Ellingham Diagram: Complete Analysis

What the Ellingham Diagram Is

A graphical plot of standard Gibbs free energy of oxide formation ( $\Delta G$ °) vs temperature for the reaction: $2xM + O_2 \rightarrow 2MxO$

Axes

X-axis: Temperature (K or °C)
Y-axis: $\Delta G$ ° of oxide formation (kJ/mol O2)

Line Slopes and Entropy

Reaction type	$\Delta S$	Slope
$2M + O_2 \rightarrow 2MO$ (solid oxide)	Negative (consumes O2 gas)	↑ Upward
$C + O_2 \rightarrow CO_2$ (gas→gas, equal moles)	≈ 0	→ Horizontal
$2C + O_2 \rightarrow 2CO$ (extra gas produced)	Positive	↓ Downward

Why 2C + O2 → 2CO Slopes Downward (Most Tested)

$2C(s) + O_2(g) \rightarrow 2CO(g)$

Moles of gas: 1 mol O2 consumed → 2 mol CO produced. Net gain = +1 mol gas. $\Delta S > 0 \Rightarrow \text{as T} \uparrow, \; T\Delta S \uparrow \Rightarrow \Delta G = \Delta H - T\Delta S \text{ becomes more negative}$

Consequence: The C→CO line crosses below metal oxide lines at high temperatures, enabling carbon to reduce those metal oxides in blast furnaces.

Ellingham Reduction Rule

A metal/element whose oxide line lies LOWER (more negative $\Delta G$ °) can reduce the oxide of a metal whose line is HIGHER.

Trend Table: Stability of Metal Oxides (Approximate Ranking, Most → Least Stable)

Metal	Oxide Stability	Position on Ellingham ( $\Delta G$ ° per mol O2)
Ca	Very high	Very low (most negative, ~−1200 kJ)
Al	Very high	Low (~−1000 kJ)
Zn	Moderate-high	Middle
Fe	Moderate	Middle-upper
Ni	Moderate	Upper-middle
Cu	Low	Near top (~−250 kJ)
Hg	Very low	Very top (barely negative/positive)
Au	Essentially zero/positive	Positive (doesn't form stable oxide)

Trend: As you go down the activity series (less reactive metals), oxide stability ↓, $\Delta G$ ° line rises on Ellingham diagram.