Electrode potentials involving atoms and ions of metals in intermediate oxidation states in aqueous solutions

Abstract

The electrode potentials of metal atoms (M+/M0 and M2+/M0 couples) were calculated from their standard electrode potentials in water minus the standard molar Gibbs energy of formation of the atoms in the gas phase, ΔfG0(Mgas0), and the Gibbs energy of hydration in solution, ΔhydG°. Electrode potentials are appreciably shifted (by 2.0–2.5 V) toward negative values relative to the potentials of the solid metals. For the univalent metals Cu0, Ag0, and Au0 (d10s1 electronic configuration), the electrode potentials of M+/M0 couples are –2.73, –1.90, and –1.82 V; for Ga0, In0, and Tl0 (d10s2p1 electronic configuration), they are –3.14, –2.45, and –1.99 V, respectively. For bivalent transition metals of period IV with the d5+ns2 configuration, the potentials of M2+/M0 couples are –2.50 V for Mn0, –2.45 V for Fe0, –2.33 V for Co0, –2.34 for Ni0, –1.29 V for Cu0, and –1.34 V for Zn0. For Cd0 and Hg0 (d10s2 configuration), the potentials are –0.87 and + 0.42 V. Estimation of the electrode potentials of bivalent metals in the intermediate and unstable oxidation states (M2+/M+ and M+/M0 couples) revealed their significant difference. In the case of Zn, Cd, and Hg, this difference is particularly noticeable, as large as 2–3 V, because of different configurations of the M+ (d10s1) and M0 (d10s2) electronic shells. Regular trends in variations of the electrode potentials in the periods and groups of the periodic table were revealed. In going from the atomic-molecular to the solid state, the chemical activity of the metals decreases. The metal surface catalyzes the reactions that are thermodynamically unfavorable in the solution volume.