Editing Ostwald process (section)

==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}}<ref>{{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 }}</ref> in the presence of a [[catalysis|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 reaction|exothermic]], making it a useful heat source once initiated:<ref name="jones1">{{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]
}}</ref>
:{{chem2|4NH3 + 5O2 -> 4NO  + 6H2O}}  (Δ''H'' = −905.2 kJ/mol)

====Side reactions====
A number of side reactions compete with the formation of nitric oxide. Some reactions convert the ammonia to N<sub>2</sub>, 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.<ref>{{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
}}</ref>
Another side reaction produces [[nitrous oxide]]:
:{{chem2|4NH3 + 4O2 -> 2N2O + 6H2O}}  (Δ''H'' = −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]].<ref name=":02">{{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}}</ref> 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.<ref name=":02" />

=== 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 (NO<sub>2</sub>)<ref>{{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}}</ref> by the reaction:

{{chem2|2NO + O2 -> 2NO2}} (Δ''H'' = -114.2 kJ/mol)<ref>{{Cite journal |last1=Grande |first1=Carlos A. |last2=Andreassen |first2=Kari Anne |last3=Cavka |first3=Jasmina H. |last4=Waller |first4=David |last5=Lorentsen |first5=Odd-Arne |last6=Øien |first6=Halvor |last7=Zander |first7=Hans-Jörg |last8=Poulston |first8=Stephen |last9=García |first9=Sonia |last10=Modeshia |first10=Deena |date=2018-08-08 |title=Process Intensification in Nitric Acid Plants by Catalytic Oxidation of Nitric Oxide |journal=Industrial & Engineering Chemistry Research |language=en |volume=57 |issue=31 |pages=10180–10186 |doi=10.1021/acs.iecr.8b01483 |issn=0888-5885|doi-access=free }}</ref> 

The reaction:

{{Chem2|2NO2 -> N2O4}}  (Δ''H'' = -57.2 kJ/mol)<ref>{{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}}</ref>

also occurs once the nitrogen dioxide has formed.<ref name=":0">{{Citation |last1=Liu |first1=Yunda |title=Static and dynamic simulation of NOx absorption tower based on a hybrid kinetic-equilibrium reaction model |date=2014-01-01 |work=Computer Aided Chemical Engineering |volume=34 |pages=363–368 |editor-last=Eden |editor-first=Mario R. |url=https://www.sciencedirect.com/science/article/pii/B9780444634337500456 |access-date=2024-05-24 |series=Proceedings of the 8 International Conference on Foundations of Computer-Aided Process Design |publisher=Elsevier |doi=10.1016/b978-0-444-63433-7.50045-6 |last2=Bluck |first2=David |last3=Brana-Mulero |first3=Francisco |isbn=978-0-444-63433-7 |editor2-last=Siirola |editor2-first=John D. |editor3-last=Towler |editor3-first=Gavin P.}}</ref>

===Conversion of nitric oxide ===
Stage two encompasses the absorption of nitrous oxides in water and is carried out in an [[absorption (chemistry)|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 [[redox|reducing]] a portion of it back to nitric oxide:<ref name="jones1" />
:{{chem2|3NO2 + H2O -> 2HNO3 + NO}} (Δ''H'' = −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,<ref name=":0" /> 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}} (Δ''H'' = −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}}  (Δ''H'' = −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}}  (Δ''H'' = −370.3 kJ/mol)