Ammonia: Structure, Preparation and Properties

Ammonia, $\text{NH}_3$, is by far the most important hydride of nitrogen. Structurally, the

nitrogen atom is $sp^3$ hybridised: three of its four hybrid orbitals form $\sigma$ bonds to the

three hydrogen atoms, and the fourth holds nitrogen's lone pair. Because a lone pair occupies more

angular space than a bonding pair (it is held by only one nucleus, so it spreads out more), the

lone pair pushes the three N-H bonding pairs closer together, compressing the ideal tetrahedral

angle of $109.5^\circ$ down to an experimentally measured $\text{H-N-H}$ angle of about

$107.8^\circ$. The resulting shape is a trigonal pyramid, with nitrogen at the apex and the

three hydrogens forming the triangular base -- not flat/planar, precisely because that lone pair

occupies real space on the far side of the nitrogen atom from the hydrogens.

Ammonia is manufactured industrially by the Haber process, the direct combination of nitrogen

and hydrogen over an iron catalyst (promoted with a small amount of molybdenum) at a high

pressure (around 200 atm) and a moderate temperature (around 720 K), since the forward reaction

is exothermic and favoured by low temperature but the reaction rate is workably fast only at

elevated temperature -- a compromise condition:

In the laboratory, ammonia is prepared far more simply by heating an ammonium salt with a base,

typically ammonium chloride with slaked lime:

Ammonia is a colourless gas with a sharp, pungent, characteristic smell, and it is exceptionally soluble in water (about 700 volumes of gas dissolve in one volume of water at room temperature)

because it readily hydrogen-bonds with water molecules and also because of its high dipole

moment. In aqueous solution it behaves as a weak base (traditionally written as ammonium

hydroxide, $\text{NH}_4\text{OH}$, though the equilibrium is better represented as

$\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$), turning red

litmus blue. Its lone pair also makes ammonia an excellent Lewis base / ligand: it readily

donates its lone pair to a wide range of metal cations to form ammine complexes (e.g.

$[\text{Cu}(\text{NH}_3)_4]^{2+}$, deep blue), and it reacts with acids to form ammonium salts

(e.g. $\text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl}$, seen visibly as dense white …