Question

When does the slope of an Ellingham line increase and when does it decrease? Consider (i) a phase change occurs with increasing temperature on the reactants side (ii) a phase change occurs with increasing temperature on the products side

Answer 1

Figure tive To Line 3 M M Line 2 Linet -ive Line 1. M Temperature M = Melting point of element IM - Melting point of oxide B = Boiling point of element 18 - Boiling point of oxile

Slopes of Ellingham lines determined by the value of the entropy change for the transformation. Since the entropy changes markedly at the melting and the boiling points of a metal as well as of its oxides, a change in the slope of the line in the Ellingham diagram may be anticipated at these temperatures. The various possibilities are shown in Figure . When the metalundergoes a phase change with rising temperature, its entropy Figure Effect of the sign of AH° and AS on Ellingham lines. Table (1) Entropy change scenario. Process Entropy change Remarks Metal melting M() +0.5 02 (G) MO (3) -S1 M(S) M (1) +S2 M (1) + 0.5 02 G). MO (s) S=-S1 - S2 More negative Oxide melting M (s) +0.5 02 (G) — MO (3) -S1 MO (s) → MO(1) +S2 M(s) + 0.5 02 (G) — MO (1) S=-S1 - S2 Less negative increases by an amount equal to the enthalpy of the phase change divided by the absolute temperature at which the change occurs. When the phase change occurs in the reactants, augmentation of the entropy of the reactants results in the slope

(m=-ASO = S - Sºp) becoming a larger positive value. Line 1 represents a typical case of phase transformation in reactant. If, on the other hand, there is a phase transformation in the product the slope becomes a smaller positive value because here the entropy of the product is augmented (S becomes more and m becomes a smaller positive value or even a negative value). In Figure (2), line 3 represents a typical case of phase transformation in the product. As regards the phase change, boiling results in a far greater augmentation of entropy than melting. Thus in the figure, sharper changes in slope occur at points marked B and B than at points marked M and M. Line 2 illustrates the special case where there is almost no entropy change associated with the oxidation reaction and where no phase transformation of the metal or its oxide occurs in the range of temperatures considered. Line 4 in the diagram pertains to another special situation which arises when oxidation causes an increase in entropy. For such reactions, Asº is positive and the slope of the Ellingham line is negative. The oxide becomes stabler as the temperature is increased. Typical examples of "line 1” behaviour are provided by magnesium, calcium, and zinc. The classic example of “line 2" behaviour is exhibited by carbon dioxide. However, this type of behaviour is not exhibited by any metal. The oxides of metals like lead and cadmium are examples of "line 3" behaviour. "Line 4” behaviour is exhibited by carbon monoxide, and volatile suboxides of several refractory metals. Figure Slopes of Ellingham lines.