Nitrous Acid

What is Nitrous Acid?

While talking about acids, you will hear of acid name HNO2 often. HNO2 is also known as nitrous acid and is an acidic compound, which is unstable and weak. It is only prepared in dilute forms and is usually cold. It is a highly useful compound in converting certain amines to diazonium compounds. The question that you might ask first is "What is the formula of nitrous acid?" The nitrous acid formula in chemistry is HNO2. Its molar mass is 47.013 g/mol. Being a planar molecule implies that the nitrogen atom is attached to oxygen atoms, through single and double bonds. Both of these oxygen molecules are electronegative in nature. Shown below is the structure of nitrous acid HNO2

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Nitrous acid tends to explode when it comes in contact with certain compounds like Phosphorus Trichloride. It is considered to be a powerful oxidizer. Although nitrous acid is not very toxic, it is still known to impact the respiratory health of humans, sometimes also aggravating the symptoms of asthma.

Physical Properties of Nitrous Acid HNO2

Liquid Form: Nitrous acid is only available in its liquid form - either dissolved in water or as a nitrate salt. 

Pale Blue: The nitrous acid solution is pale blue in colour and gets its colour from N2O3, which is a blue solid and acidic in nature.

Density: Nitrous acid HNO2 has a density of about 1 g/mL with its weight of about 47.013 g/mol

Boiling Point: Nitrous Acid boiling point is 158-degree centigrade. 

Chemical Properties of Nitrous Acid HNO2

Monobasic: Since Nitrous acid HNO2 has only one hydrogen ion which it can donate to a base in case of a reaction between an acid and a base, it is known as a monobasic acid. It can release only one proton, i.e., H+ in solution. 

Unstable: Nitrous acid is highly unstable in nature, and so mostly found in its liquid form. It has to be freshly prepared every time it has to be used. 

Weak Acid: Nitrous acid is a weak acid and so does not completely dissociate when in water.

Oxidation Number: Since the oxidation number of the N in HNO2 is 3+, it can act as both an oxidizing agent and a reducing agent.

Nitrous acid oxidizes hydrogen sulphide to form sulphur:


Reaction With Bases: Since nitrous acid is acidic in nature, it reacts with bases to form salts. It usually forms nitrite salts like sodium nitrite. 

2 HNO2 + Na2CO3 → 2 NaNO2 + HCO3

Structure of Nitrous Acid

The formula of nitrous acid is represented by HNO2. Since it is an acid, the hydrogen molecule goes outside of one of the oxygen molecules. So, we can put nitrogen in the centre with two oxygens with Hydrogen outside one of them. If we count the number of electrons, Hydrogen has 1, nitrogen has 5, and each oxygen molecule has 6, making it 12. This means we have a total of 18 electrons. After the nitrous acid structure HNO2 is drawn, we can see all of the atoms have full outer shells except nitrogen. A double bond can be shared between nitrogen and oxygen, which means each atom has a formal charge of 0.

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Nitrous Acid Uses

Nitrous acid is a useful compound in several industries and organic chemistry laboratories to produce other chemicals. For example, nitrous acid reaction with amine can form diazonium salts. When 1 degree amine reacts with nitrous acid, it gives us alkyl diazonium chloride. When diazonium salts are put in water, nitrogen and hydrochloric acid escape, leaving us with primary alcohol. Uses of nitrous acid HNO2 are widely recognized in the field of chemistry. Diazonium salts are also used in the making of certain dyes. 

Decomposition of Nitrous Acid

The nitrous acid chemical formula is HNO2, and there are two primary ways in which nitrous acid can decompose. One method is when nitrous acid decomposes to give us water, nitrogen dioxide and nitric acid. Here is the balanced equation for the same: 

2HNO2 → NO2 + NO + H2O

If you know the molecular formula of nitrous acid, you would also know another way in which nitrous acid can decompose is to give us nitric acid, nitrous oxide and water. Here is the balanced equation for the same:

4HNO2 → 2HNO3 + N2O + H2O

FAQ (Frequently Asked Questions)

1. Is Nitric Acid the Same as Nitrous Acid?

A1. No. Nitric acid and nitrous acid are different acidic compounds. While nitric acid is represented as HNO2, Nitrous acid is represented as HNO3. Nitric acid is also a stronger acid than nitrous acid because the former completely dissociates in the water while a weak acid is in equilibrium with its conjugate base in water. The NO3- ion has excess negative charge, which is actually spread over 3 oxygen atoms while excess negative charge present in NO2- is spread over two oxygen atoms. Nitric acid is used for manufacturing nitrogen-based compounds like nylon, for nitrate salts and dyes, and other drugs.

2. How is Nitrous Acid Prepared?

After acidification of certain aqueous solutions of sodium nitrite, we can prepare nitrous acid. This is usually done at very low temperatures. It reacts either as an oxidizing agent, or a reducing agent. So, its nitrogen atom may either lose or gain electrons in reactions with other substances. Nitrous acid can also be naturally found in the earth's atmosphere. When nitric oxide (NO) reacts with water droplets in the atmosphere, it helps in regulating the ozone content in the atmosphere. Since nitrous acid is unstable, it has to be freshly prepared and in-situ. Here is a common method of preparation of nitrous acid:

NaNo2 + HCl = HNO2 + NaCl