The changes in Gibbs energy that occur when one moleofoxygen isused may be plotted against temperature for a number of reaction of metals to form
their oxides. Such a graph is shown in Figure below and is called an Ellingham diagram for oxides. Similarly, we can plot Ellingham diagrams for halides.
(1) Ellingham diagram normallyconsist of plots of dfG° vs T for the formation of oxides of elements, i.e. for the reaction.
(2) The graphs for metal oxide all slope upwards because the change in Gibbs energybecomes less negative with increase in temperature.
(3) Each plot follows a straight line unless there is some change in phase.
(4) When the temperature is increased, a point will be reached when the line crosses dG = 0 line. Below this temperature the dfG° of oxide is negative and hence the oxide is stable.Above this temperature dfG° of the oxide is positive and hence the oxide becomes unstable and decomposes on its own into metal and oxygen.
Chemical refining methods:
(2) Parting Process:
(6) Vapour phase refining:
