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
Ammonia is converted to nitric acid in 2 stages.
Stage 1
The Ostwald process begins with anhydrous
ammonia. Ammonia burns in
oxygen at temperature about 900 °C (1,650 °F) and pressure up to 8 standard atmospheres (810 kPa)[4] in the presence of a
catalyst such as
platinum gauze with 10%
rhodium, platinum metal on fused silica wool, copper or nickel,[5] to form
nitric oxide (nitrogen(II) oxide) and
water (as steam). This reaction is strongly
exothermic, making it a useful heat source once initiated:[6]
(ΔH = −905.2 kJ/mol)
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:
This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.[7]
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).[6] 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:[6]
(ΔH = −114 kJ/mol)
(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:
(ΔH = −348 kJ/mol).[In Absorption Tower].
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:
(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.
^GB 190200698,
Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", published January 9, 1902, issued March 20, 1902
^GB 190208300,
Ostwald, Wilhelm, "Improvements in and relating to the Manufacture of Nitric Acid and Oxides of Nitrogen", published December 18, 1902, issued February 26, 1903
^Kroneck, Peter M. H.; Torres, Martha E. Sosa (2014). The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Dordrecht: Springer. p. 215.
ISBN978-94-017-9268-4.