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]

${\displaystyle {\ce {4 NH3 (g) + 5 O2 (g) -> 4 NO (g) + 6 H2O (g)))}$ (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]

${\displaystyle {\ce {2 NO (g) + O2 (g) -> 2 NO2 (g)))}$ (ΔH = −114 kJ/mol)

${\displaystyle {\ce {3 NO2 (g) + H2O (l) -> 2 HNO3 (aq) + NO (g)))}$ (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:

${\displaystyle {\ce {4 NO2 (g) + O2 (g) + 2 H2O (l) -> 4 HNO3 (aq)))}$ (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:

${\displaystyle {\ce {4 NH3 + 6 NO -> 5 N2 + 6 H2O))}$

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:

${\displaystyle {\ce {2 NH3 (g) + 4 O2 (g) + H2O (l) -> 3 H2O (g) + 2 HNO3 (aq)))}$ (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,

${\displaystyle {\ce {NH3 (g) + 2 O2 (g) -> H2O + HNO3 (aq)))}$ (ΔH = −370.3 kJ/mol)