In Ellingham diagrams plots of `Delta_(f)G` (oxide formation) show positive slope except for the formation of `CO(g)` from coke. Why ?

Explanation

In the following reaction ,
`underset("Coke")(2C(s))+underset(1"mol")(O_(2)(g))to underset(2"mol")(2CO(g))`
`DeltaH` is negative (exothermic reaction) and `DeltaS` is positive (moles of gasesous substance increasing). The value of `DeltaG` (which is, `DeltaH-TDeltaS`) becomes more and more negative by increasing temperature, The slop of graph becomes negative.
In all other cases, `O_(2)` gas is consumed to produce solid metal oxides. Here, entropy decreases (becomes negative). `TDetlaS` dominates `DeltaH` and the value of `DeltaG` increases constantly, thereby increasing the slop of the graph.