Trend: Reactivity and Oxide Stability
Trend: ↑ Reactivity → ↑ Oxide stability → ↓ ° (more negative) → Lower line on Ellingham diagram
| Metal | Activity Series Position | Oxide Stability | Extraction Method |
|---|---|---|---|
| Na, K, Ca, Mg | Very reactive (top) | Very stable oxides | Electrolysis of molten salt |
| Al | Reactive | Very stable (Al2O3) | Electrolysis (Hall-Heroult) |
| Zn, Fe | Moderately reactive | Moderately stable | Carbon reduction (high T) |
| Cu, Ag, Au | Least reactive (bottom) | Unstable oxides | Self-reduction / direct |
Periodic Trends Affecting Ellingham Lines
Down a group (e.g., Li → Na → K):
- Reactivity ↑ → oxide stability ↑ → line moves down on Ellingham ↓
- Requires more energy (electrolysis) to extract
Across a period (e.g., Na → Mg → Al → Si):
- Metallic character decreases → oxide stability decreases (Na2O > MgO > Al2O3 > SiO2 [acidic])
- Methods shift: electrolysis → carbon reduction possible for less active metals
Why Carbon Cannot Reduce Al2O3 (Trend-based)
Al is in Period 3, Group 13. Its electronegativity (1.61) is higher than Na (0.93) but lower than C (2.55). However, Al2O3 ° formation is approximately −1580 kJ/mol — far more negative than the C→CO reaction (~−400 kJ/mol at 1000°C). The large energy gap means no thermal process can overcome this.