Chemistry:Ostwald process
The Ostwald process is a chemical process used for making nitric acid (HNO3). Wilhelm Ostwald developed the process, and he patented it in 1902.[1][2] The Ostwald process is a mainstay of the modern chemical industry, and it provides the main raw material for the most common type of fertilizer production.[3] Historically and practically, the Ostwald process is closely associated with the Haber process, which provides the requisite raw material, ammonia (NH3).
Description of the Ostwald Process
Stage 1
Ammonia is converted to nitric acid in 2 stages. It is oxidized by heating with oxygen in the presence of a catalyst such as platinum with 10% rhodium, platinum metal on fused silica wool, copper or nickel,[4] to form nitric oxide (nitrogen(II) oxide) and water (as steam). This reaction is strongly exothermic, making it a useful heat source once initiated:[5]
- [math]\ce{ 4 NH3 (g) + 5 O2 (g) -> 4 NO (g) + 6 H2O (g) }[/math] (ΔH = −905.2 kJ/mol)
Stage 2
Stage two encompasses two reactions and is carried out in an absorption apparatus containing water. Initially nitric oxide is oxidized again to yield nitrogen dioxide (nitrogen(IV) oxide).[5] This gas is then readily absorbed by the water, yielding the desired product (nitric acid, albeit in a dilute form), while reducing a portion of it back to nitric oxide:[5]
- [math]\ce{ 2 NO (g) + O2 (g) -> 2 NO2 (g) }[/math] (ΔH = −114 kJ/mol)
- [math]\ce{ 3 NO2 (g) + H2O (l) -> 2 HNO3 (aq) + NO (g) }[/math] (ΔH = −117 kJ/mol)
The NO is recycled, and the acid is concentrated to the required strength by distillation.
And, if the last step is carried out in air:
- [math]\ce{ 4 NO2 (g) + O2 (g) + 2 H2O (l) -> 4 HNO3 (aq) }[/math] (ΔH = −348 kJ/mol).[In Absorption Tower].
Typical conditions for the first stage, which contribute to an overall yield of about 98%, are:
- pressure is between 4–10 standard atmospheres (410–1,000 kPa; 59–150 psi) and
- temperature is about 870–1,173 K (600–900 °C; 1,100–1,700 °F).
A complication that needs to be taken into consideration is a side-reaction in the first step that reverts the nitric oxide back to nitrogen:
- [math]\ce{ 4 NH3 + 6 NO -> 5 N2 + 6 H2O }[/math]
This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.[6]
Overall reaction
The overall reaction is the sum of the first equation, 3 times the second equation, and 2 times the last equation; all divided by 2:
- [math]\ce{ 2 NH3 (g) + 4 O2 (g) + H2O (l) -> 3 H2O (g) + 2 HNO3 (aq) }[/math] (ΔH = −740.6 kJ/mol)
Alternatively, if the last step is carried out in air, the overall reaction is the sum of equation 1, 2 times the equation 2, and equation 4; all divided by 2.
Without considering the state of water,
- [math]\ce{ NH3 (g) + 2 O2 (g) -> H2O + HNO3 (aq) }[/math] (ΔH = −370.3 kJ/mol)
References
- ↑ Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", GB patent 190200698, published January 9, 1902, issued March 20, 1902
- ↑ Ostwald, Wilhelm, "Improvements in and relating to the Manufacture of Nitric Acid and Oxides of Nitrogen", GB patent 190208300, published December 18, 1902, issued February 26, 1903
- ↑ Kroneck, Peter M. H.; Torres, Martha E. Sosa (2014) (in en). The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Dordrecht: Springer. pp. 215. ISBN 978-94-017-9268-4.
- ↑ Foist, Laura. "The Ostwald Process & Catalytic Oxidation of Ammonia". https://study.com/academy/lesson/the-ostwald-process-catalytic-oxidation-of-ammonia.html.
- ↑ 5.0 5.1 5.2 Alan V. Jones; M. Clemmet; A. Higton; E. Golding (1999). Alan V. Jones. ed. Access to chemistry. Royal Society of Chemistry. p. 250. ISBN 0-85404-564-3. https://archive.org/details/accesstochemistr00jone.
- ↑ Harry Boyer Weiser (2007). Inorganic Colloid Chemistry -: The Colloidal Elements. Read Books. p. 254. ISBN 978-1-4067-1303-9.
External links
- Nitrogen & Phosphorus (General Chemistry course), Purdue University
- Drake, G; "Processes for the Manufacture of Nitric Acid" (1963), International Fertiliser Society (paysite/password)
- Manufacturing Nitrates: the Ostwald process Carlton Comprehensive High School; Prince Albert; Saskatchewan, Canada.
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