO2 | 0.41 | 6.85 |
| S + 2e– ⇌ S2− | 0.44 | 7.44 |
| Cu+ + e– ⇌ Cu | 0.52 | 8.79 |
| Fe3+ + e– ⇌ Fe2+ | 0.77 | 13.02 |
| NO3+ + 2H+ + e– ⇌ NO2(g) + H2O | 0.80 | 13.53 |
| Ag+ + e– ⇌ Ag | 0.80 | 13.53 |
| Hg2+ + 2e– ⇌ Hg | 0.85 | 14.37 |
| MnO2(s) + 4H+ + 2e– ⇌ Mn2+ + 2H2O | 1.22 | 20.63 |
| O2 + 4H+ + 4e– ⇌ 2H2O | 1.23 | 20.80 |
| MnO4– + 8H+ + 5e– ⇌ Mn2+ + 4H2O | 1.51 | 25.53 |
| Au+ + e– ⇌ Au | 1.69 | 28.58 |
| Ce4+ + e– ⇌ Ce3+ | 1.72 | 29.05 |
| Pt+ + e– ⇌ Pt | 2.64 | 44.64 |
*
