ELLINGA ELLINGAM DIAGRAMS M DIAGRAMS

Ellingham diagrams are plots of Δf Go with respect to T for reactions, like formation of oxides, sulfides etc. of various elements.

The main application of Ellingham diagram is in the metallurgy industry, where it helps to select the best reducing agent for various ores in the extraction process.

For a particular reaction , ΔH and ΔS remain almost constant even on changing temperature.

In general, the plot of ΔG vs T, is a straight line with a positive slope.

In Ellingam diagram ,an oxide at a lower position is more stable than an oxide at a higher position.A reaction for which ΔG is -ve ,can be made spontaneous by coupling it with another reaction ,which has more –ve ΔG value.We can use Ellingam diagram to select a suitable reducing agent for extraction of a metal from its ore.Any element, whose plot is at a lower position in Ellingam diagram ,can reduce the oxide of the element, which occupies a higher position.Larger the difference in ΔG values for the two reactions coupled together ,easier is the reduction reaction.

In some cases , there is an abrupt change in the slope, this is due to change in phase (solid → liquid ) or ( liquid → gaseous ).

Metal/Oxide

Melting pointoC

Boiling pointoC

Al 660 2467

Cu 1083 2567

Cu2O 1201 2000

Fe 1535 2750

FeO 1377 3414

Mg 650 1107

MgO 2852 3600

Zn 419.5 907

ZnO 1975 2360

Limitations of Ellingam DiagramIt does not say anything about the kinetics

of the reduction process ie how fast or how slow the process will be.

Interpretation of ΔG is based on equilibrium constant

ΔG = -RTlnKIt is presumed that reactants and products are at equilibrium ,which is not always true, as reactants and products may be solids.

QUESTIONS1. Under which condition (a)can Mg reduce Al2O3

(b)can Al reduce FeO?2. Out of C or CO, which one is a better reducing agent at

673K? why?3. Why Zn is not extracted from ZnO using CO as reducing

agent?4. ΔG of Cr2O3 is -540kj/mol and ΔG of Al2O3 is -827kj/molIs reduction of Cr2O3 possible with Al?5. Out of C or CO,which one is a better reducing agent for ZnO? Why?

ANSWERS1.

i. Below 1350oC Mg can reduce Al2O3

ii. Above 1350oC Al can reduce MgO.2. CO is a better reducing agent at 673K,because Δf

Go for reaction 2CO+O2=2CO2Is more –ve than Δf Go

for the reaction 2C+O2=2CO or C+O2=CO2

3. The reaction may require very high temperature.4. Yes5. Carbon. Since Δf Go C,CO is more –ve and the

graph lies at a lower position than that for (Zn,ZnO)in Ellingam Diagram.

Thank you