Trihydrides, EH₃ (E=N, P, As, Sb and Bi)

   Each group 15 element forms a trihydride, selected properties of which are given in Table 1.1; the lack of data for BiH₃ stems from its instability. The variation in boiling points ( Table 1.1) is one of the strongest pieces of evidence for hydrogen bond formation by nitrogen. Further evidence comes from the fact that NH₃ has a greater value of Δ_vapH^o and surface tension than the later trihydrides.   Thermal stabilities of these compounds decrease down the group (BiH₃ decomposes above 228 K), and this trend is reflected in the bond enthalpy terms.  Ammonia is the only trihydride to possess a negative value of Δ_fH^o (Table 1.1).

  Ammonia is obtained by the action of H₂O on the nitrides of Li or Mg  by heating [NH₄]‏ salts with base  or by reducing a nitrate or nitrite in alkaline solution with Zn or Al.

Trihydrides of the later elements are best made by method as in below or by acid hydrolysis of phosphides, arsenides, antimonides or bismuthides. Phosphine can also be made as below, [PH₄]I being prepared from P₂I₄.

    The industrial manufacture of NH₃ involves the Haber process (reaction below), and the manufacture of the H₂ required contributes significantly to the overall cost of the process.

   The Haber process is a classic application of physicochemical principles to a system in equilibrium. The decrease in number of moles of gas means that Δ_rS^o(298 K) is negative. For industrial viability, NH₃ must be formed in optimum yield and at a reasonable rate; increasing the temperature increases the rate of reaction, but decreases the yield since the forward reaction is exothermic. At a given temperature, both the equilibrium yield and the reaction rate are increased by working at high pressures; the presence of a suitable catalyst also increases the rate; the rate determining step is the dissociation of N₂ into N atoms m chemisorbed onto the catalyst. The optimum reaction conditions are T = 723 K, P = 202 600 kPa, and Fe₃O₄ mixed with K₂O, SiO₂ and Al₂O₃ as the heterogeneous catalyst; the Fe₃O₄ is reduced to give the catalytically active α-Fe. The NH₃ formed is either liquefied or dissolved in H₂O to form a saturated solution of specific gravity 0.880.

   Ammonia is a colourless gas with a pungent odour; Table 1.1 lists selected properties and structural data for the  trigonal pyramidal molecule the barrier to inversion for which is very low (24 kJ mol^-1). Oxidation products of NH₃ depend on conditions. Reaction below occurs on combustion in O₂, but at ≈1200K in the presence of a Pt/Rh catalyst and a contact time of ≈1 ms, the less exothermic reaction  takes place. This reaction forms part of the manufacturing process for HNO₃ .

  The solubility of NH₃ in water is greater than that of any other gas, doubtless because of hydrogen bond formation between NH₃ and H₂O. The equilibrium constant (at 298 K) shows that nearly all the dissolved NH₃ is non-ionized, consistent with the fact that even dilute solutions retain the characteristic smell of NH₃. Since K_w = 10^-14, it follows that the aqueous solutions of [NH₄]⁺‏ salts of strong acids (e.g. NH₄Cl) are slightly acidic.

   Ammonium salts are easily prepared by neutralization reactions. Industrial syntheses are carried out using the Solvay process both ammonium sulfate and nitrate are important fertilizers, and NH₄NO₃ is a component of some explosives

    Detonation of NH₄NO₃ may be initiated by another explosion, and ammonium perchlorate is similarly metastable with respect to oxidation of the [NH₄]‏⁺ cation by the anion; NH₄ClO₄ is used in solid rocket propellants, e.g. in the booster rockets of the space shuttle. ‘Technical ammonium  carbonate’ (used in smelling salts) is actually a mixture of [NH₄][HCO₃] and [NH₄][NH₂CO₂] (ammonium carbamate); the latter is prepared by passing NH₃, CO₂ and steam into a lead chamber, and smells strongly of NH₃ because carbamic acid is an extremely weak acid (scheme below). Pure   carbamic acid (H₂NCO₂H) has not been isolated; the compound dissociates completely at 332 K.

  Ammonium salts often crystallize with lattices similar to those of the corresponding K‏, Rb‏ or Cs‏ salts. The [NH₄]⁺‏ ion can be approximated to a sphere with r_ion = 150 pm, a value similar to that of Rb‏.