Ostwald process
{{Short description|Chemical process for producing nitric acid}}
The Ostwald process is a chemical process used for making nitric acid (HNO3).{{cite encyclopedia |author=Thiemann, Michael |author2=Scheibler, Erich |author3=Wiegand, Karl Wilhelm |year=2005|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley-VCH|place=Weinheim|doi=10.1002/14356007.a17_293|chapter=Nitric Acid, Nitrous Acid, and Nitrogen Oxides|isbn=978-3-527-30673-2}} 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.{{Cite book |last1=Kroneck |first1=Peter M. H. |title=The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment |last2=Torres |first2=Martha E. Sosa |publisher=Springer |year=2014 |isbn=978-94-017-9268-4 |location=Dordrecht |pages=215 |language=en}} Historically and practically, the Ostwald process is closely associated with the Haber process, which provides the requisite raw material, ammonia (NH3). This method is preferred over other methods of nitric acid production, in that it is less expensive and more efficient.{{Cite web |title=Ostwald Process |url=https://unacademy.com/content/upsc/study-material/chemistry/ostwald-process/ |access-date=2024-09-05 |website=Unacademy |language=en-US}}
Reactions
Ammonia is converted to nitric acid in 2 stages.
= Initial oxidation of ammonia=
The Ostwald process begins with burning ammonia. Ammonia burns in oxygen at temperature about {{convert|900|°C}} and pressure up to {{convert|8|atm}}{{Cite book |editor=Considine, Douglas M. |title=Chemical and process technology encyclopedia |year=1974 |publisher=McGraw-Hill |location=New York |isbn=978-0-07-012423-3 |pages=[https://archive.org/details/chemicalprocesst00newy/page/769 769–72] |url=https://archive.org/details/chemicalprocesst00newy/page/769 }} in the presence of a catalyst such as platinum gauze, alloyed with 10% rhodium to increase its strength and nitric oxide yield, platinum metal on fused silica wool, copper or nickel to form nitric oxide (nitrogen(II) oxide) and water (as steam). This reaction is strongly exothermic, making it a useful heat source once initiated:{{cite book
| title = Access to chemistry
| url = https://archive.org/details/accesstochemistr00jone
| url-access = limited
| author1 = Alan V. Jones
| author2 = M. Clemmet
| author3 = A. Higton
| author4 = E. Golding
| editor = Alan V. Jones
| publisher = Royal Society of Chemistry
| year = 1999
| isbn = 0-85404-564-3
| page = [https://archive.org/details/accesstochemistr00jone/page/n273 250]
}}
:{{chem2|4NH3 + 5O2 -> 4NO + 6H2O}} (ΔH = −905.2 kJ/mol)
==Side reactions==
A number of side reactions compete with the formation of nitric oxide. Some reactions convert the ammonia to N2, such as:
:{{chem2|4NH3 + 6NO -> 5N2 + 6H2O}}
This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.{{cite book
| title = Inorganic Colloid Chemistry -: The Colloidal Elements
| author = Harry Boyer Weiser
| publisher = Read Books
| year = 2007
| isbn = 978-1-4067-1303-9
| page = 254
}}
Another side reaction produces nitrous oxide:
:{{chem2|4NH3 + 4O2 -> 2N2O + 6H2O}} (ΔH = −1105 kJ/mol)
== Platinum-rhodium catalyst ==
The platinum and rhodium catalyst is frequently replaced due to decomposition as a result of the extreme conditions which it operates under, leading to a form of degradation called cauliflowering.{{Cite journal |last1=Hannevold |first1=Lenka |last2=Nilsen |first2=Ola |last3=Kjekshus |first3=Arne |last4=Fjellvåg |first4=Helmer |date=2005-04-28 |title=Reconstruction of platinum–rhodium catalysts during oxidation of ammonia |url=https://www.sciencedirect.com/science/article/pii/S0926860X05000669 |journal=Applied Catalysis A: General |volume=284 |issue=1 |pages=163–176 |doi=10.1016/j.apcata.2005.01.033 |bibcode=2005AppCA.284..163H |issn=0926-860X|url-access=subscription }} The exact mechanism of this process is unknown, the main theories being physical degradation by hydrogen atoms penetrating the platinum-rhodium lattice, or by metal atom transport from the centre of the metal to the surface.
