Nitrous Acid Formula, Structure, and Key Reactions for Exams
What is Nitrous Acid?
While talking about acids, you will hear of acid name HNO2 often. HNO2 is also known as nitrous acid and is an acidic compound, which is unstable and weak. It is only prepared in dilute forms and is usually cold. It is a highly useful compound in converting certain amines to diazonium compounds. The question that you might ask first is "What is the formula of nitrous acid?" The nitrous acid formula in chemistry is HNO2. Its molar mass is 47.013 g/mol. Being a planar molecule implies that the nitrogen atom is attached to oxygen atoms, through single and double bonds. Both of these oxygen molecules are electronegative in nature. Shown below is the structure of nitrous acid HNO2:
[Image Will be Uploaded Soon]
Nitrous acid tends to explode when it comes in contact with certain compounds like Phosphorus Trichloride. It is considered to be a powerful oxidizer. Although nitrous acid is not very toxic, it is still known to impact the respiratory health of humans, sometimes also aggravating the symptoms of asthma.
Physical Properties of Nitrous Acid HNO2
Liquid Form: Nitrous acid is only available in its liquid form - either dissolved in water or as a nitrate salt.
Pale Blue: The nitrous acid solution is pale blue in colour and gets its colour from N2O3, which is a blue solid and acidic in nature.
Density: Nitrous acid HNO2 has a density of about 1 g/mL with its weight of about 47.013 g/mol
Boiling Point: Nitrous Acid boiling point is 158-degree centigrade.
Chemical Properties of Nitrous Acid HNO2
Monobasic: Since Nitrous acid HNO2 has only one hydrogen ion which it can donate to a base in case of a reaction between an acid and a base, it is known as a monobasic acid. It can release only one proton, i.e., H+ in solution.
Unstable: Nitrous acid is highly unstable in nature, and so mostly found in its liquid form. It has to be freshly prepared every time it has to be used.
Weak Acid: Nitrous acid is a weak acid and so does not completely dissociate when in water.
Oxidation Number: Since the oxidation number of the N in HNO2 is 3+, it can act as both an oxidizing agent and a reducing agent.
Nitrous acid oxidizes hydrogen sulphide to form sulphur:
H2S+2HNO2→2H2O+2NO+S
Reaction With Bases: Since nitrous acid is acidic in nature, it reacts with bases to form salts. It usually forms nitrite salts like sodium nitrite.
2 HNO2 + Na2CO3 → 2 NaNO2 + HCO3
Structure of Nitrous Acid
The formula of nitrous acid is represented by HNO2. Since it is an acid, the hydrogen molecule goes outside of one of the oxygen molecules. So, we can put nitrogen in the centre with two oxygens with Hydrogen outside one of them. If we count the number of electrons, Hydrogen has 1, nitrogen has 5, and each oxygen molecule has 6, making it 12. This means we have a total of 18 electrons. After the nitrous acid structure HNO2 is drawn, we can see all of the atoms have full outer shells except nitrogen. A double bond can be shared between nitrogen and oxygen, which means each atom has a formal charge of 0.
[Image Will be Uploaded Soon]
Nitrous Acid Uses
Nitrous acid is a useful compound in several industries and organic chemistry laboratories to produce other chemicals. For example, nitrous acid reaction with amine can form diazonium salts. When 1 degree amine reacts with nitrous acid, it gives us alkyl diazonium chloride. When diazonium salts are put in water, nitrogen and hydrochloric acid escape, leaving us with primary alcohol. Uses of nitrous acid HNO2 are widely recognized in the field of chemistry. Diazonium salts are also used in the making of certain dyes.
Decomposition of Nitrous Acid
The nitrous acid chemical formula is HNO2, and there are two primary ways in which nitrous acid can decompose. One method is when nitrous acid decomposes to give us water, nitrogen dioxide and nitric acid. Here is the balanced equation for the same:
2HNO2 → NO2 + NO + H2O
If you know the molecular formula of nitrous acid, you would also know another way in which nitrous acid can decompose is to give us nitric acid, nitrous oxide and water. Here is the balanced equation for the same:
4HNO2 → 2HNO3 + N2O + H2O
FAQs on Nitrous Acid: Complete Guide for Chemistry Students
1. What is nitrous acid and what is its chemical formula?
Nitrous acid is a weak, monoprotic acid known for being unstable and existing only in solution or in the gas phase. It is not found as a pure substance. Its chemical formula is HNO₂, representing one hydrogen atom, one nitrogen atom, and two oxygen atoms.
2. How is nitrous acid typically prepared for laboratory use?
Due to its instability, nitrous acid cannot be stored and must be prepared fresh within the reaction mixture, a process known as in-situ preparation. It is commonly generated by acidifying an aqueous solution of a nitrite salt, such as sodium nitrite (NaNO₂), with a strong mineral acid like hydrochloric acid (HCl), usually at ice-cold temperatures (0-5°C). The reaction is:
NaNO₂ + HCl → HNO₂ + NaCl
3. What is the structure of nitrous acid (HNO₂)?
Nitrous acid has a planar molecular structure. The atoms are connected in the order H-O-N=O. It exists as a mixture of two geometric isomers:
- trans-Nitrous acid: The H-O bond is oriented away from the N=O double bond. This form is more stable.
