Inert-pair effect: potentials of the N → N−2 couples

Standard potentials for reducing the group oxidation state of groups 13–15 by two (25 °C, acid); at period 6 (Tl, Pb, Bi) they jump above +1.25 V.


$\ce{Ga}$	13	4	+3 → +1	$\ce{Ga^3+ + 2e- -> Ga+}$	−0.724	derived from two CRC potentials
$\ce{In}$	13	5	+3 → +1	$\ce{In^3+ + 2e- -> In+}$	−0.443	CRC electrochemical series
$\ce{Tl}$	13	6	+3 → +1	$\ce{Tl^3+ + 2e- -> Tl+}$	1.252	CRC electrochemical series
$\ce{C}$	14	2	+4 → +2	$\ce{CO2(g) + 2H+ + 2e- -> CO(g) + H2O}$	−0.104	computed from ΔfG°
$\ce{Ge}$	14	4	+4 → +2	$\ce{GeO2 + 2H+ + 2e- -> GeO + H2O}$	−0.118	CRC electrochemical series
$\ce{Sn}$	14	5	+4 → +2	$\ce{SnO2 + 4H+ + 2e- -> Sn^2+ + 2H2O}$	−0.094	CRC electrochemical series
$\ce{Pb}$	14	6	+4 → +2	$\ce{PbO2 + 4H+ + 2e- -> Pb^2+ + 2H2O}$	1.455	CRC (site potential table)
$\ce{P}$	15	3	+5 → +3	$\ce{H3PO4 + 2H+ + 2e- -> H3PO3 + H2O}$	−0.276	CRC electrochemical series
$\ce{As}$	15	4	+5 → +3	$\ce{H3AsO4 + 2H+ + 2e- -> HAsO2 + 2H2O}$	0.560	CRC electrochemical series
$\ce{Sb}$	15	5	+5 → +3	$\ce{Sb2O5 + 6H+ + 4e- -> 2SbO+ + 3H2O}$	0.581	CRC electrochemical series
$\ce{Bi}$	15	6	+5 → +3	$\ce{Bi2O4 + 4H+ + 2e- -> 2BiO+ + 2H2O}$	1.593	CRC electrochemical series

Ga³⁺/Ga⁺ is derived from Ga³⁺/Ga (−0.549 V) and Ga⁺/Ga (−0.2 V) and is uncertain by about 0.1 V. The species compared are not all of one kind (aqua ions or oxides), so only the large jumps are meaningful.

Sources

P. Vanýsek, “Electrochemical series”, CRC Handbook of Chemistry and Physics, 91st ed. (2010), Table 1.
J. D. Cox, D. D. Wagman, V. A. Medvedev, CODATA Key Values for Thermodynamics (1989) (ΔfG° of CO, CO₂ and H₂O for the carbon couple).

Used in these lessons

Boron, aluminium, carbon, silicon and the inert pair