How is nitric acid prepared by Ostwald’s process?

**Preparation of Nitric Acid by Ostwald’s Process**

The **Ostwald process** is an industrial method used for the large-scale production of **nitric acid (HNO₃)**, starting from **ammonia (NH₃)**. This process was developed by the German chemist **Wilhelm Ostwald** in the early 20th century. It involves several oxidation reactions of ammonia and is widely used because of its efficiency.

Here is a step-by-step explanation of the process:

### 1. **Catalytic Oxidation of Ammonia**
Ammonia is first oxidized to form **nitric oxide (NO)** in the presence of a catalyst, typically platinum or platinum-rhodium gauze. This reaction is highly exothermic.

\[
4NH₃(g) + 5O₂(g) \xrightarrow{\text{Pt or Pt-Rh catalyst at 800-900°C}} 4NO(g) + 6H₂O(g) + \text{Heat}
\]

### 2. **Oxidation of Nitric Oxide**
The nitric oxide (NO) formed in the first step is then further oxidized by oxygen in the air to form **nitrogen dioxide (NO₂)**.

\[
2NO(g) + O₂(g) \rightarrow 2NO₂(g)
\]

### 3. **Absorption of Nitrogen Dioxide in Water**
The nitrogen dioxide (NO₂) is then absorbed in water, where it reacts to form **nitric acid (HNO₃)** and **nitric oxide (NO)**. The nitric oxide formed in this step is recycled back into the process for further oxidation.

\[
3NO₂(g) + H₂O(l) \rightarrow 2HNO₃(aq) + NO(g)
\]

### Summary of the Process:
1. **Oxidation of ammonia**: Ammonia is converted into nitric oxide (NO).
2. **Oxidation of nitric oxide**: Nitric oxide is converted into nitrogen dioxide (NO₂).
3. **Absorption**: Nitrogen dioxide is dissolved in water to produce nitric acid (HNO₃).

The final product is typically a solution of nitric acid with a concentration of about 68%. Higher concentrations can be obtained by distillation or by dehydration using sulfuric acid.

This process is essential in the production of fertilizers, explosives, and many other chemical products.