Concept · committed · confidence 0.92
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The reversible gas–solid reaction CO₂ + C ⇌ 2CO, named after the French chemist Octave Boudouard, who studied the equilibrium between carbon monoxide and carbon dioxide over carbon systematically around 1900. The reaction is endothermic (ΔH° ≈ +172 kJ/mol), meaning it is driven forward (toward CO) by increasing temperature and by Le Chatelier’s principle is also favored by decreasing pressure (2 moles of gas produced from 1 mole). At atmospheric pressure the equilibrium crosses from CO₂-dominant to CO-dominant at approximately 700 °C (‘the Boudouard crossover’); above ~900–950 °C the equilibrium strongly favors CO (>95 mol% CO in the gas phase at 1 atm). Below ~400 °C the reverse reaction (2CO → CO₂ + C, the Biot–Stoney reaction or ‘carbon deposition’ reaction) is thermodynamically favored, but is kinetically sluggish at low temperatures. In ironmaking, the Boudouard equilibrium is the central mechanism by which a carbonaceous charge (charcoal or coke) sustains a CO-rich reducing atmosphere: CO₂ produced when CO reduces iron oxides (FeₓOᵧ + CO → Fe + CO₂) is continuously regenerated back to CO by reaction with solid carbon, closing the loop. This self-regenerating atmosphere is what makes carbon-based shaft furnaces (bloomery and blast furnace alike) thermodynamically efficient as iron smelters.
Aliases
- Boudouard equilibrium
- CO2–C equilibrium
- Carbon gasification equilibrium
- CO2 + C ⇌ 2CO
Domain
Physical chemistry / Thermodynamics / Pyrometallurgy
See also
- Direct Reduction of Iron Oxides
- Ellingham diagram
- Bloomery Iron Smelting
- Carbon monoxide
- Carbothermic reduction
Claims
- Decreasing pressure shifts the equilibrium toward CO (Le Chatelier’s principle: 2 mol gas produced from 1 mol gas); increasing pressure favors CO₂. (confidence 0.98)
- In a charcoal- or coke-charged iron smelting furnace, the Boudouard reaction continuously regenerates CO from the CO₂ produced by iron oxide reduction (FeₓOᵧ + CO → Fe + CO₂), creating a self-sustaining reducing atmosphere above ~700 °C. (confidence 0.95)
- The Boudouard equilibrium line appears on the Richardson–Jeffes Ellingham diagram, intersecting the iron oxide reduction lines and visually demonstrating the temperature windows within which CO is both thermodynamically stable and capable of reducing iron oxides. (confidence 0.95)
- The Boudouard reaction (CO₂ + C ⇌ 2CO) was systematically characterized by Octave Boudouard in a series of experiments published in 1900 in the Annales de chimie et de physique. (confidence 0.95)
- At atmospheric pressure (1 atm), the Boudouard equilibrium crosses from CO₂-dominant to CO-dominant at approximately 700 °C; above ~950 °C, CO exceeds 95 mol% in the equilibrium gas phase. (confidence 0.92)
- mol at standard conditions (298 K). (confidence 0.92)
- Below approximately 400 °C, the reverse reaction (2CO → CO₂ + C, sometimes called carbon deposition or the Biot reaction) is thermodynamically favored, but the kinetics are slow enough that carbon deposition from CO at low temperatures is a well-known but slow industrial nuisance rather than a rapid equilibration process. (confidence 0.85)
- mol applies to charcoal (amorphous carbon) vs. graphite (draft) (confidence 0.5) — ⚠ non-blocking verification: JANAF tables are for graphite as the reference state. The enthalpy of amorphous carbon (charcoal) differs slightly from graphite (~1–3 kJ/mol difference in carbon standard enthalpy). For the purposes of ironmaking thermodynamics this difference is negligible, but for precise thermochemical calculations a correction should be applied.
Connections
Incoming
- Prerequisite knowledge ← Bloomery Iron Smelting — To correctly manage a bloomery smelt — specifically to understand why the CO/CO₂ ratio in the furnace atmosphere matters, why tuyere placement and bellows rate control temperature relative to the ~700 °C Boudouard crossover, and why charcoal acts as a reductant and not merely a fuel — one must understand the Boudouard equilibrium. Without this, operators cannot reason about why excess air re-oxidizes iron or why the furnace atmosphere must be maintained above ~700 °C in the reduction zone.
- Prerequisite knowledge ← Direct Reduction of Iron Oxides — Direct Reduction of Iron Oxides is conceptually inseparable from the Boudouard reaction: the CO reductant that drives the iron oxide reduction sequence (Fe₂O₃ → Fe₃O₄ → FeO → Fe) is generated and maintained by the Boudouard equilibrium above ~700 °C. Understanding direct reduction requires knowing both the iron oxide reduction thermodynamics and the carbon–CO–CO₂ equilibrium that regenerates the reductant.
Sources
- boudouard-1900-étude-de-léquilibre-c-co₂-2co · Boudouard, O. (1900) Étude de l’équilibre C + CO₂ = 2CO. Annales de chimie et de physique, 7th series, vol. 19, pp. 153–241.
- chase-1998-nist-janaf-thermochemical-tables · Chase, M.W. Jr. (1998) NIST-JANAF Thermochemical Tables. 4th ed., Journal of Physical and Chemical Reference Data, Monograph No. 9, NIST, Gaithersburg, MD..
- fruehan-1998-the-making-shaping-and-treating-of-steel (draft) · Fruehan, R.J. (ed.) (1998) The Making, Shaping, and Treating of Steel. 9780930767020.
- kubaschewski-1979-metallurgical-thermodynamics (draft) · Kubaschewski, O.; Alcock, C.B. (1979) Metallurgical Thermochemistry. ISBN:0080208975. https://books.google.com/books?id=OXNNAQAAIAAJ
- porter-2009-phase-transformations-in-metals-and-allo (draft) · Porter, D.A.; Easterling, K.E.; Sherif, M.Y. (2009) Phase Transformations in Metals and Alloys.
- turkdogan-1980-physical-chemistry-of-high-temperature-t (draft) · Turkdogan, E.T. (1980) Physical Chemistry of High Temperature Technology. 012704650X.