Ammonia

Preparation

Small amounts of ammonia are present naturally in air and soil, produced by the decay of nitrogen-containing organic matter such as urea:

On a small scale, ammonia is generated by treating an ammonium salt with a strong base such as caustic soda or calcium hydroxide, which displaces the free gas:

Industrially, ammonia is manufactured almost exclusively by the Haber process:

Since this reaction is exothermic and proceeds with a decrease in the number of gas moles, Le Chatelier's principle predicts that high pressure favours the forward reaction. In practice, the optimum industrial conditions are a pressure of about $200 \times 10^5$ Pa (roughly 200 atm), a temperature near 700 K, and a catalyst of iron oxide promoted with small amounts of $K_2O$ and $Al_2O_3$ to speed up the approach to equilibrium (earlier plants used plain iron as the catalyst with molybdenum as the promoter).

The flow diagram for this process shows a closed recycling loop: streams of $H_2$ and $N_2$ are compressed together (to about 20 MPa) and fed into the catalyst chamber, where iron oxide promoted with $Al_2O_3$ and $K_2O$ converts part of the mixture to ammonia at around 700 K. The gas leaving the catalyst bed — a mixture of unreacted $N_2$, $H_2$ and the newly formed $NH_3$ — passes into a cooled vessel fitted with a coiled condenser, where the ammonia liquefies out as liquid $NH_3$ at the bottom while the unreacted $N_2$ and $H_2$ are pumped back up and recycled into the incoming compressed feed, so the cycle repeats continuously with nothing wasted.

Properties

Ammonia is a colourless gas with a sharp, pungent odour, freezing at 198.4 K and boiling at 239.7 K. As in water, extensive hydrogen bonding links the molecules together in the solid and liquid states, which is why its melting and boiling points run higher than its modest molecular mass alone would suggest.

The $NH_3$ molecule is trigonal pyramidal, with the nitrogen atom sitting at the apex above three hydrogen atoms; nitrogen carries three bond pairs and one lone pair of electrons, and this lone pair occupies the position where a fourth bond would otherwise sit, pushing the molecule into its pyramidal shape (the $N$–$H$ bond length is 101.7 pm and the $H$–$N$–$H$ bond angle is 107.8$^\circ$, slightly compressed from the ideal tetrahedral angle by the lone pair's greater repulsion).

Ammonia gas dissolves very readily in water, and the resulting solution is only weakly basic, owing to the formation of hydroxide ions:

With acids it forms ammonium salts such as $NH_4Cl$ and $(NH_4)_2SO_4$. As a weak base it will precipitate the hydroxides (or, for some metals, the hydrated oxides) of many metal ions out of their salt solutions — for example:

$FeCl_3(aq) + NH_4OH(aq) \rightarrow Fe_2O_3.xH_2O(s)\downarrow_{\text{brown}} + NH_4Cl(aq)$ …