Extraction of metal from oxide = reduction: MxOy → xM + y/2 O2 (Delta G must be negative for feasibility). A metal M1 can reduce M2's oxide if M1's oxide formation has more negative Delta G (M1 is more reactive). The Ellingham diagram plots Delta G of formation of oxides (per mole O2) vs temperature. Key principles: (1) Oxide with lower (more negative) Delta G is more stable — that metal cannot be reduced by the other. (2) Any metal whose line is BELOW can reduce the oxide whose line is ABOVE. (3) Temperature effect: higher temperature generally favours reduction because -TDelta S becomes more significant.