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Oscar Lanzi
Oscar Lanzi
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Whether the libeslines are exactly concurrent depends on how the data were obtained. In the case of carbon oxidation, if we draw on measured thermodynamic data for oxidizing $\ce{CO}$ to $\ce{CO2}$ and also for oxidizing $\ce{C}$ to $\ce{CO2}$, then we could calculate the "data" for oxidizing $\ce{C}$ to $\ce{CO}$ by doubling the first reaction and subtracting the second (assuming all reactions are based on one mole of oxygen, which is standard for Ellingham diagrams), in which case the three lines are forced to be concurrent.

But if we draw on independent measurements for all three reactions and they are all subject to experimental errors, the errors may not combine mathematically in the same way as the theoretical reaction stoichiometries. The error (per mole of oxygen) that occurred when $\ce{C}$ was oxidized to $\ce{CO}$ may not be exactly twice the $\ce{C/CO2}$ error minus the $\ce{CO/CO2}$, in which case the three independently determined free energy lines would also not match up. If all three experiments had a similar amount of experimental error, then the deviation from concurrency in the diagram would provide a rough measure of this error. So having the lines not meet exactly could actually provide an additional bit of information behind the data.

Whether the libes are exactly concurrent depends on how the data were obtained. In the case of carbon oxidation, if we draw on measured thermodynamic data for oxidizing $\ce{CO}$ to $\ce{CO2}$ and also for oxidizing $\ce{C}$ to $\ce{CO2}$, then we could calculate the "data" for oxidizing $\ce{C}$ to $\ce{CO}$ by doubling the first reaction and subtracting the second (assuming all reactions are based on one mole of oxygen, which is standard for Ellingham diagrams), in which case the three lines are forced to be concurrent.

But if we draw on independent measurements for all three reactions and they are all subject to experimental errors, the errors may not combine mathematically in the same way as the theoretical reaction stoichiometries. The error (per mole of oxygen) that occurred when $\ce{C}$ was oxidized to $\ce{CO}$ may not be exactly twice the $\ce{C/CO2}$ error minus the $\ce{CO/CO2}$, in which case the three independently determined free energy lines would also not match up. If all three experiments had a similar amount of experimental error, then the deviation from concurrency in the diagram would provide a rough measure of this error. So having the lines not meet exactly could actually provide an additional bit of information behind the data.

Whether the lines are exactly concurrent depends on how the data were obtained. In the case of carbon oxidation, if we draw on measured thermodynamic data for oxidizing $\ce{CO}$ to $\ce{CO2}$ and also for oxidizing $\ce{C}$ to $\ce{CO2}$, then we could calculate the "data" for oxidizing $\ce{C}$ to $\ce{CO}$ by doubling the first reaction and subtracting the second (assuming all reactions are based on one mole of oxygen, which is standard for Ellingham diagrams), in which case the three lines are forced to be concurrent.

But if we draw on independent measurements for all three reactions and they are all subject to experimental errors, the errors may not combine mathematically in the same way as the theoretical reaction stoichiometries. The error (per mole of oxygen) that occurred when $\ce{C}$ was oxidized to $\ce{CO}$ may not be exactly twice the $\ce{C/CO2}$ error minus the $\ce{CO/CO2}$, in which case the three independently determined free energy lines would also not match up. If all three experiments had a similar amount of experimental error, then the deviation from concurrency in the diagram would provide a rough measure of this error. So having the lines not meet exactly could actually provide an additional bit of information behind the data.

Source Link
Oscar Lanzi
Oscar Lanzi
  • 62.5k
  • 4
  • 96
  • 187

Whether the libes are exactly concurrent depends on how the data were obtained. In the case of carbon oxidation, if we draw on measured thermodynamic data for oxidizing $\ce{CO}$ to $\ce{CO2}$ and also for oxidizing $\ce{C}$ to $\ce{CO2}$, then we could calculate the "data" for oxidizing $\ce{C}$ to $\ce{CO}$ by doubling the first reaction and subtracting the second (assuming all reactions are based on one mole of oxygen, which is standard for Ellingham diagrams), in which case the three lines are forced to be concurrent.

But if we draw on independent measurements for all three reactions and they are all subject to experimental errors, the errors may not combine mathematically in the same way as the theoretical reaction stoichiometries. The error (per mole of oxygen) that occurred when $\ce{C}$ was oxidized to $\ce{CO}$ may not be exactly twice the $\ce{C/CO2}$ error minus the $\ce{CO/CO2}$, in which case the three independently determined free energy lines would also not match up. If all three experiments had a similar amount of experimental error, then the deviation from concurrency in the diagram would provide a rough measure of this error. So having the lines not meet exactly could actually provide an additional bit of information behind the data.