What Is Nitrous Acid Definition Formula Preparation Reactions and Uses
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:
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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.
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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 HNO2 Structure Properties and Chemical Behavior
1. What is nitrous acid?
Nitrous acid is a weak, unstable acid with the chemical formula HNO2 that contains nitrogen in the +3 oxidation state. It is known as a weak monoprotic acid because it donates one proton (H+) in aqueous solution.
Key points about nitrous acid:
- It exists mainly in aqueous solution and is not stable as a pure compound.
- It partially dissociates in water: HNO2(aq) ⇌ H+(aq) + NO2-(aq).
- It plays an important role in diazotization reactions and organic chemistry.
2. What is the chemical formula and structure of nitrous acid?
The chemical formula of nitrous acid is HNO2, and its structure is best represented as H–O–N=O. In this structure:
- Nitrogen is bonded to one hydroxyl group (–OH) and one double-bonded oxygen.
- The molecule contains one acidic hydrogen attached to oxygen.
- Nitrogen has an oxidation state of +3.
3. Is nitrous acid a strong or weak acid?
Nitrous acid (HNO2) is a weak acid because it only partially ionizes in water. Its dissociation equilibrium is:
HNO2(aq) ⇌ H+(aq) + NO2-(aq)
Important points:
- It does not fully dissociate like strong acids such as HCl or HNO3.
- It has a relatively small acid dissociation constant (Ka).
- Because it is weak, the solution contains both undissociated HNO2 and ions.
4. How is nitrous acid prepared in the laboratory?
Nitrous acid is prepared in the laboratory by reacting a nitrite salt with a dilute mineral acid, usually sodium nitrite with hydrochloric acid. The balanced equation is:
NaNO2(aq) + HCl(aq) → HNO2(aq) + NaCl(aq)
Key details:
- The reaction is carried out at low temperature because HNO2 is unstable.
- Nitrous acid is formed in situ (in solution) and not isolated.
- It decomposes easily if heated or concentrated.
5. Why is nitrous acid unstable?
Nitrous acid is unstable because it readily decomposes into nitric oxide, nitrogen dioxide, and water. The balanced decomposition reaction is:
3HNO2(aq) → HNO3(aq) + 2NO(g) + H2O(l)
Reasons for instability:
- Nitrogen in the +3 oxidation state is intermediate and can undergo oxidation and reduction (disproportionation).
- The molecule easily breaks down, especially at higher temperatures.
- It is therefore used immediately after preparation.
6. What is the difference between nitrous acid and nitric acid?
The main difference between nitrous acid and nitric acid is that HNO2 is a weak acid with nitrogen in the +3 oxidation state, while HNO3 is a strong acid with nitrogen in the +5 oxidation state. Key differences:
- Strength: HNO2 is weak; HNO3 is strong.
- Oxidation state of nitrogen: +3 in HNO2, +5 in HNO3.
- Stability: HNO2 is unstable; HNO3 is relatively stable.
- Conjugate base: NO2- (nitrite) vs. NO3- (nitrate).
7. What is the conjugate base of nitrous acid?
The conjugate base of nitrous acid is the nitrite ion (NO2-). When HNO2 donates a proton, the reaction is:
HNO2(aq) ⇌ H+(aq) + NO2-(aq)
In this acid–base reaction:
- HNO2 acts as a Brønsted–Lowry acid.
- NO2- is the corresponding conjugate base.
- The pair HNO2/NO2- is a conjugate acid–base pair.
8. What are the uses of nitrous acid in chemistry?
Nitrous acid is mainly used in diazotization reactions and the preparation of aromatic diazonium salts. Its key uses include:
- Formation of diazonium salts from primary aromatic amines.
- Synthesis of azo dyes in dye chemistry.
- Analytical and synthetic organic chemistry reactions.
9. What type of reaction does nitrous acid undergo?
Nitrous acid commonly undergoes disproportionation reactions, where nitrogen is simultaneously oxidized and reduced. A typical balanced reaction is:
3HNO2(aq) → HNO3(aq) + 2NO(g) + H2O(l)
In this reaction:
- Some nitrogen (+3) is oxidized to +5 in HNO3.
- Some nitrogen (+3) is reduced to +2 in NO.
- This makes HNO2 chemically unstable.
10. How do you calculate the pH of a nitrous acid solution?
To calculate the pH of a nitrous acid solution, use its acid dissociation constant (Ka) and apply the weak acid equilibrium expression. Steps:
- Write the equilibrium: HNO2(aq) ⇌ H+(aq) + NO2-(aq).
- Use the expression: Ka = [H+][NO2-]/[HNO2].
- Set up an ICE table and solve for [H+].
- Calculate pH using pH = −log[H+].
