Nitrous acid

Key takeaways

Nitrous acid belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nitrous acid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nitrous acid from memory before moving on to harder problems.

Reference excerpt

Nitrous acid (molecular formula HNO2) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite (NO−2) salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

Structure In the gas phase, the planar nitrous acid molecule can adopt both a syn and an anti form. The anti form predominates at room temperature, and IR measurements indicate it is more stable by around 2.3 kJ/mol.

Decomposition and preparation

Free, gaseous nitrous acid is unstable, rapidly disproportionating to nitric oxides:

2 HNO2 → NO2 + NO + H2O In aqueous solution, the nitrous acid also disproportionates, for a net reaction producing nitric oxide and nitric acid:

3 HNO2 → 2 NO + HNO3 + H2O Consequently applications of nitrous acid usually begin with mineral acid acidification of sodium nitrite. The acidification is usually conducted at ice temperatures, and the HNO2 consumed in situ. Nitrous acid equilibrates with dinitrogen trioxide in water, so that concentrated solutions are visibly blue:

N2O3 + H2O ⇌ 2 HNO2 Addition of dinitrogen trioxide to water is thus another preparatory technique.

Chemical applications Nitrous acid is the main chemophore in the Liebermann reagent, used to spot-test for alkaloids. At high acidities (pH ≪ 2), nitrous acid is protonated to give water and nitrosonium cations.

Reduction With I− and Fe2+ ions, NO is formed:

2 HNO2 + 2 KI + 2 H2SO4 → I2 + 2 NO + 2 H2O + 2 K2SO4 2 HNO2 + 2 FeSO4 + 2 H2SO4 → Fe2(SO4)3 + 2 NO + 2 H2O + K2SO4 With Sn2+ ions, N2O is formed:

2 HNO2 + 4 HCl + 2 SnCl2 → 2 SnCl4 + N2O + 3 H2O With SO2 gas, NH2OH is formed:

2 KNO2 + 6 H2O + 4 SO2 → 3 H2SO4 + K2SO4 + 2 NH2OH With Zn in alkali solution, NH3 is formed:

5 H2O + KNO2 + 3 Zn → NH3 + KOH + 3 Zn(OH)2 With N2H+5, both HN3 and (subsequently) N2 gas are formed:

HNO2 + [N2H5]+ → HN3 + H2O + H3O+ HNO2 + HN3 → N2O + N2 + H2O Oxidation by nitrous acid has a kinetic control over thermodynamic control, this is best illustrated that dilute nitrous acid is able to oxidize I− to I2, but dilute nitric acid cannot.

I2 + 2 e− ⇌ 2 I− Eo = +0.54 V NO−3 + 3 H+ + 2 e− ⇌ HNO2 + H2O Eo = +0.93 V HNO2 + H+ + e− ⇌ NO + H2O Eo = +0.98 V It can be seen that the values of Eocell for these reactions are similar, but nitric acid is a more powerful oxidizing agent. Based on the fact that dilute nitrous acid can oxidize iodide into iodine, it can be deduced that nitrous is a faster, rather than a more powerful, oxidizing agent than dilute nitric acid.

Organic chemistry Nitrous acid is used to prepare diazonium salts:

HNO2 + ArNH2 + H+ → ArN+2 + 2 H2O where Ar is an aryl group. Such salts are widely used in organic synthesis, e.g., for the Sandmeyer reaction and in the preparation azo dyes, brightly colored compounds that are the basis of a qualitative test for anilines. Nitrous acid is used to destroy toxic and potentially explosive sodium azide. For most purposes, nitrous acid is usually formed in situ by the action of mineral acid on sodium nitrite: It is mainly blue in colour

NaNO2 + HCl → HNO2 + NaCl 2 NaN3 + 2 HNO2 → 3 N2 + 2 NO + 2 NaOH Reaction with two α-hydrogen atoms in ketones creates oximes, which may be further oxidized to a carboxylic acid, or reduced to form amines. This process is used in the commercial production of adipic acid. Nitrous acid reacts rapidly with aliphatic alcohols to produce alkyl nitrites, which are potent vasodilators:

(CH3)2CHCH2CH2OH + HNO2 → (CH3)2CHCH2CH2ONO + H2O The carcinogens called nitrosamines are produced, usually not intentionally, by the reaction of nitrous acid with secondary amines:

HNO2 + R2NH → R2N-NO + H2O In the Abidi alkyne synthesis, a trisubstituted isopropylidene olefin reacts with NaNO2/HOAc-H2O with overall loss of the elements of methane to give a methyl substituted alkyne. The reaction mechanism of this highly unusual process is complex and uncertain, with proposals advanced by Corey and then Zard.

Atmosphere of the Earth Nitrous acid is involved in the ozone budget of the lower atmosphere, the troposphere. The heterogeneous reaction of nitric oxide (NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric aerosols, the product readily photolyses to hydroxyl radicals.

DNA damage and mutation Treatment of Escherichia coli cells with nitrous acid causes damage to the cell's DNA including deamination of cytosine to uracil, and these damages are subject to repair by specific enzymes. Also, nitrous acid causes base substitution mutations in organisms with double-stranded DNA.

Illustrations

Worked examples

Example 1 — a first encounter with Nitrous acid

Start with the simplest possible case. Write down what Nitrous acid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nitrous acid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nitrous acid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nitrous acid

In research: Nitrous acid appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nitrous acid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nitrous acid is common in secondary-school and first-year university syllabi. It links to neighbouring topics Nitrogen(III) compounds, Nitrogen cycle, Nitrogen oxoacids, so understanding it makes those chapters shorter.
In everyday life: Look for Nitrous acid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Nitrous acid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Nitrous acid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Nitrous acid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Nitrous acid in simple terms?

Nitrous acid (molecular formula HNO2) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite (NO−2) salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter".

Why does Nitrous acid matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nitrous acid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nitrous acid.