A Study on the Reaction of Nitrous Acid- Decomposition

Nitrous acid is a weak and unstable acid. It has the ability to either donate a proton or form a covalent bond with an electron pair. It is found in the form of salts and esters. It is formed under cold, acidic conditions, and it decomposes to form nitric oxide (NO) and nitric acid (HNO3). The structure of nitrous acid contains two oxygen atoms to a nitrogen atom, one bonded with a single bond and the other with a double bond. In the lower atmosphere, when nitric oxide reacts with water, nitrous acid is formed naturally.

Decomposition of Nitrous acid

Nitrous acid is usually prepared in situ by acidifying amines with hydrochloric acid and then adding sodium or potassium nitrate. During the decomposition of nitrous acid, toxic and hazardous fumes of nitric oxide are produced. 

Generally, after decomposition, the nitrous oxide usually produces nitric acid that is HNO3, nitric oxide (NO) and water.

1. During warm conditions or in concentrated solutions, three molecules of nitrous oxide decompose to form 2 molecules of nitric oxide, nitric acid and water.

                      3HNO2  →   HNO3 +2NO + H2O

2. When 2 molecules of nitrous acid decompose, it gives nitrogen dioxide, nitric oxide and water.

                        2HNO2  →  NO2 +NO + H2O

3. In an aqueous solution, the nitrogen dioxide generated is further reacted with water to form nitric acid and nitrous acid.

                        2NO2 + H2O   →    HNO3 + HNO2

4. When oxidation of nitric oxide occurs, it gives nitric acid.

                         2HNO2 + O2 → 2HNO3

Decomposition Reaction

Decomposition is when a single compound or reactant undergoes a reaction and breaks down into two or simpler compounds. In day-to-day chemistry, decomposition reactions take place very often. Even the food we eat is decomposed into water, heat and water. The polymers like proteins, carbohydrates and fats are broken down into simpler units.

As per chemistry, there are three types of decomposition reactions, and they are thermolysis decomposition, photolysis decomposition and electrolysis decomposition reactions.

1. Thermolysis Decomposition Reaction

When the heat is responsible for breaking down the bonds between the compounds leading to decomposition, then the reaction is called a thermal decomposition reaction.

As heat is involved during the reaction, this reaction is termed an endothermic reaction.

2. Photolysis Decomposition Reaction

This type of decomposition reaction is done in the presence of sunlight. When a reactant breaks down into two or more products due to the light present during the reaction, then it is called a photolysis decomposition reaction. 

3. Electrolysis Decomposition Reaction

When a reaction takes place in an aqueous solution, and an electric current is passed through the solution, then the compound gets decomposed. This reaction is called the electrolysis decomposition reaction.

• Double decomposition Reaction

The double decomposition takes place when one part of the compound is broken down or changed to form two new compounds. It is used in the metal extraction process and thermite welding process.

Examples of Decomposition Reaction

Here are some examples of decomposition reactions-

1. During warm conditions or in concentrated solutions, three molecules of nitrous oxide decompose to form 2 molecules of nitric oxide, nitric acid and water.

3HNO2   →  HNO3 +2NO + H2O

  2. Calcium Carbonate Decomposition Reaction where the calcium is decomposed to form quicklime (calcium oxide) and CO2.

                                      heat

                  CaCO3(s)      →       CaO(s) + CO2

 3) Electrolysis decomposition of water- Here, as electric current is passed through the aqueous solution, the water is split into hydrogen and oxygen.

2H2O (l)   →  2H2 (g) + O2 (g)

4) Silver Chloride Decomposition Reaction is when silver chloride is kept open to airand light, then it starts to turn grey as it has decomposed itself into silver

2AgCl(s) → 2Ag(s) + Cl2 (g)

5) Metal oxides and carbon dioxide are formed when the carbonates undergo decomposition

         CaCO3 (s) →   CaO (s) + CO2 (g) 

          Na2CO3(s)  →  Na2O (s) +CO2 (g)

Conclusion

Nitrous acid is an unstable acid that can act as both oxidising and reducing agents. It is used in various laboratory practices, which include the identification of primary, secondary and tertiary amines. It is used in the reaction with diazonium salts which in turn is used in the manufacturing of azo dyes in the dye industry. Nitric oxide and nitrogen dioxide are generated after nitrous acid’s decomposition.

Decomposition reactions are the reactions that we experience in our day-to-day life, including food digestion and treating our wounds which is usually done using hydrogen peroxide. The decomposition of organic waste or other materials with the help of digestive enzymes etc. We humans rely on the decomposition process for renewal and healthy life.