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.what is Ellingham diagrams .please explain its concept easily?

Asked by Albert Mathew Mathew 8th July 2013, 8:52 PM
Answered by Expert
?G = ?H T ?S
The enthalpy (?H) is a measure of the actual energy that is liberated when the reaction occurs (the heat of reaction). If it is negative, then the reaction gives off energy, while if it is positive the reaction requires energy. The entropy (?S) is a measure of the change in the possibilities for disorder in the products compared to the reactants
An Ellingham diagram is a plot of ?G versus temperature.
Since ?H and ?S are essentially constant with temperature unless a phase change occurs.
The free energy versus temperature plot can be drawn as a series of straight lines, where ?S is the slope and ?H is the y-intercept. The slope of the line changes when any of the materials involved melt or vaporize.
  • Free energy of formation is negative for most metal oxides, and so the diagram is drawn with ?G=0 at the top of the diagram, and the values of ?G shown are all negative numbers. 
  • Temperatures where either the metal or oxide melt or vaporize are marked on the diagram
  • The Ellingham diagram shown is for metals reacting to form oxides
  • The majority of the lines slope upwards, because both the metal and the oxide are present as condensed phases (solid or liquid). 
  • The reactions are therefore reacting a gas with a condensed phase to make another condensed phase, which reduces the entropy.
  •  A notable exception to this is the oxidation of solid carbon. The line for the reaction C+O2? CO2 is a solid reacting with a mole of gas to produce a mole of gas, and so there is little change in entropy and the line is nearly horizontal. 
  • For the reaction 2C+O? 2CO we have a solid reacting with a gas to produce two moles of gas, and so there is a substantial increase in entropy and the line slopes rather sharply downward. 
  • Similar behavior can be seen in parts of the lines for lead and lithium, both of which have oxides that boil at slightly lower temperatures than the metal does.
Answered by Expert 8th July 2013, 11:28 PM
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