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.

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

Nitrous acid | nitrc(iii) acid | HNO₂

Physical properties of HNO₂

An unstable acid - Decomposition of nitrous acid

HNO₂ decomposes to HNO₃, NO, H₂O when temperature is increased in the solution.

A weak acid

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

Colourless

Chemical properties of HNO₂

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

Preparation of nitrous acid

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

Heating sodium nitrate (NaNO₃) | decomposition

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

Dissolving in water

Solid sodium nitrite is dissolved in water to prepare aqueous sodium nitrite. No reaction is occurred in this step.

Cold dilute HCl and NaNO₂ reaction

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

Nitric acid | nitric(v) acid | HNO₃

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, nitrous acid starts to decomposes to nitrogen dioxide, oxygen and water. Nitrogen dioxide (NO₂) is a brown gas.

Questions

What is more acid? 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 a one reason why nitric acid is much acidic than nitrous acid.