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Nitrous acid is a weak and unstable acidic compound found in solutions of nitrate salts. Its chemical formula is HNO2, and its molar mass is 47.013 g/mol.
- Nitrous acid is formed from the mixture of nitrous gas and nitric acid containing some proportion of oxygen.
- It is obtained by distilling dry nitrate of lead fitted with a receiver.
- The acid is used for the conversion of certain amines to diazonium compounds.
- Nitrous acid was discovered by Carl Wilhelm Scheele, who called it phlogisticated acid of niter.
- If you are not aware, the acid is in the list of the most important medications needed in a basic health system by WHO.
Key Terms: Nitrous acid, Nitric Acid, Nitrate Salts, Amines, Diazonium Salts, Nitric Oxide, Nitrogen Dioxide, Sodium Nitrite, Properties of Nitrous Acid
What is Nitrous Acid?
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Nitrous acid is an acidic compound that can be formed by reacting potassium or sodium nitrite with hydrochloric acid (HCl). It can be produced by mixing nitrous gas and nitric acid.
- Nitrous acid is a powerful oxidizer that explodes when it comes in contact with compounds such as Phosphorus Trichloride (PCl3).
- It is not toxic in nature, but it may impact human health and sometimes may cause diseases like asthma.
- The diazonium salts produced with the help of nitrous acid are used as reagents in azo coupling reactions to form azo dyes.
- In the gas phase, the acid is found in two forms, syn and anti.
- Being electronegative in nature, nitrous acid is used as a laboratory agent.
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Nitrous Acid Structure
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Nitrous acid, which has the chemical formula HNO2, is a planar molecule in which one nitrogen atom is attached to two oxygen atoms by a single and double bond. Since the compound is acidic in nature, hydrogen stays outside of one of the oxygen atoms.
- In the HNO2 molecule, nitrogen has 5, each oxygen has 6, and hydrogen has 1 electron.
- Thus, a total of 18 electrons are present in one HNO2 molecule.
- A double bond is formed between oxygen and nitrogen, which signifies that each atom has 0 formal charges.
- In the O-N-O bond, the p-orbitals of nitrogen and two oxygens are in the same plane while the π bonded electrons get delocalized in between these p-orbitals.
- Thus, both the bonds show the double bond characteristic.
- The hydrogen is bonded with the single-bonded oxygen in such a manner that it can make trans and cis isomers in the gas phase.
- It is found that trans isomer is more stable than cis isomer by 2.3 kJ/mol.
Nitrous Acid
Properties of Nitrous Acid
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The properties of nitrous acid are divided into physical and chemical properties which are as follows:
Physical Properties of Nitrous Acid
Nitrous Acid has several physical properties. Some of them are given below.
- Form of Availability: Nitrous acid is available in liquid form only. It can be found either in nitrate salt or dissolved in water.
- Colour: The solution of nitrous acid is pale blue. The blue colour is due to the acidic and solid N2O3 in the solution.
- Boiling Point: The boiling point of nitrous acid is 158o C.
- Density: Nitrous acid HNO2 has a density of approximately 1 g/mL and a molecular weight of 47.01 g/mol.
Some other physical properties are given in the table below:
| Category | Data |
|---|---|
| Specific Density | 1.35 g/cm3 |
| Odour | Bitter or pungent |
| Appearance | Pale blue solution |
| Systematic name | Dioxonitric(III) acid |
| Melting Point | Only known in solution or as gas |
Chemical Properties of Nitrous Acid
Chemical properties of Nitrous Acid are
- Nitrous acid is a strong, volatile and monobasic acid.
- The acid starts boiling at a temperature of 82o C, and has a specific gravity of 1.45.
- Since HNO2 contains only one atom of hydrogen, it can release an H+ ion (proton) into the solution.
- In the vapour state, when nitrous acid comes in contact with heat it remains unchanged.
- Unlikely, when reacting with water, bubbles form along with nitrous gas.
- Nitrous acid must be freshly prepared whenever required for use and cannot be stored for long.
- It supports the combustion of charcoal or phosphorus irrespective of its irrespirable (in a fuming state) property.
- The oxidation state of nitrogen in HNO2 is +3, thus acting as both a reducing agent and an oxidizing agent.
- Being acidic in nature, nitrous acid reacts with bases to form salts.
- HNO2 oxidizes hydrogen sulphide to produce sulphur as
H2S + 2HNO2 → 2H2O + 2NO + S
- HNO2 forms nitrite salts such as sodium nitrite.
