Ostwald process

The Ostwald process is a chemical process for producing nitric acid, which was developed by Wilhelm Ostwald (patented 1902). It is a mainstay of the modern chemical industry. Historically and practically it is closely associated with the Haber process, which provides the requisite raw material, ammonia.

Description

Ammonia is converted to nitric acid in two stages. It is oxidized (in a sense "burnt") by heating with oxygen in the presence of a catalyst such as platinum with 10% rhodium, to form nitric oxide and water. This step is strongly exothermic, making it a useful heat source once initiated:^[1]

4 NH₃ (g) + 5 O₂ (g) → 4 NO (g) + 6 H₂O (g) (ΔH = −950 kJ/mol)

Stage two (combining two reaction steps) is carried out in the presence of water in an absorption apparatus. Initially nitric oxide is oxidized again to yield nitrogen dioxide:^[1]

2 NO (g) + O₂ (g) → 2 NO₂ (g) (ΔH = −114 kJ/mol)

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:^[1]

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.

Alternatively, if the last step is carried out in air:

4 NO₂ (g) + O₂ (g) + 2 H₂O (l) → 4 HNO₃ (aq)

Typical conditions for the first stage, which contribute to an overall yield of about 96%, are:

• pressure between 4 and 10 atmospheres (approx. 400-1010 kPa or 60-145 psig) and
• temperature is about 1173 K (approx. 900 °C or 1652 °F.).

A complication that needs to be taken into consideration involves a side-reaction in the first step that reverts the nitrogen back to N₂:

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.^[2]

External links

• Physics Daily
• Nitrogen & Phosphorus (General Chemistry course), Purdue University
• Drake, G; "Processes for the Manufacture of Nitric Acid" (1963), International Fertiliser Society
• Manufacturing Nitrates: the Ostwald process Carlton Comprehensive High School; Prince Albert; Saskatchewan, Canada.

References

1. ^ ^{a b c} Alan V. Jones; M. Clemmet; A. Higton; E. Golding (1999). Alan V. Jones. ed. Access to chemistry. Royal Society of Chemistry. p. 250. ISBN 0854045643. 
2. ^ Harry Boyer Weiser (2007). Inorganic Colloid Chemistry -: The Colloidal Elements. Read Books. p. 254. ISBN 1406713031.