Trioxonitrate(V) Acid: Nitric Acid

Trioxonitrate(V) acid (nitric acid), HNO₃, is an essential starting material for the manufacture of organic nitro-compounds used in the production of drugs, fertilizers, dyes and explosives. It is used in the production of nitrochalk fertilizers and TNT (methyl-2,4,6-trinitrobenzene) explosives especially. Aqua regia is another important use of trioxonitrate(V) acid where a mixture of one part trioxonitrate(V) acid and three parts of hydrochloric acid is used as solvent for platinum and gold.

Although nitrogen(IV) oxide is commonly used as a nitrating agent and an acid, it is also used as an oxidizing agent in the production of polymers like nylon and terylene.

Trioxonitrate(V) acid is prepared in the laboratory by the reaction of concentrated teraoxosulphate(VI) acid with trioxonitrate(V).

The Conc. tetraoxosulphate(VI) acid will displace trioxonitrate(V) acid from potassium trionitrate(V).

KNO₃(s) + H₂SO₄(aq) → KHSO₄(aq) + HNO₃(g)

Sodium trioxonitrate(V) will also react with concentrated tetraoxosulphate(VI) acid to form the trioxonitrate(V) acid.

NaNO₃(s) + H₂SO₄(aq) → NaHSO₄(aq) + HNO₃(g)

Trioxonitrate(V) acid is produced industrially by the catalytic oxidation of ammonia to nitrogen(II) oxide.
Here, excess air is reacted with ammonia in the presence of platinum-rhodium catalyst at 700°C.

4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)

The nitrogen(IV) oxide is cooled and further oxidized with excess air to produce nitrogen(IV) oxide.

2NO(g) + O₂(g) → 2NO₂(g)

The nitrogen(IV) oxide is then dissolved in a spray of hot water in the presence of more air to form up to 50% trioxonitrate(V) acid.

4NO₂(g) + 2H₂O(l) + O₂(g) → 4HNO₃(aq)

Note: The formation of nitrogen(IV) oxide is exothermic and could generate enough energy to maintain the temperature require by the platinum-rhodium catalyst.

Also, a higher concentration of trioxonitrate(V) acid is obtained in modern plants by the absorption of nitrogen(IV) oxide under pressure.

Acidic Properties

•Reaction with metals,

Very diluted (about 1%) form of the acid reacts with metals such as magnesium, calcium or manganese to liberate hydrogen gas.

Mg(s) + 2HNO₃(aq) → Mg(NO₃)₂(aq) + H₂(g)

It rarely form the hydrogen gas at higher concentration as the hydrogen is immediately oxidised by the acid to form water.

•Reaction with bases,

It undergoes neutralization reaction with bases and alkalis to form salt and water only. The ionization reaction with alkalis is shown in the chemical equation of reaction:

OH^-(aq) + H⁺(aq) → H₂O(l)

While the ionization reaction with bases can be expressed as,

O^2-(s) + 2H⁺(aq) → H₂O(l)

An example of reaction with alkalis is the neutralization reaction with sodium hydroxide to form sodium trioxonitrate(V) and water.

NaOH(aq) + HNO₃(aq) → NaNO₃(aq) + H₂O(l)

Calcium oxide base reacts with two moles of the acid to form calcium trioxonitrate(V) and water.

CaO(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l)

•Reaction with trioxocarbonate(IV),

It liberate carbon(IV) oxide when reacted with metallic trioxocarbonate(IV) as shown in the ionization reaction:

CO₃^2-(s) + 2H⁺(aq) → H₂O(l) + CO₂(g)

Calcium trioxocarbonate(IV) will liberate carbon(IV) oxide when reacted with the acid.

CaCO₃(s) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g)

As an oxidizing agent

The acid accepts electrons to be reduced to a range of products that could be NO₂, NO, N₂, HNO₂, NH₂OH, N₂O, NH₄⁺, N₂H₄; depending on the temperature, concentration of the acid and the strenght of the reducing agent. Nitrogen(II) oxide is formed when the oxidizing agent is dilute trioxonitrate(V) acid. Here, there is a transition from oxidation state +5 to +4.

NO₃^-(aq) + 4H⁺(aq) + 3e^- → NO(g) + 2H₂O(g)

Concentrated acid will produce nitrogen(IV) oxide with oxidation number change from +5 to +4

NO₃^-(aq) + 2H⁺(aq) + e^- → NO₂(g) + H₂O(l)

It will in its hot concentrated form oxidize non-metals to their highest oxides that can revert to their corresponding acids when reacted with water.

Carbon will oxidize to carbon(IV) oxide.

C + 4HNO₃(aq) → CO₂(g) + 2H₂O(l) + 4NO₂(g)

Sulphur is also heated to oxidize to tetraoxosulphate(VI) acid.

S(s) + 6HNO₃(aq) → H₂SO₄(aq) + 2H₂O(l) + 6NO₂(g)

Heating red phosphorus will result to the formation of tetraoxophosphate(V) acid.

P(s) + 5HNO₃(aq) → H₃PO₄(aq) + H₂O(l) + 5NO₂(g)

And, iodine can be boiled with trioxonitrate(V) acid to trioxoiodate(V) acid.

I₂(s) + 10HNO₃(aq) → 2HIO₃(aq) + 4H₂O(l) + 10NO₂(g)

With metals,

Trioxonitrate(V) acid is also a strong oxidizing agent that reacts with reducing agents such as hydrogen sulphide and iron(II) salts in a redox reaction to form oxidized products.

Hydrogen sulphide is oxidized to sulphur as nitrogen(IV) oxide is formed.

H₂S(g) + 2HNO₃(aq) → S(s) + 2H₂O(l) + 2NO₂(g)

Iron(II) salts are oxidized to iron(III) salts with nitrogen(II) oxide.

6Fe²⁺(aq) + 8H⁺(aq) + 2NO₃^- → 6Fe³⁺(aq) + 4H₂O(l) + 2NO(g)

The nitrogen(II) oxide is further oxidised to nitrogen(IV) oxide when exposed to air.

2NO(g) + O₂(g) → 2NO₂(g)

Nitration reactions

The acid dissociates to form nitryl cation (nitronium ion), NO₂⁺, in the presence of concentrated tetraoxosulphate(VI) acid. The nitronium ion can replace hydrogen ions in many organic compounds that include benzene, phenol and methyl benzene.

C₆H₆(l) + HNO₃(aq) → C₆H₅NO₂(l) + H₂O(l)

This process is called nitration and it is of great industrial significance.

Decomposition reaction

The acid is decomposed slowly at room temperature especially in the presence of sunlight, or rapidly by heating to form nitrogen(IV) oxide and oxygen.

4HNO₃(aq) → 2H₂O(l) + 4NO₂(g) + O₂(g)