2 HNO2 + Na2CO3 → 2 NaNO2 + HCO3
Preparation of Nitrous Acid
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Nitrous acid can be prepared by reacting potassium or sodium nitrite with hydrochloric acid (HCl). It is a weak and unstable compound prepared in situ.
Preparation of Nitrous acid
Uses of Nitrous Acid
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Various uses of nitrous acid are as follows:
Identification of Amines
Nitrous acids were once used for the identification of amines. To check the type of amine, firstly amine is acidified with HCl (Hydrochloric acid). Then potassium or sodium nitrite is added to the solution. Thus the nitrous acid formed is then reacted with the amine to identify its type (whether primary, secondary, or tertiary).
Preparation of Diazonium Salts
Nitrous acid is a powerful oxidizer used to convert amines into diazonium compounds. When a diazonium compound is placed in water, nitrogen, and hydrochloric acid, it becomes a primary alcohol.
Preparation of Oximes
The acid-on reaction with two α-hydrogen atoms in ketones will result in the formation of oximes. The compound is further oxidized to produce carboxylic acid or reduced to form amines.
Others
Nitrous acid is used in the synthesis of organic compound in Sandmeyer reaction. It is used to remove the toxicity of explosive compound sodium azide.
Reactions of Nitrous Acid
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The nitrous acid behaves like both a reducing agent and oxidizing agent by gaining or losing electrons respectively.
Nitrous acid as a reducing agent
When nitrous acid acts as a reducing agent and reacts with I2 or Br2 in the aqueous solution to form nitric acid. Its oxidation number increases from + 3 to +5.
HNO2 + I2 + H2O → HNO3 + 2HI
HNO2 + Br2 + H2O → HNO3 + 2HBr
Nitrous acid as an oxidizing agent
Nitrous acid reacts with hydrogen sulphide (H2S) and gives nitric oxide (NO) as a final product. In this reaction, HNO2 acts as an oxidizing agent, and the oxidation number of nitrogen decreases from +3 to +2.
2 HNO2 + H2S → 2NO + 2H2O + S
Decomposition of Nitrous Acid
The decomposition reaction of nitrous acid is as follows:
- Nitrous acid forms nitric acid, nitric oxide, and water in concentrated or warm solutions.
3 HNO2 → HNO3 + 2 NO + H2O
- Nitrous acid decomposes into nitric oxide, nitrogen dioxide, and water
2 HNO2 → NO2 + NO + H2O
- Nitrogen oxide, on further reaction with water, forms nitrous acid and nitric acid in an aqueous solution.
2 NO2 + H2O → HNO3 + HNO2
- Nitric oxide, on oxidation by air, produces nitric acid.
2 HNO2 + O2 → 2 HNO3
Things to remember
- Nitrous acid is an unstable acid found in a cold, acidic solution or in the nitric salts.
- It decomposes into nitrogen (NO2), nitric oxide (NO) and water.
- The compound behaves like both a reducing agent and an oxidizing agent.
- It is used as an oxidizer in liquid fuel rockets.
- Since it is an unstable compound, it is mostly found in its liquid form.
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Sample Questions
Ques. Show the decomposition reaction of nitrous acid. Name the products obtained in the reaction? (3 marks)
Ans. Nitrous acid get decomposes into nitrogen dioxide (NO2) and nitric oxide (NO). The reaction can be represented as:
2 HNO2 → NO2 + NO + H2O
NO2 further reacts with water to produce nitrous acid and nitric acid.
Ques. Why is nitrous acid a weaker acid than nitric acid? (2 marks)
Ans. Nitric acid (HNO3) is a stronger acid than a nitrous acid. This is because the conjugate base of nitrous acid is much less stable than the conjugate base of nitric acid. Thus we can conclude that: Acidity of Acid ∝ Stability of conjugate base of acid
Ques. A compound containing nitrogen reacts with nitrous acid. A colourless and odourless gas is evolved along with an organic compound. Write the reaction. Name the nitrogen-containing compound, gas ,and compound produced as a product? (2 marks)
Ans. From the given details, we can say that the nitrogen compound is a primary amine. When nitrous acid reacts with a primary amine, Nitrogen gas (N2) is evolved which is colourless and odourless. Along with N2 gas, ethanol is produced in the product. The above reaction can be represented as:
CH3CH2NH2 + HNO2 → CH3CH2OH + H2O + N2 ↑ (ethanol)
Ques. The Ka value of nitrous acid is 4x10-4. It is measured to have a pH of 2.8. Find the ratio of the concentration of HNO2 to NO2-? (4 marks)
Ans. Firstly, write the dissociation process of Nitrous acid (HNO2).
