Nitric Acid: Structure and Preparation

Nitric acid, $\text{HNO}_3$, is manufactured on an industrial scale from ammonia by the

Ostwald process, a three-stage catalytic oxidation. First, ammonia is catalytically oxidised

by air over a platinum-rhodium gauze catalyst at about 500 K and moderate pressure:

The nitric oxide so formed is then further oxidised by more air, on cooling, to nitrogen dioxide:

Finally the $\text{NO}_2$ is absorbed in water, giving nitric acid and regenerating some NO

(which is recycled back into the second stage):

The dilute acid obtained this way (around 68% by mass) can be concentrated by distillation with

concentrated sulphuric acid, which acts as a dehydrating agent. In the laboratory, nitric acid is

prepared on a small scale by heating potassium or sodium nitrate with concentrated sulphuric

acid:

Structurally, the nitrogen atom in $\text{HNO}_3$ is $sp^2$ hybridised, giving the molecule a

planar (trigonal, roughly $120^\circ$ bond angles) shape. Nitrogen forms one ordinary single bond

to the $-\text{OH}$ oxygen and is bonded to the other two oxygens through a resonance-delocalised

system: one canonical structure shows a $\text{N=O}$ double bond and a $\text{N-O}^-$ single bond

carrying a formal negative charge, while a second, equally valid canonical structure has these two

assignments reversed. Because both structures contribute equally, the true molecule is a

resonance hybrid, and experimentally the two non-hydroxyl N-O bonds (each about 122 pm) are found

to be of equal length, intermediate between a typical N-O single bond (~136 pm) and a typical

N=O double bond (~115 pm) -- direct

experimental evidence that the true bonding is delocalised rather than one fixed double bond and

one fixed single bond.

Chemically, $\text{HNO}_3$ is a strong, monobasic acid, fully ionising in water to give

$\text{H}_3\text{O}^+$ and $\text{NO}_3^-$. It is also a powerful oxidising agent, particularly

in its concentrated, hot form, oxidising many metals (even relatively unreactive ones like

copper) and non-metals; the specific reduction product ($\text{NO}_2$, $\text{NO}$, $\text{N}_2\text{O}$,

or even $\text{NH}_4^+$) depends on the concentration of the acid and the reducing strength of

the substance being oxidised. A well-known consequence of this oxidising power is that …