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Nitric Acid Preparation- Introduction to Nitric Acid, Laboratory Preparation & Commercial Methods of Preparation

Nitric Acid Preparation- Introduction to Nitric Acid, Laboratory Preparation & Commercial Methods of Preparation

We often buy precious metals like gold, silver and platinum in the form of ornaments due to their non-reactive nature, and they are referred to as noble metals. In fact, these metals do not even react with very strong acids like sulphuric acid. Probably you might have recalled the reaction of metals with acid. These metals like gold and silver don’t react due to their inert nature and we need a special type of solution to dissolve these noble metals. What is the name of that solution? Absolutely! That is known as “Aqua Regia” which is a Latin name and the English translation to this is “Royal Water”. Do you know what the composition of aqua regia is? Aqua regia is a fuming mixture of two important acids, one is hydrochloric acid and another one is nitric acid which is mixed in a ratio of 3:1 respectively. Nitric acid is a strong oxidant and highly corrosive in nature, and therefore should be used very carefully. Let's dive in and explore how this important mineral acid is prepared.

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Table of contents

  • Introduction to nitric acid
  • Laboratory preparation of nitric acid
  • Commercial methods of preparation of nitric acid
  • Practice problems
  • Frequently asked questions-FAQs

Introduction to nitric acid

Nitric acid is an important oxo-acid of nitrogen which is a commercially important reagent in the manufacture of drugs, explosives, fertilisers etc. It was named aqua-fortis (which means stronger than water) by chemists. The chemical formula of nitric acid is HNO3. It is a monobasic planar molecule in which a nitrogen atom is sp2 hybridised which releases one hydronium ion when dissolved in water.

It is a colourless fuming liquid but develops a yellowish colour due to its decomposition into nitrogen dioxide (dark brown) in the presence of sunlight.

4HNO3(l)2H2O(l)+O2(g)+4NO2(g)

Laboratory Preparation of Nitric Acid

Nitric acid is prepared in the laboratory by mixing alkaline nitrate salt with concentrated nitric acid at a temperature below 200 ℃. In this reaction, the salt of the more volatile acid is displaced by the less volatile acid to form nitric acid.

KNO3(aq)+H2SO4(aq)KHSO4(aq)+HNO3(aq)

Commercial methods of preparation of nitric acid

Nitric acid can be commercially prepared from two different methods:

Birkeland and Eyde method:

  • In this method oxidation of nitrogen gas takes place in the first step at high temperature in the presence of an electric arc which leads to the formation of nitric oxide.
  • This reaction is reversible and endothermic in nature.

N2(g)+O2(g)  \rlhar 2NO(g)

  • This reaction is generally favoured at high temperatures because according to Le Chatelier’s Principle. when the temperature is increased in the case of a reversible endothermic reaction, it shifts the reaction in the forward direction and therefore leads to the formation of more nitric oxide.
  • Nitric oxide formed during the process is cooled and then allowed to react with oxygen molecules to form nitrogen dioxide gas.

2NO(g)+O2(g)2NO2(g)

  • Vapours of nitrogen dioxide produced are allowed to pass through water and result in the formation of a mixture of nitric acid (HNO3) and nitrous acid (HNO2).

2NO2(g)+H2OHNO3(aq)+HNO2(aq)

  • Nitrous acid is unstable and gets decomposed into nitric acid along with nitric oxide and water. This is a disproportionation reaction.

4HNO2(aq)2HNO3(g)+NO(g)+H2O(l)

  • Nitric oxide produced during the reaction can be utilised again for producing nitric acid.

Ostwald’s process:

  • This is an important commercial method used for the preparation of nitric acid.
  • In the first step of preparation of nitric acid, ammonia is allowed to react with air (which is free from carbon dioxide and dust particles) in the ratio 1:10 by volume which is oxidised to nitric oxide at a temperature range of (750 ℃-900 ℃) in the presence of a platinum gauze which acts as a catalyst. This reaction is exothermic in nature, producing heat which is utilised for maintaining the temperature of the catalyst.

4NH3(g)+5O2(g)4NO(g)+6H2O(l)+Heat

  • Nitric oxide formed in the oxidation of ammonia is allowed to react with air, which further oxidises to nitrogen dioxide.

2NO(g)+O2(g)2NO2(g)

  • Nitrogen dioxide produced in the reaction is cooled and dissolved in water which results in the formation of nitric acid along with the nitric oxide.

3NO2(g)+H2O (l)2HNO3(aq)+NO (g)

  • Nitric oxide formed during the process can be reused for the formation of nitric acid. In this method of nitric acid formation, the concentration of nitric acid is less about (50-60)% by volume. So, further, the concentration of acid can be increased upto 98% by volume when allowed to pass through the sulphuric acid followed by distillation. Sulphuric acid here acts as a dehydrating agent and removes the moisture present in the reaction.

Note: Birkeland and Eyde's method is normally obsolete because this method of nitric acid preparation is costlier and the amount of nitric oxide produced is less. Therefore, Ostwald's method is used for the preparation widely.

