Aqueous ammonia

Ammonia (NH₃) is a colorless flammable gas (boiling point, -33.34 °C; melting point, -77.73 °C, critical temperature 132.22 °C, critical pressure 11.345 MPa, critical density 234.3 kg ˣ m^-3; [3685 ]) with a characteristic unpleasant and toxic odor. It is polar with dipole moment 1.5 D. In aqueous solution, it acts as a weak base and may be used as a general-purpose cleaner. Ammonia–water complexes, where NH₃ molecules are are weakly hydrogen-bonded to surface water molecules are found on the surface of aqueous ammoniacal solutions [3687 ].

Ammonia is very soluble in water (it is the most soluble gas) where it reacts to form ammonium ions and hydroxide ions at appropriate pH. A saturated solution is highly corrosive and contains about 0.31 kg ammonia per kg solution at 25 °C, and has a density of about 0.88 g ˣ ml^-1 (> 13 M). Solutions give off ammonia gas and concentrations cannot be considered as precise. These species are shown in the above right figure ^a where all the hydrogen atoms in both structures are equivalently positioned. The ammonia molecule may invert (like a wind-blown umbrella, see also the hydrogen ion). This inversion has an energy barrier of about 24 kJ ˣ mol^-1 [3686] but in contrast to the H₃O⁺ ion, the inversion is prevented by acceptor hydration at the nitrogen atom. The thermodynamic properties of ammonia-water mixtures have been reported [IAPWS, 3684 ].

H₂O + NH₃ OH^- + NH₄⁺ pK_{b (NH3)} = 4.755 at 25 °C [3681]

H₂O + NH₄⁺H₃O⁺ + NH₃ DH° = +51.92 kJ ˣ mol^-1 at 25 °C [3680 ]

pK_{a (NH4+)}= -Log₁₀([H₃O⁺] ˣ [NH₃]/ [NH₄⁺]) = 9.245 at 25 °C

pH = 9.245 + Log₁₀([NH₃]/ [NH₄⁺]) at 25 °C

The relationship of pK_{a (NH4+)} to temperature is pK_{a (NH4+)} = 0.09018 + 2729.92/T where T is in Kelvin [3681 ].

Ammonium (NH₄⁺) is a very weak acid, with an equilibrium constant (5.7 ˣ 10^-10 M) approximately 70 times lower than physiological (buffered) hydrogen ion concentration (4.0 ˣ 10^-8 M), so that when ammonia is produced in the body, it is immediately mostly converted (~98 %) to ammonium ions, but when added to pure unbuffered water it mostly remains as dissolved NH₃.

Ammonia is a reducing agent and can be produced by the electro-synthesis of ammonia [3871]:

N₂ + 6 H⁺ + 6 e^-2 NH₃ E° = +0.55 V

N₂ + 8 H⁺ + 6 e^- 2 NH₄⁺ E° = +0.27 V

Ammonia may be oxidized 4 NH₃ + 3 O₂ 2 N₂ + 6 H₂O

H3N···H-OH hydrogen bond in the gas phase — H₃N···H-OH hydrogen bond in the gas phase

cis- and trans- structures of the H3N···H-OH hydrogen bond — cis- and trans- H₃N···H-OH hydrogen bonds

When attached to the water molecule, both the ammonia molecule and the water molecule lose some of their symmetry and the ammonia molecule cannot invert. The two very shallow energy minima cis- and trans- structures of the H₂N-H ···OH₂ hydrogen bond in the gas phase are given on the left. The energy difference (0.14 J ˣ mol^-1) is not significant when compared with that of the hydrogen bond (25 kJ ˣ mol^-1) and the thermal energy (2.5 kJ ˣ mol^-1) leaving the H₂O and NH₃ molecules to freely rotate around the hydrogen bond. The (partially constrained) molecular parameters have also been determined with the CCSD(T)-F12A method and the VTZ-F12 basis sets with similar results. [3682 ]

H3N+-H···H2OH2 hydrogen bond in the gas phase — H₃N⁺-H···OH₂ hydrogen bond in the gas phase

In aqueous solution, both NH₃ and NH₄⁺ are hydrated. NH₄⁺ can form four strong, tetrahedrally-placed and long-lived, donor hydrogen bonds to H₂O molecules [136] (see right, N-H, 1.01686 Å, N···O, 2.90274 Å). This structure is found to be much more stable than a cluster with an NH₃in the center and an H₃O⁺ on the surface [194]. NH₃ only forms one moerately-strong acceptor hydrogen bond (see the NH₃ ···OH₂ cis- trans- figure, above left) with the three much weaker donor H₂N-H···OH₂ (ultraweak) hydrogen bonds losing out to much stronger HO-H···OH₂ interactions [3688]. In the magic ion NH₄⁺(H₂O)₂₀, the NH₄⁺ ion sits centrally in the water dodecahedron with the four hydrogen bonds from the central ammonium ion equivalent (see the connectivity map), so helping to explain the apparent increased structural stability of this ion relative to H₃O⁺(H₂O)₂₀. Exchange of hydrogen bond partners by the central NH₄⁺ explains its faster than expected rotation [855 ].

Note that a single H₂O hydrogen-bonded NH₄⁺ ion (H₃N⁺-H···OH₂) forms one of the strongest hydrogen bonds known at 92.5 kJ ˣ mol^-1 [447 ]. In this structure, the hydrogen bond H₃N⁺-H···OH₂ shrinks by 0.15 Å (and the N-H covalent bond expands by 0.018 Å) between the tetra-coordinated and the single-coordinated NH₄⁺, in line with the stronger bonding of the latter.

Under high pressures, such as occur in large planetary systems like Neptune and their moons such as Titan, Callisto, and Ganymede, mixtures of water and ammonia may form a complex phase diagram with many crystal forms [3683 ]. The multiplicity of these is not only due to same reasons as for the complexity of the water phase diagram but plus the additional complexity introduced by the multi-component-system present. A total of twelve mixed phases exist containing both NH₃ and H₂O molecules under various pressure–temperature conditions; ammonia monohydrate (NH₃:H₂O = 1:1, six phases), ammonia dihydrate (NH₃:H₂O, 1:2, four phases), and ammonia hemihydrate (NH₃:H₂O, 2:1, two phases) [3861 ].

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Footnotes

^a The molecular structures on this page were calculated in the gas phase, using the Restricted Hartree-Fock wave function (RHF) using the 6-31G** basis set. This is in line with other calculated structure factors given in this website (e.g., the values for the water molecule and its clusters). [Back]