4_3 Nitrous Acid and Nitric Acid: Preparation, Structure and Reactions

Nitrogen forms two important oxyacids. In nitrous acid,

\mathrm{HNO_2}

, nitrogen is +3. In nitric acid,

\mathrm{HNO_3}

, nitrogen is in its highest oxidation state, +5. Nitric acid is one of the most important industrial chemicals, used for fertilizers, explosives and dyes. Both acids are oxidizing agents, but nitrous acid can also act as a reducing agent, because its nitrogen sits halfway up the oxidation-state ladder.

Nitrous Acid (HNO₂)

Nitrous acid is a weak acid that exists only in cold, dilute solution. It is made by acidifying a nitrite solution at 0 °C, for example with barium nitrite and dilute sulphuric acid, which also removes the barium as insoluble barium sulphate. Even at room temperature it soon disproportionates, meaning the same element is both oxidized and reduced:

\mathrm{3HNO_2 \rightarrow HNO_3 + 2NO + H_2O} \qquad (+3 \rightarrow +5 \text{ and } +2)

• As an oxidizing agent: it oxidizes iodide to iodine,

  \mathrm{2HNO_2 + 2HI \rightarrow 2NO + I_2 + 2H_2O}

  , and also oxidizes

  \mathrm{SO_2}

  and tin(II) chloride.
• As a reducing agent: it decolourizes acidified potassium manganate(VII) and bromine water, and is itself oxidized to nitric acid.
• With amino groups: it reacts with

  \mathrm{-NH_2}

  groups and releases nitrogen gas. This reaction is used to destroy urea and to make diazonium salts in organic chemistry.

NC(N)=O.ON=O.ON=O>>N#N.N#N.O=C=O.O.O.O

Making Nitric Acid

In the laboratory, nitric acid is made by heating potassium or sodium nitrate with concentrated sulphuric acid, a less volatile acid, and distilling off the more volatile nitric acid:

\mathrm{KNO_3 + H_2SO_4 \rightarrow KHSO_4 + HNO_3}

.

Industrially it is made by the Ostwald process, which starts from ammonia made by the Haber process. Ammonia and air pass over a platinum–rhodium gauze at about 850–900 °C, which oxidizes the ammonia to nitrogen monoxide. The NO is cooled and reacts with more air to form

\mathrm{NO_2}

. The

\mathrm{NO_2}

is then absorbed in water, and the NO formed in that step is recycled:

\begin{aligned} \ce{4NH3 + 5O2 &->[Pt/Rh] 4NO + 6H2O} \\ \ce{2NO + O2 &-> 2NO2} \end{aligned}

O=[N+][O-].O=[N+][O-].O=[N+][O-].O>>O[N+](=O)[O-].O[N+](=O)[O-].[N]=O

An older method, the Birkeland–Eyde process, combined nitrogen and oxygen directly in an electric arc at about 3000 °C. It used so much electricity that the Ostwald process replaced it.

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Structure and Properties of Nitric Acid

In the

\mathrm{HNO_3}

molecule, a planar nitrogen atom bonds to an OH group and two other oxygens, and the N–O bonds to those two oxygens are equivalent because of resonance. Pure nitric acid is a colourless, fuming liquid. In light it slowly decomposes to

\mathrm{NO_2}

, which colours old samples yellow, so it is stored in brown bottles. It is a strong acid, fully ionized in water, and reacts normally with bases, basic oxides and carbonates to form nitrates.

Nitric Acid as an Oxidizing Agent

The nitrate ion is a powerful oxidizer, so nitric acid oxidizes non-metals to their highest oxides or oxyacids. It converts carbon to

\mathrm{CO_2}

, sulphur to

\mathrm{H_2SO_4}

and phosphorus to

\mathrm{H_3PO_4}

. With metals, nitrogen rather than hydrogen is usually reduced. The product depends on the metal and on the concentration of the acid: the more dilute the acid and the more reactive the metal, the further nitrogen is reduced.

Typical reduction products of nitric acid

Metal and conditions	Main nitrogen product	N oxidation state
Cu, Ag, Hg with concentrated acid	NO₂	+4
Cu with dilute acid	NO	+2
Zn with moderately dilute acid	N₂O	+1
Zn or Sn with very dilute acid	NH₄NO₃	−3
Mg or Mn with very dilute acid	H₂ (unusual)	—

Some metals do not react. Gold and platinum are not attacked at all. Iron, chromium, aluminium, cobalt and nickel are made passive by concentrated nitric acid, which coats them in a thin, protective oxide film.

Aqua regia ("royal water") is a mixture of 3 volumes of concentrated hydrochloric acid to 1 volume of concentrated nitric acid. It dissolves gold and platinum, which neither acid can dissolve alone. Nitric acid oxidizes the metal, while chloride ions trap the metal ions as stable complexes such as

\mathrm{[AuCl_4]^-}

. Removing the ions this way pulls the reaction forward. The mixture also contains reactive nitrosyl chloride and chlorine.

Nitric acid's main uses are making ammonium nitrate fertilizer, explosives such as nitroglycerine and TNT, nylon and other polymers, and dyes, and in etching and cleaning metals.

Tips for remembering

• Nitrous is in the middle, so it goes both ways: nitrogen in HNO₂ is +3, so it can be reduced (oxidizing agent) or oxidized to +5 (reducing agent).
• Metals rarely give H₂ with nitric acid: the nitrate ion is a stronger oxidizer than H⁺, so nitrogen oxides form instead of hydrogen.
• Aqua regia: 3 HCl to 1 HNO₃: nitric acid oxidizes the gold, and chloride traps it as AuCl₄⁻, which pulls the reaction forward.