Nitric Acid and Nitrous Acid | HNO3, HNO2

Nitric acid and nitrous acid are mono basic oxoacids of nitrogen. But they are very different in chemical and physical properties. Nitric acid is widely used in chemical industry for producing other chemicals. Also nitric acid is used as an strong acid and oxidizing acid in laboratories. Though both are acids, their acidic strength are different.

In this tutorial, we are leaning about characteristics, preparation, differences and some reactions of HNO3 and HNO2 acids.


  • Nitrous acid | Nitrc(iii) acid | HNO2 Structure
    • Physical and chemical properties of HNO2 acid
    • Preparation of nitrous acid
  • Nitric acid | Nitric(v) acid | HNO3
    • Physical properties of nitric acid
    • Preparation of nitric acid
    • Decomposition of nitric acid
  • Difference between nitric acid and nitrous acid
  • Nomenclature of Nitrous acid, Nitric acid, Nitrite ion

When you are dealing with acids such as nitric acid, you should be very careful to avoid any injuries because nitric acid is an oxidizing acid.

Nitrous acid | Nitrc(III) acid | HNO2 Structure

Nitrous acid contains one hydrogen atom, one nitrogen atom and two oxygen atoms. According to the lewis structure of HNO2 acid, nitrogen atom is the center atom and there is one O-H bond, one N-O bond and one N=O bond. Also, there is a lone pair on nitrogen atom.

Physical and chemical properties of HNO2 acid

An unstable acid - Decomposition of nitrous acid

Nitrous acid is not a stable acid as nitric acid. Therefore, Nitrous acid is prepared when it is required because storage is not effective. Nitrous acid (HNO2) decomposes to Nitric acid (HNO3), Nitric oxide (NO) and water (H2O). When temperature of the aqueous Nitrous acid solution is increased, decomposition rate is also increased as a result.

decomposition of nitrous acid to HNO3, NO, H2O

A weak acid

HNO2 does not dissociate completely in the water. Therefore its' aqueous solution has a H+ concentration is very less when it compare with undissociated HNO2 acid concentration.

Colourless aqueous solution

Aqueous nitrous acid solution is colourless.

Oxidation number and redox reactions of HNO2 acid

Nitrous acid can be oxidized because nitrogen atom is at +3 oxidation state. Therefore HNO2 acid can reduce acidic potassium permanganate (H+ / KMnO4).

Preparation of nitrous acid

Due to the instability of nitrous acid, it is not stored in the laboratory. At a specific requirement, we prepare the nitrous acid as below.

Step 1: Heating sodium nitrate (NaNO3) | decomposition of sodium nitrate

Solid sodium nitrate (NaNO3) is heated to decompose to solid sodium nitrite (NaNO2) and oxygen (O2) gas. This reaction is a redox reaction (oxidation - reduction reaction)

sodium nitrate NaNO3 decompose when heating

Step 2: Dissolving of solid sodium nitrite in water

Produced solid sodium nitrite is dissolved in water to prepare aqueous sodium nitrite solution. There is no reaction in this step, only a dissolving.

sodium nitrite dissolving in water

Step 3: Add cold dilute HCl acid solution to aqueous NaNO2 solution to reaction

Add cold dilute HCl acid to aqueous sodium nitrite solution. It gives dilute nitrous acid and NaCl solution.

aqueous NaNO2 and cold dilute HCl reaction

Nitric acid | Nitric(v) acid | HNO3

As we have studied about Nitrous acid, he we will look a brief summary about Nitric acid.

Physical properties of nitric acid

  • A strong acid
  • Boiling point is 860C.
  • Oxidizing acid
  • Concentrated HNO3 has a strong oxidizing capability and diluted HNO3 has a certain ability.
  • Colourless solution. But when nitric acid solution bottles exposed to the sunlight, HNO3 decomposes and produce nitrogen dioxide (NO2). Therefore bottles of nitric in the laboratories are in yellow colour.

Preparation of nitric acid

Concentrated sulfuric acid and sodium nitrate reaction | H2SO4 + NaNO3(s)

Add concentrated H2SO4 acid to the solid NaNO3(s) and distill the mixture.

sodium nitrate and sulfuric acid reaction | preparation of HNO3

In industrial production of HNO3

Ostwald process is used in industry to production of nitric acid. Ammonia and oxygen are used as raw materials in the presence of Pt / Rh catalyst. Higher temperatures and pressure are applied in the production to get a successful yield.

Decomposition of nitric acid

When the temperature of the acid solution increases, nitric acid starts to decomposes to nitrogen dioxide, oxygen and water. Nitrogen dioxide (NO2) is a brown gas.

thermal decomposition of nitric acid

Difference between nitric acid and nitrous acid

Nitric acid is a strong acid which dissociate completely in the water to H+ ions and NO3-. But, nitrous acid is a weak acid and partially dissociates to H+ ions and NO2-. This happens because conjugate base (NO3-) of nitric acid is more stable. pKa of nitrous acid is 3.3 at 180C. Also, nitrous acid decomposes readily than nitric acid.

Also, oxidation numbers of nitrogen atoms are different. In HNO2, nitrogen atom is at +3 oxidation state . In HNO3, nitrogen atom is at +5.

Nomenclature of Nitrous acid, Nitric acid, Nitrite ion

  • Nitrous acid - HNO2
  • Nitric acid - HNO3
  • Nitrite ion - NO2-

Have Questions?

What has more acidic properties? nitrous acid or nitric acid

Nitric acid is a strong acid. But nitrous acid is a weak acid. Therefore acidic strength of nitric acid is much higher than nitrous acid.

Why nitric acid is much acidic than nitrous acid?

When oxidation number increases, acidic characteristics increases. Oxidation number of nitrogen in nitric acid is +5 while it is +3 in nitrous acid. That is one reason why nitric acid is much acidic than nitrous acid.

oxidation state of nitrogen in HNO3?

Oxidation state of HNO3 is +5. So nitrogen atom cannot be oxidized furthermore. Due to +5 oxidation state, nitric acid shows strong acidic properties.

what is HNO2 in chemistry?

HNO2 is an oxo acid of nitrogen and it is a weak acid. HNO2 is also named as nitrous acid or nitric(III) acid.

Nitrous acid is used very much in organic chemistry such as amine and HNO2 reaction.

How nitric acid and nitrous acid solution is formed?

Aqueous nitrous acid is decomposed to nitric acid when temperature is increased. So after preparation of nitrous acid, nitric acid and nitrous acid solution mixture can be given.

hno3 + hno2 what will happen?

A strong acidic solution is given.

HNO2 organic or inorganic ?

HNO2 does not contain carbon as a element. So HNO2 is an inorganic compound.

nitrous acid strong or weak?

Nitrous acid is a weak monobasic acid.

why hno2 is unstable?

HNO2 decomposes to HNO3, NO and water easily at room temperature. This decomposition increases with temperature. Therefore, we cannot store nitrous acid for long time in the laboratory. When we require HNO2, at that moment HNO2 is prepared.

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