TheOstwald process is achemical process used for makingnitric acid (HNO3).[1] The Ostwald process is a mainstay of the modernchemical industry, and it provides the main raw material for the most common type of fertilizer production.[2] Historically and practically, the Ostwald process is closely associated with theHaber 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.[3]
Ammonia is converted to nitric acid in 2 stages.
The Ostwald process begins with burningammonia. Ammonia burns inoxygen at temperature about 900 °C (1,650 °F) and pressure up to 8 standard atmospheres (810 kPa)[4] in the presence of acatalyst such asplatinum gauze, alloyed with 10%rhodium to increase its strength and nitric oxide yield, platinum metal on fused silica wool, copper or nickel to formnitric oxide (nitrogen(II) oxide) andwater (as steam). This reaction is stronglyexothermic, making it a useful heat source once initiated:[5]
A number of side reactions compete with the formation of nitric oxide. Some reactions convert the ammonia to N2, such as:
This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.[6]Another side reaction producesnitrous oxide:
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 calledcauliflowering.[7] 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.[7]
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)[8] by the reaction:
2NO + O2 → 2NO2 (ΔH = -114.2 kJ/mol)[9]
The reaction:
2NO2 → N2O4 (ΔH = -57.2 kJ/mol)[10]
also occurs once the nitrogen dioxide has formed.[11]
Stage two encompasses the absorption of nitrous oxides in water and is carried out in anabsorption apparatus, aplate column containing water.[citation needed] This gas is then readily absorbed by the water, yielding the desired product (nitric acid in a dilute form), whilereducing a portion of it back to nitric oxide:[5]
The NO is recycled, and the acid is concentrated to the required strength bydistillation.
This is only one of over 40 absorption reactions of nitrous oxides recorded,[11] with other common reactions including:
And, if the last step is carried out in air:
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:
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,
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Wilhelm Ostwald developed the process, and he patented it in 1902.[12][13]