HNO2 + H2O ⇆ H3O+ + NO2-
Given, pH = 2.8
Ka = 4 x 10-4
Calculate the value of pKa,
pKa = - log Ka
= - log (4 x 10-4)
= 3.38 ≈ 3.4
To calculate the ratio [HNO2]/[NO2-] we have the equation as:
- pH = pKa + log [NO2-]/[HNO2]
- log [NO2-]/[HNO2] = pH - pKa
- log [NO2-]/[HNO2] = 2.8 - 3.4
- log [NO2-]/[HNO2] = -0.6
- log [HNO2]/[NO2-] = 0.6
- [HNO2]/[NO2-] = 10^0.6
- [HNO2]/[NO2-] = 3.98 ≈ 4
Hence the ratio of the concentration of HNO2 to NO2- is 4.
Ques. Why is nitrous acid unstable in nature? (2 marks)
Ans. Nitrous acid forms nitric acid (HNO3), nitric oxide (NO), and water in concentrated or warm solutions. It can act as either a reducing or oxidizing agent. On reacting with substances reactions, nitrogen can lose or gain electrons. That’s why nitrous acid is unstable.
Ques. What is the oxidation state of nitrogen in nitrous acid? Show reaction when nitrous acid acts as oxidizing and reducing agent respectively? (3 marks)
Ans. The oxidation state of Nitrogen in HNO2 is +3.
Explanation: Let the oxidation state of nitrogen is x. The oxidation state of oxygen and hydrogen is -2 and +1 respectively. Thus we have,
1 + x + 2(-2) = 0
x = +3
- Nitrous acid as an oxidising agent
HNO2 oxidizes hydrogen sulphide to produce sulphur:
H2S + 2 HNO2 → 2 H2O + 2 NO + S
In this reaction, the oxidation state of nitrogen decreases from +3 to +2.
- Nitrous acid as a reducing agent
HNO2 + I2 + H2O → HNO3 + 2HI
Or
HNO2 + Br2 + H2O → HNO3 + 2HBr
In this reaction, the oxidation state of nitrogen increases from +3 to +4.
Ques. List down the uses of nitrous acid in the field of chemistry? (3 marks)
Ans. Nitrous acid has several uses, some of the uses are given below.
- Nitrous acid is used to determine the type of amine present in the solution.
- It is used to prepare diazonium salts in reaction with an amine. Diazonium salt on further reaction forms azo-colouring aromatic phenols and amines.
- It is used in the manufacture of dyes, artificial silk, and plastics.
- As an oxidizer, it is used as a rocket propellant.
Ques. How nitrous acid is used in the determination of different types of amines? (3 marks)
Ans. The nitrous acid formed by adding sodium nitrite in the solution of amine and hydrochloric acid further reacts with the amine in different conditions to identify its type (whether primary, secondary, or tertiary).
- In primary amines: On reacting with a primary amine, produces colourless and odourless nitrogen gas and ethanol as a final product.
- In secondary amines: Yellow oil of nitrosamine is produced that signifies the presence of a secondary amine. No gas is evolved in this reaction.
- In tertiary amines: Tertiary amines on reaction with nitrous acid gives the colourless solution that signifies the presence of tertiary amines.
Ques. Is Nitric Acid harmful? How? (2 marks)
Ans. Nitric Acid is a highly toxic material. If gets contacted with the skin and other tissues, it leaves a distinctive yellow scar. It can cause severe damage and in some extreme cases, it may cause death.
Ques. How nitrous acid is prepared from the atmosphere? (2 marks)
Ans. When water and nitrogen dioxide react on different surfaces, like air particles, nitrous acid is created in the atmosphere. Due to the instability of the free nitrous acid, sunlight breaks it down, producing hydroxyl free radicals.
Ques. How can you prepare primary and secondary amines from nitrous acid? (3 marks)
Ans. The primary and secondary amines can be prepared from nitrous acid as follows:
- Primary Amines: The evaluation of odourless and colourless nitrogen gas as the end product signifies the presence of primary amines. Many different compounds are produced as end products along with nitrogen gas. To calculate the number of primary amines present in the solution we have to calculate the moles of nitrogen. The final products are alcohol that is generated by replacing the amine group (-NH2) with the alcohol group (-OH).
-
Secondary Amines: In reaction with secondary amines, no gas is evolved. Yellow oil of nitrosamine signifies the presence of a secondary amine. The final product formed is known as carcinogens.
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