State the important features of Ellingham diagram. Also state its limitations.

(N/A) Ellingham diagram normally consists of plots of $\Delta_{f} G^{\circ}$ $vs$ $T$ for the formation of oxides of common metals and reducing agents,$i.e.$,for the reaction given below:
$2xM_{(s)} + O_{2_{(g)}} \rightarrow 2M_{x}O_{(s)}$
In this reaction,gas is consumed in the formation of oxide; hence,molecular randomness decreases,which leads to a negative value of $\Delta S$. As a result,the sign of the $T\Delta S$ term in the equation becomes positive. Subsequently,$\Delta_{f} G^{\circ}$ shifts towards a higher side despite rising $T$. The result is a positive slope in the curve for most of the reactions for the formation of $M_{x}O_{(s)}$.
$(b)$ Each plot is a straight line and slopes upwards except when some change in phase ($s \rightarrow l$ or $l \rightarrow g$) takes place. The temperature at which such change occurs is indicated by an increase in the slope on the positive side ($e.g.$,in the $Zn, ZnO$ plot,the melting is indicated by an abrupt change in the curve).
$(c)$ When temperature is raised,a point is reached in the curve where it crosses the $\Delta_{r} G^{\circ} = 0$ line. Below this temperature,$\Delta_{r} G^{\circ}$ for the formation of oxide is negative,so $M_{x}O$ is stable. Above this point,the free energy of formation of oxide is positive,and the oxide $M_{x}O$ will decompose on its own.
$(d)$ Similar diagrams are constructed for sulfides and halides also. From them,it becomes clear why the reduction of $M_{x}S$ is difficult.
Limitations of Ellingham Diagram:
$(i)$ The graph simply indicates whether a reaction is possible or not,$i.e.$,the tendency of reduction with a reducing agent is indicated. This is because it is based only on thermodynamic concepts. It does not explain the kinetics of the reduction process. It cannot answer questions like how fast reduction can proceed.
$(ii)$ It is interesting to note that $\Delta H$ (enthalpy change) and $\Delta S$ (entropy change) values for any chemical reaction remain nearly constant even on varying temperature. So,the only dominant variable in the equation $\Delta G = \Delta H - T\Delta S$ becomes $T$. However,$\Delta S$ depends much on the physical state of the compound.
$(iii)$ Since entropy depends on disorder or randomness in the system,it will increase if a compound melts $(s \rightarrow l)$ or vaporizes $(l \rightarrow g)$ since molecular randomness increases on changing the phase from solid to liquid or from liquid to gas.
$(iv)$ The interpretation of $\Delta_{r} G^{\circ}$ is based on $K$ $(\Delta G^{\circ} = -RT \ln K)$. Thus,it is presumed that the reactants and products are in equilibrium: $M_{x}O + A_{red} \rightleftharpoons xM + A_{red}O$. This is not always true because the reactant/product may be solid. In commercial processes,reactants and products are in contact for a short time.