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Ostwald Process

Last updated date: 16th Jul 2024
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Understanding the Ostwald Process

The Ostwald process is considered as one of the most chemical Processes or common methods used for the manufacturing of Nitric Acid. The Process of Ostwald was developed in the year 1902 by a German chemist named Wilhelm Ostwald. In 1909 He was later awarded the Nobel prize for his research. Over the years this process has gained popularity as it is the easiest way to produce Nitric Acid which is also widely used in many areas such as in the production of fertilizers as well as inorganic and organic Nitrates or Nitro compounds.In the Ostwald Process, Usually, ammonia is transformed into nitric acid. Vanadium pentoxide which is denoted by V2O5 is often used as a catalyst during the Processing of nitric acid in this method. There are many steps involved in this process. We shall discuss them.

What is the Ostwald Process? 

The Ostwald process is defined as a chemical Process used for making nitric acid which is written as HNO3. Wilhelm Ostwald developed this Process, and in 1902 he patented it. This Process is a mainstay of the modern chemical industry, and it also provides the main raw material for the most common type of fertilizer production in the world. If we look at it historically and practically, then we can see that this Process is closely associated with the Haber Process, which provides the requisite raw material, Ammonia.

History of Ostwald Process

Before the method of the Ostwald Process, industrial nitrogen was the nitrates that were treated with sulphuric acid to produce nitrogen. The Ostwald Process in addition to Haber’s Process that is used for fixing nitrogen was essentially used for World War I. Later the method was adopted for producing fertilizers since the nitrogen consumption increased. Ostwald patented and profited the Process later in 1902. The introduction of stainless steel promoted higher yield by enhancing and supporting higher pressure. 

Description of the Process

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The Process is divided into two stages:

Stage 1 

In this stage Ammonia is converted to nitric acid in 2 particular stages. By heating it is oxidized with oxygen in the presence of a catalyst such as platinum with 10% rhodium, which fuses the platinum metal on silica wool, copper or nickel, to form nitric oxide(nitrogen(II) oxide and water as steam. This reaction is considered as strongly exothermic Process, making it a useful heat source once initiated:

4NH3(g) + 5O2(g) → 4NO(g) + 6H2O (g)

ΔH = −905.2 kJ/mol

Stage 2

In this Stage two there encompasses two reactions and is carried out in an absorption apparatus which contains water. Nitric oxide is oxidized initially again to yield nitrogen dioxide which is nitrogen(IV) oxide.This gas is readily absorbed then by the water, by yielding the desired product nitric acid, albeit in a dilute form, while reducing a portion of it back to nitric oxide as shown below:

2NO(g) + O2(g) → 2NO2(g)

ΔH = −114 kJ/mol

3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g)

ΔH = −117 kJ/mol

The NO or nitric oxide is recycled, and the acid is concentrated to the required strength by the distillation Process.

If the last step is Alternatively carried out in air:

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

ΔH = −348 kJ/mol

Stage 3 

This step is established by the absorption of nitrogen dioxide. The oxygen and nitrogen dioxide that are present in the air react with water to form nitric acid. The nitrogen dioxide that is present in the secondary oxidation chamber has to be introduced within the absorption tower. When nitrogen dioxide is passed through the tower, water is sprinkled and thus the nitric acid is obtained. 

The nitric acid that is obtained within the tower is dilute in nature which is why it is recycled through the tower again and again so that more NO2 is absorbed and after the Process of recycling, the nitric acid becomes 68 percent concentrated. 

Step 4

For the purpose of concentrating the HNO3 even further, the vapours of nitric acid are passed over concentrated H2SO4 that is a dehydrating agent that absorbs the water from nitric acid, thus resulting in concentrated HNO3

First phase typical conditions, which overall contribute to a yield of about 98%, are:

In standard atmospheres pressure is between 4–10 (410–1,000 kPa; 59–150 psi) and

The temperature is around 870–1,073 K (600–800 °C; 1,100–1,500 °F). A complication that is to be considered is that it involves a side-reaction in the first step that reverts the nitric oxide back to nitrogen is shown below:

4NH3 + 6NO → 5N2 + 6H2O

This is considered as a secondary reaction which is minimised by reducing the time the gas mixtures are in contact with the catalyst. Looking at the Overall reaction first equations overall reaction is the sum of the, 3 times the second equation, and 2 times the last equation; and at the end, all divided by 2:

2NH3(g) + 4O2(g) + H2O(l) → 3H2O(g) + 2HNO3(aq)

ΔH = −740.6 kJ/mol

In the air, if alternatively, if the last step is carried out, then the overall reaction is the sum of equation 1, and 2 times the equation 2, and equation 4;at the end again all divided by 2, Without considering the state of water,

NH3(g) + 2O2(g) → H2O + HNO3(aq)

