The key difference between nitric acid and nitrous acid is that nitric acid molecule contains three oxygen atoms bound to a central nitrogen atom whereas nitrous acid molecule contains two oxygen atoms bound to a central nitrogen atom.
Nitric and nitrous acids are inorganic acids of nitrogen. Both these acids contain nitrogen, oxygen and hydrogen atoms.
CONTENTS
1. Overview and Key Difference
2. What is Nitric Acid
3. What is Nitrous Acid
4. Side by Side Comparison – Nitric Acid vs Nitrous Acid in Tabular Form
5. Summary
What is Nitric Acid?
Nitric acid has the chemical formula HNO3. It is a very corrosive and dangerous acid. Moreover, it can have either a dilute or concentrated chemical nature. Either way, it has nitric acid molecules dissolved in water. The reaction between nitrogen dioxide and water forms nitric acid. There are two types of nitric acid: fuming nitric acid and concentrated nitric acid.
Figure 01: Resonance Structures of Nitric Acid Molecule
Fuming nitric acid is a commercial grade of nitric acid that has a very high concentration and a high density. It contains 90-99% HNO3. We can prepare this liquid by adding excessive nitrogen dioxide to nitric acid. It forms a colourless, yellowish or brownish fuming liquid that is highly corrosive. Therefore, this acid solution has gaseous molecules in combination with water; there is no water in it. The fume of this acid rises off the surface of the acid; this leads to its name, “fuming”. The chemical formula of this compound is HNO3-xNO2.
Concentrated nitric acid is simply a solution containing more nitric acid in less water. That means the concentrated form of this acid contains a less amount of water compared to the amount of solutes in it. In commercial scale, 68% or upwards is considered as concentrated nitric acid. Moreover, the density of this solution is 1.35 g/cm3. This much concentration does not produce fumes, but the very high concentration of this acid may give off-white coloured fumes. We can produce this liquid by reacting nitrogen dioxide with water.
What is Nitrous Acid?
Nitrous acid is an inorganic acid having the chemical formula HNO2. It is a weak acid and a monoprotic acid. This acid occurs in the solution state, in the gas phase, and in the form of nitrile salt. This acid is useful in making diazonium salts which are reagents in azo coupling reactions to give azo dyes.
Figure 02: Structure of Nitrous Acid
Nitrous acid solutions appear in pale blue colour. The conjugate base of this acid is nitrile ion. In its gas phase, nitrous acid is in the planar geometry, and it can adopt both cis and trans forms. The trans isomer predominates at room temperature, and it is stable than the cis isomer.
Nitrous acid can be prepared by the acidification of aqueous solutions of sodium nitrite with a mineral acid. We can conduct the acidification process at ice temperatures, and the HNO2 is consumed in situ conditions. Free nitrous acid molecules are unstable and tend to decompose rapidly. Moreover, we can produce nitrous acid via dissolving dinitrogen trioxide in water.
What is the Difference Between Nitric Acid and Nitrous Acid?
Nitric acid and nitrous acid are inorganic acids containing nitrogen atoms. The key difference between nitric acid and nitrous acid is that the nitric acid molecule contains three oxygen atoms bound to a central nitrogen atom whereas nitrous acid molecule contains two oxygen atoms bound to a central nitrogen atom.
Moreover, an easily identifiable difference between nitric acid and nitrous acid is that the nitric acid is a colourless, pale-yellow, or red fuming liquid while nitrous acid is a pale-blue colour solution. In addition to that, nitric acid is a strong acid than nitrous acid.
Below infographic tabulates the differences between nitric acid and nitrous acid.
Summary – Nitric Acid vs Nitrous Acid
Nitric acid and nitrous acid are inorganic acids containing nitrogen atoms. The key difference between nitric acid and nitrous acid is that the nitric acid molecule contains three oxygen atoms bound to a central nitrogen atom whereas nitrous acid molecule contains two oxygen atoms bound to a central nitrogen atom.
Reference:
1. Nitrous Acid. (n.d.). ScienceDirect. Retrieved October 13, 2020, Available here.
Image Courtesy:
1. “Nitric-acid-resonance-A” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
2. “Nitrous acid acsv” By Calvero – Own work (Public Domain) via Commons Wikimedia