- cis-Nitrous acid: The H-O bond is oriented on the same side as the N=O double bond.
4. What is the main difference between nitrous acid (HNO₂) and nitric acid (HNO₃)?
While both are oxoacids of nitrogen, they differ significantly:
- Chemical Formula: Nitrous acid is HNO₂, while nitric acid is HNO₃.
- Acid Strength: Nitrous acid is a weak acid as it only partially dissociates in water. Nitric acid is a strong acid that dissociates completely.
- Oxidation State of Nitrogen: In nitrous acid, nitrogen has an oxidation state of +3. In nitric acid, it is +5.
- Stability: Nitrous acid is highly unstable and readily decomposes, whereas nitric acid is much more stable.
5. What are the key chemical properties of nitrous acid?
The most important chemical properties of nitrous acid include:
- Acidity: It behaves as a weak acid in aqueous solutions, with a pKa of approximately 3.3.
- Instability: It undergoes disproportionation (self-oxidation and reduction) to form nitric acid (HNO₃), nitric oxide (NO), and water (H₂O).
- Dual Redox Nature: It can act as both an oxidizing agent (e.g., oxidizing iodides to iodine) and a reducing agent (e.g., reducing permanganate ions), because its nitrogen atom is in an intermediate oxidation state (+3).
6. Why is nitrous acid considered an unstable compound that must be prepared in-situ?
Nitrous acid's instability stems from the intermediate oxidation state (+3) of its nitrogen atom. This makes it prone to a process called disproportionation, where it simultaneously oxidizes and reduces itself. In this reaction (3HNO₂ → HNO₃ + 2NO + H₂O), some nitrogen atoms are oxidized to the +5 state (in HNO₃) while others are reduced to the +2 state (in NO). This inherent tendency to decompose is why it cannot be isolated or stored and must be generated directly within the reaction vessel (in-situ) for immediate use.
7. How does nitrous acid's reaction with amines help distinguish between primary, secondary, and tertiary types?
The reaction with nitrous acid is a classic chemical test to differentiate amines based on their structure:
- Primary Amines (R-NH₂): Aliphatic primary amines react to produce a diazonium salt that is highly unstable and immediately decomposes to release nitrogen gas (seen as bubbles or effervescence). Aromatic primary amines form a more stable diazonium salt at low temperatures (0-5°C).
- Secondary Amines (R₂NH): React with nitrous acid to form a yellow, oily N-nitrosamine, which is insoluble in the aqueous solution.
- Tertiary Amines (R₃N): Do not have a hydrogen atom on the nitrogen to react in the same way. They typically form a soluble nitrite salt with the acid without releasing gas or forming an oily layer, thus showing no visible reaction under these conditions.
8. What is the primary industrial application of nitrous acid?
The most significant industrial application of nitrous acid is in the synthesis of azo dyes. This process involves a crucial first step called diazotization, where nitrous acid (prepared in-situ) reacts with a primary aromatic amine (like aniline) in cold conditions. This reaction forms an aromatic diazonium salt. This salt then acts as an electrophile in a coupling reaction with an electron-rich aromatic compound (like phenol or aniline) to produce a highly coloured azo compound, which forms the basis of many synthetic dyes used in textiles and pigments.
9. How can one explain the dual role of nitrous acid as both an oxidizing and reducing agent?
The dual redox nature of nitrous acid is explained by the +3 oxidation state of its nitrogen atom. This state is intermediate between higher states (like +5 in nitric acid) and lower states (like +2 in nitric oxide).
- As an oxidizing agent: Nitrous acid can accept electrons and get reduced. For example, it oxidizes hydrogen iodide (HI) to iodine (I₂), while its own nitrogen is reduced from +3 to +2 (in NO).
- As a reducing agent: It can donate electrons and get oxidized when reacting with a stronger oxidizing agent. For example, it reduces potassium permanganate (KMnO₄), and its nitrogen is oxidized from +3 to +5 (in the nitrate ion, NO₃⁻).
10. How does nitrous acid (HNO₂) differ from other common nitrogen compounds like nitrous oxide (N₂O) and hyponitrous acid (H₂N₂O₂)?
These compounds are often confused but are fundamentally different:
- Nitrous Acid (HNO₂): An unstable weak acid where nitrogen is in the +3 oxidation state. It is used in organic synthesis.
- Nitrous Oxide (N₂O): A stable, relatively unreactive neutral gas, commonly known as laughing gas. Here, nitrogen is in the +1 oxidation state. It is not an acid.
- Hyponitrous Acid (H₂N₂O₂): An extremely weak acid and unstable compound where nitrogen is also in the +1 oxidation state. It is the acid counterpart to nitrous oxide but is chemically distinct from both HNO₂ and N₂O.