ΔH = −370.3 kJ/mol


Before getting into the different steps, we can quickly understand the principle or mechanism behind this Process. The ammonia conversion to nitric acid simply occurs as a result of an oxidation reaction. This oxidation reaction gives us the corresponding nitric oxide which we need. Further, when nitric acid oxidation is oxidized nitrous gases are formed and those gases can trap water molecules too. We obtain  As we obtain nitric acid. Catalytic oxidation involves O2 and is used where ammonia will give rise to the desired product. While the process is carried out, there are particular reaction chambers where ammonia is fed from one direction and air through various different paths. Possibilities are there of side reactions occurring as well. If we go to subsequent ammonia the oxidation Process will get some other reactions. Usually, it happens in the case of dinitrogen. Ammonia is created by the removal of dinitrogen. If we try to oxidize the ammonia, dinitrogen will be given back, there can also be other oxidized forms as well. In all these, optimizing the reaction condition becomes very important, otherwise, many gases can get together and form with the desired NO. Therefore It is, it is necessary to avoid side reactions. Moving on, the next stage involves the oxidation of NO2 which can also dimerize to give an end result as N2O4. This reaction in the stage is only favoured at low temperatures. Meanwhile, this process which is known as the Ostwald Process is also closely related to Born Haber's Processor cycle.

Primary Oxidation Explained

A catalytic chamber is used for the oxidation of ammonia and the catalyst that is used for this Process is platinum gauze and the temperature of this chamber is 600 degrees. Oxidation of nitric acid is reversible as well as exothermic. A decrease in temperature can also lead to furthering the reaction as explained by Le Chatelier’s principle. In primary oxidation, 95 per cent of ammonia is regenerated into nitric acid.  

The nitric oxide gas that is obtained by the oxidation of NH3 is hot and in order to reduce the temperature, it is then passed via a heat exchanger where the temperature of NO is reduced to 150 degrees celsius. 

Secondary Oxidation Explained 

NO after cooling down is then transferred to another oxidizing tower where it is oxidized to NO2 at around 50 degrees celsius. 

Importance of Haber’s Process in Ostwald Process

The commercial synthesis of ammonia which is a raw material used in the Ostwald Process is called Haber’s Process. This Process overcomes the difficulty in breaking down the energy in nitrogen and hydrogen with the help of ammonia thus helping in the Ostwald Process as ammonia is the feedstock for this reaction.  

FAQs on Ostwald Process

1. What catalyst is used in the Ostwald Process?

Ammonia is converted to nitric acid in two stages. It is oxidized by heating with oxygen in the presence of a catalyst which is platinum that has to be mixed with 10 per cent rhodium fused with copper or nickel to form nitric oxide. Vanadium oxide catalyst is also used in the contact process. 

2. Explain the Ostwald Process.

The volume of dry ammonia and dry air in the ratio 10:1 is mixed, compressed and then passed into a platinum gauze which is the catalyst to the reaction at a temperature of 800 degrees celsius. The nitric oxide is oxidized to form nitric dioxide. The nitric dioxide is absorbed in the water to react with oxygen and air in the water to result in nitric acid. 

3. Why is the Ostwald Process considered important?

The Ostwald Process was initially important because the Germans needed it during the first world war for the purposes of explosives since it can also be used for producing explosives on a large scale.  The ammonium nitrate that is produced in this process is used for crops and lawns as fertilizer. 

4. What is the formula of Nitric Acid?

In this, the nitrogen atom is bonded by an equivalent bond to a hydroxyl group and the remaining oxygen atoms. It serves a role as a reagent and protic solvent. It is also the conjugate acid of nitrate. Nitric acid molecule consists of 3 oxygen atoms, 1 nitrogen atom and 1 hydrogen atom. One oxygen atom in the nitric acid molecule is doubly bonded to the nitrogen atom while the other oxygen atom is bonded to the hydrogen atom and central nitrogen atom. In the nitric acid molecule, the last oxygen atom has a charge of -1 that is singly bonded to the central nitrogen atom. The nitrogen atom at the centre of the molecule is is a part of four covalent bonds, it has a charge of +1 which results in the net charge of a nitric acid molecule being 0 since the negative charge and the positive charge of the oxygen atom and nitrogen atom respectively cancel each other. It can be delocalized by resonance. 

5. What is the principle behind the Ostwald Process? 

The conversion of ammonia to nitric acid is a result of oxidation. This oxidation results in nitric oxide. Furthermore, nitric oxide can be oxidized to result in nitrous gasses that trap molecules of water. The catalytic oxidation including oxygen is used where ammonia will result in the product that will provide nitric acid. Optimizing the conditions of the reaction is very essential or else many other gasses can be formed with NO. It is important to avoid side reactions. Oxidation of NO2 can also be dimerized to give N2O4 and this reaction is only favourable in lower temperatures.