Using the interactive Ellingham diagram tool
- Displaying reactions
The diagram consists of two screens. On the first screen, you can select the reactions you wish to include on the Ellingham diagram. First of all, select the system or systems you want to investigate. This will be dictated by the gaseous phases present in the system. Position the cursor over the name of each type of compound to highlight the elements for which data is available.
Then you can select the metals involved in the reactions. In order to keep the diagram simple you may select up to two elements at one time, along with carbon and hydrogen.
Press the “see Ellingham diagram” button to proceed to the second screen, where the selected reactions are displayed on the Ellingham diagram.
On the second screen, the standard free energy of each reaction, in kJ mol-1 of reacting gas, is plotted as a function of temperature in Kelvin. Each selected metal may have several reactions, or none, associated with each gaseous phase.
A change in gradient of any line may be associated with a phase change, either melting or boiling of the metal which is indicated by an m or b at the point, or melting or boiling of the resulting compound, indicated by (m) or (b).
The temperature range over which data is available for each line is shown in purple. Each line is extrapolated to the absolute zero of temperature (thin green line) for simple comparison of the standard enthalpy changes for different reactions.
- Obtaining precise reaction data
In order to obtain specific data for a particular reaction, its line on the diagram may be selected by clicking anywhere along its length. The selected line is shown in brown and a crosshair appears at one end. Moving the slider bar along the x-axis of the diagram can move this crosshair to the temperature of interest, and an accurate readout of the standard free energy of the selected reaction is given at the chosen temperature.
Data including the equilibrium constant for the reaction, the equilibrium partial pressure of the reacting gas and the equation of the line in the form A + BT is also given. These values are obtained by simple operations on the standard free energy value - see section on Partial pressure of reacting gas.
A different line may be selected by simply clicking anywhere along its length.
- Non-standard reactions
If the metal is not in its standard state, the free energy change for the reaction changes, along with the partial pressure of reacting gas at equilibrium. Entering the activity of the metal in the blue-coloured box and pressing the “Compute” button can find non-standard reaction data at the selected temperature.
- Gas mixtures
If the oxidation reactions of carbon or hydrogen are selected, the Ellingham lines for each shown in black or red respectively, the ratio of the pressures of the gases CO and CO2, or the gases H2 and H2O is given as well as the pressure of O2 gas.
The diagram can be restarted, so that different reactions can be seen, by pressing the “Restart” button.
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