Practice Problems

Q. Select the reaction which does not take place in the preparation of nitric acid by the Ostwald method.

a. 4NH3+5O24NO+6H2O+Heat
b. 3NO2+H2OHNO3+NO
c. 2NO+O2 2NO2
d. N2+O2\rlhar 2NO

Answer: (D)

In the first step of preparation of nitric acid, ammonia is allowed to react with air (which is free from carbon dioxide and dust particles) in the ratio 1:10 by volume which is oxidised to nitric oxide at a temperature range of (750 ℃-900 ℃) in the presence of a platinum gauze which acts as a catalyst. This reaction is exothermic in nature, producing heat which is utilised for maintaining the temperature of the catalyst.

4NH3(g)+5O2(g)4NO(g)+6H2O(l)+Heat

Nitric oxide formed in the oxidation of ammonia is allowed to react with air which is further oxidised to nitrogen dioxide.

2NO(g)+O2(g)2NO2(g)

Nitrogen dioxide produced in the reaction of nitric oxide is cooled and dissolved in water which results in the formation of nitric acid along with the nitric oxide.

3NO2(g)+H2O2HNO3(aq)+NO(g)

Nitric oxide formed during the process can be reused again for the formation of nitric acid.

So, only N2+O2⇌ 2NO reaction is not taking place in Ostwald’s method in preparation of nitric acid.

Q. What is the method for the concentration of nitric acid prepared by Ostwald’s method?

a. Adding H2SO4 and distillation of the dilute acid prepared
b. Heating of dilute acid
c. Addition of CaO which absorbs moisture
d. Both B and C

Answer: (A)

Solution: In Ostwald’s method of nitric acid formation, the concentration of nitric acid is less about (50-60)% by volume and the concentration of acid can be increased up to 98% (v/v) when allowed to pass through the sulphuric acid followed by distillation. Sulphuric acid here acts as a dehydration agent and removes the moisture present in the reaction.

Q. Which of the following is the correct option with respect to nitric acid?

a. Nitric acid is a linear molecule.
b. Hybridisation of nitrogen in nitric acid is sp3.
c. Nitric acid is a planar molecule.
d. There is no lone pair present on the nitrogen atom present in nitric acid.

Answer: (C)

Solution: Nitric acid is a planar molecule with sp2 hybridisation of the central nitrogen atom and there is one lone pair present on the central nitrogen atom.

Q. Select the correct option that can be used for the commercial method for the preparation of nitric acid.

a. Haber’s process
b. Ostwald method
c. Birkland and Eyde process
d. Both B and C

Answer: (D)

Solution: Both Ostwald’s method and Birkland and Eyde's process can be used for the preparation of nitric acid but the Birkeland and Eyde's method is normally obsolete because this method of nitric acid preparation is costlier and the amount of nitric oxide produced is less. Therefore, Ostwald's method is used for the preparation widely.

Frequently asked questions

Q. What is the difference between the Birkland and Eyde method and the Ostwald method of preparation of nitric acid?
Answer:
The difference between the Birkland and Edye method of preparation and the Ostwald process are:

Birkland and Eyde's process

Ostwald's method

In this method, oxidation of nitrogen gas is taking place in the first step at a high temperature in the presence of an electric arc which leads to the formation of nitric oxide.

This reaction is reversible.

N2(g)+O2(g) ⇌ 2NO(g)

In the first step of preparation of nitric acid, ammonia is allowed to react with air which is free from carbon dioxide and dust particles) in the ratio of 1:10 by volume which further oxidises to nitric oxide at a temperature range of (750 ℃-900 ℃) in the presence of a platinum gauze which acts as a catalyst.

4NH3(g)+5O2(g)

4NO(g)+6H2O(l)+Heat

This reaction is endothermic in nature.

This process is exothermic in nature.

Q. What happens when we dissolve the nitric acid in water?
Answer:
Nitric acid is a water-soluble monobasic acid that when dissolved in water releases one H+ ion per molecule making the overall solution acidic in nature having a pH less than 7 at a temperature of 25 ℃.

HNO3(aq)+H2O(l)H3O+(aq)+NO3-(aq)

Q. What are some important uses of nitric acid?
Answer:
Some important uses of nitric acid includes:

  • It is used to make ammonium nitrates, which are used to make fertilisers, and dye.
  • It's used to make explosives like TNT and nitroglycerin.
  • In liquid-fueled rockets it is used as fuel.
  • It's utilised to get rid of warts in their purest form.
  • In electrochemistry, it is employed as a chemical doping agent.

Q. What will happen if dilute nitric acid accidentally falls on our hand?
Answer:
When dilute nitric acid comes in contact with human skin, it causes yellow stains. This is due to it reacting with keratin, a protein found in human skin. The xanthoproteic reaction is a common name for this reaction. This method is also used to determine the protein present in the sample and is known as the xanthoproteic test.

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