Producing Nitric Acid

Producing nitric acid: The Ostwald process

The Ostwald process is used to produce nitric acid from ammonia. Nitric acid can then be used in reactions that produce fertilizers. Ammonia is converted to nitric acid in a three-step process.

Firstly ammonia is oxidised by heating it with oxygen, in the presence of a platinum (\(\text{Pt}\)) catalyst, to form nitrogen monoxide (\(\text{NO}\)) and water. This step is strongly exothermic, which makes it a useful heat source. The reaction that takes place is:

\(4\text{NH}_{3}(\text{g}) + 5\text{O}_{2}(\text{g})\) \(\to\) \(4\text{NO}(\text{g}) + 6\text{H}_{2}\text{O}(\text{g})\)

In the second step, nitrogen monoxide is oxidised again to yield nitrogen dioxide (\(\text{NO}_{2}\)) according to the following reaction:

\(2\text{NO}(\text{g}) + \text{O}_{2}(\text{g})\) \(\to\) \(2\text{NO}_{2}(\text{g})\)

In step three, nitrogen dioxide is absorbed by water to produce nitric acid (\(\text{HNO}_{3}\)) as follows:

\(3\text{NO}_{2}(\text{g}) + \text{H}_{2}\text{O}(\text{l})\) \(\to\) \(2\text{HNO}_{3}(\text{aq}) + \text{NO}(\text{g})\)

Nitrogen monoxide, also known as nitrogen oxide or nitric oxide, is a by-product of this reaction. The nitrogen monoxide is recycled and the acid is concentrated to the required strength (e.g., for use in further chemical processes).