= Secondary oxidation =
The nitric oxide (NO) formed in the prior catalysed reaction is then cooled down from around 900˚C to roughly 250˚C to be further oxidised to nitrogen dioxide (NO2){{cite web|author=Afolayan Ayodele S |date=7 December 2007 |title=Design of a Plant to Produce 20,000 Litres per Day of Nitric Acid From Ammonia and Air (Using Oswald Process) |website=Repository Futminna |access-date=24 May 2024 |url=http://repository.futminna.edu.ng:8080/jspui/bitstream/123456789/24229/1/OCRENGR0183891.pdf}} by the reaction:
The reaction:
{{Chem2|2NO2 -> N2O4}} (ΔH = -57.2 kJ/mol){{Cite web |date=24 May 2024 |title=21.1 The Effect of Temperature on the NO2/N2O4 Equilibrium |url=https://chemed.chem.purdue.edu/genchem/demosheets/21.1.html |access-date=24 May 2024 |website=chemed.chem.purdue.edu}}
=Conversion of nitric oxide =
Stage two encompasses the absorption of nitrous oxides in water and is carried out in an absorption apparatus, a plate column containing water.{{Cn|date=May 2024}} This gas is then readily absorbed by the water, yielding the desired product (nitric acid in a dilute form), while reducing a portion of it back to nitric oxide:
:{{chem2|3NO2 + H2O -> 2HNO3 + NO}} (ΔH = −117 kJ/mol)
The NO is recycled, and the acid is concentrated to the required strength by distillation.
This is only one of over 40 absorption reactions of nitrous oxides recorded, with other common reactions including:
:{{Chem2|3N2O4 + 2H2O -> 4HNO3 + 2NO}}
And, if the last step is carried out in air:
:{{chem2|4NO2 + O2 + 2H2O -> 4HNO3}} (ΔH = −348 kJ/mol).
= 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:
:{{chem2|2NH3 + 4O2 + H2O -> 3H2O + 2HNO3}} (ΔH = −740.6 kJ/mol)
Alternatively, if the last step is carried out in the air, the overall reaction is the sum of equation 1, 2 times equation 2, and equation 4; all divided by 2.
Without considering the state of the water,
:{{chem2|NH3 + 2O2 -> H2O + HNO3}} (ΔH = −370.3 kJ/mol)
History
{{Expand section|date=May 2024}}
Wilhelm Ostwald developed the process, and he patented it in 1902.{{cite patent |inventor-last=Ostwald |inventor-first=Wilhelm |inventorlink=Wilhelm Ostwald |title=Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides |country-code=GB |patent-number=190200698 |description=|issue-date=March 20, 1902 |publication-date=January 9, 1902}}{{cite patent |inventor-last=Ostwald |inventor-first=Wilhelm |inventorlink=Wilhelm Ostwald |title=Improvements in and relating to the Manufacture of Nitric Acid and Oxides of Nitrogen |country-code=GB |patent-number=190208300 |description= |issue-date=February 26, 1903 |publication-date=December 18, 1902}}
See also
References
{{reflist|30em}}
External links
- [http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch10/group5.php Nitrogen & Phosphorus] (General Chemistry course), [http://chemed.chem.purdue.edu Purdue University]
- Drake, G; [http://www.fertiliser-society.org/Proceedings/UK/Prc78.HTM "Processes for the Manufacture of Nitric Acid"] (1963), [http://www.fertiliser-society.org International Fertiliser Society] (paysite/password)
- [http://digipac.ca/chemical/mtom/contents/chapter3/fritzhaber_2.htm Manufacturing Nitrates: the Ostwald process] Carlton Comprehensive High School; Prince Albert; Saskatchewan, Canada.