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.

Content

• 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

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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.

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)

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.

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.

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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 + NaNO_3(s)

Add concentrated H₂SO₄ acid to the solid NaNO_3(s) and distill the mixture.

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.

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.

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Nomenclature of Nitrous acid, Nitric acid, Nitrite ion

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

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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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