Electrochemical Principle of Metallurgy

Just as thermodynamic principles guide the choice of a chemical reducing agent (Section 1.4), ELECTROCHEMICAL principles find their own applications in metallurgical processes -- and become essential specifically for the most reactive metals. The reduction of the oxides of active metals such as sodium and potassium by carbon is thermodynamically NOT feasible (their oxide-formation lines sit far too low on the Ellingham diagram for carbon's line ever to cross below them at any practically reachable temperature); such metals must instead be extracted from their ores by ELECTROCHEMICAL methods. In this technique the metal salt is taken either in a FUSED (molten) form or in solution, and the metal ion present is reduced either by treating it with a suitable chemical reducing agent or, more commonly for the most reactive metals, by ELECTROLYSIS.

The Gibbs free energy change for an electrolytic reduction process is given by ΔG0 = -nFE0, where n is the number of electrons transferred in the reduction, F is the Faraday constant, and E0 is the standard electrode potential of the redox couple concerned. Since ΔG0 = -nFE0, a POSITIVE E0 makes ΔG0 NEGATIVE, i.e. the reduction is spontaneous -- so, in designing an electrochemical extraction, the overall redox reaction is deliberately set up so that the net e.m.f. (electromotive force) of the combined reaction comes out positive. …