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Thermodynamics of materials

Which statement correctly describes the line for the carbon reaction 2C(s) + O₂(g) → 2CO(g) in an Ellingham diagram?

  1. ALike C + O₂ → CO₂, ΔS° is nearly 0, so the line is horizontal
  2. BLike metal oxidation, ΔS° is negative, so the line slopes upward and carbon becomes a weaker reducing agent at higher temperature
  3. CThe gas increases from 1 mol to 2 mol, so ΔS° is positive, the line slopes downward, and carbon becomes a stronger reducing agent at higher temperature
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Answer: C. The gas increases from 1 mol to 2 mol, so ΔS° is positive, the line slopes downward, and carbon becomes a stronger reducing agent at higher temperature

In 2C + O₂ → 2CO, the gas increases from 1 mol of O₂ to 2 mol of CO, so ΔS° is positive (about +0.18 kJ/(mol·K)) and the ΔG° line slopes downward. As a result, at higher temperatures the CO line lies below the lines of many metal oxides, and carbon becomes able to reduce those oxides. This is why iron ore can be reduced with coke in a blast furnace. In C + O₂ → CO₂ the number of gas molecules does not change and ΔS° is nearly 0, so it is this reaction whose line is nearly horizontal. An upward slope is the case for metal oxidation, in which gaseous O₂ is taken into a solid.

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More Thermodynamics of materials questions

  1. In an Ellingham diagram, the formation reactions of all oxides are drawn with ΔG° normalized per mol of O₂. What is the main advantage of doing so?
  2. At 1000℃, take ΔG° for 4/3 Al + O₂ → 2/3 Al₂O₃ as −850 kJ/mol and ΔG° for 4/3 Cr + O₂ → 2/3 Cr₂O₃ as −530 kJ/mol (both per mol of O₂). Which combination of the ΔG° of 2/3 Cr₂O₃ + 4/3 Al → 2/3 Al₂O₃ + 4/3 Cr and the direction in which the reaction proceeds is correct? Assume all substances are pure.
  3. At 1000 K, ΔG° for 2Ni(s) + O₂(g) → 2NiO(s) is −298 kJ/mol. Which value is closest to the equilibrium oxygen partial pressure at which Ni and NiO coexist? Treat Ni and NiO as pure solids (activity 1), and take the standard pressure as 1 bar, the gas constant as 8.314 J/(mol·K), and ln 10 = 2.303.
  4. At 1000 K, take ΔG° for 2H₂(g) + O₂(g) → 2H₂O(g) as −385 kJ/mol and ΔG° for 2Ni(s) + O₂(g) → 2NiO(s) as −298 kJ/mol. When nickel is annealed in an H₂–H₂O gas mixture, which value is closest to the partial pressure ratio pH₂O/pH₂ at which Ni and NiO are in equilibrium (Ni is not oxidized if the ratio is smaller than this)? Take the gas constant as 8.314 J/(mol·K), and treat Ni and NiO as pure solids.
  5. Which statement about the activity of a component in a solution (a liquid or a solid solution) is correct?