Anhydrous nitric acid
3D model (JSmol)
|Molar mass||108.01 g/mol|
|Density||1.642 g/cm3 (18 °C)|
|Melting point||41 °C (106 °F; 314 K) |
|Boiling point||47 °C (117 °F; 320 K) sublimes|
|reacts to give HNO3|
|Solubility||soluble in chloroform |
negligible in CCl4
|−35.6·10−6 cm3/mol (aq)|
|planar, C2v (approx. D2h)|
N–O–N ≈ 180°
|178.2 J K−1 mol−1 (s)|
355.6 J K−1 mol−1 (g)
Std enthalpy of
|−43.1 kJ/mol (s)|
+11.3 kJ/mol (g)
Gibbs free energy (ΔfG˚)
|Main hazards||strong oxidizer, forms strong acid in contact with water|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Dinitrogen pentoxide is the chemical compound with the formula N2O5. Also known as nitrogen pentoxide, N2O5 is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It is an unstable and potentially dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by NO2BF4 (nitronium tetrafluoroborate).
N2O5 is a rare example of a compound that adopts two structures depending on the conditions: most commonly it is a salt, but under some conditions it is a polar molecule:
- [NO2+][NO3−] ⇌ N2O5
Syntheses and properties
- P4O10 + 12 HNO3 → 4 H3PO4 + 6 N2O5
- N2O5 + H2O → 2 HNO3
Solid N2O5 is a salt, consisting of separated anions and cations. The cation is the linear nitronium ion NO2+ and the anion is the planar nitrate NO3− ion. Thus, the solid could be called nitronium nitrate. Both nitrogen centers have oxidation state +5.
The intact molecule O2N–O–NO2 exists in the gas phase (obtained by subliming N2O5) and when the solid is extracted into nonpolar solvents such as CCl4. In the gas phase, the O–N–O angle is 133° and the N–O–N angle is 114°. When gaseous N2O5 is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70 °C.
Reactions and applications
- N2O5 + Ar–H → HNO3 + Ar–NO2
where Ar represents an arene moiety.
For this use, dinitrogen pentoxide has been largely replaced by nitronium tetrafluoroborate [NO
4]−. This salt retains the high reactivity of NO2+, but it is thermally stable, decomposing at about 180 °C (into NO2F and BF3). The reactivity of the NO2+ can be further enhanced with strong acids that generate the "super-electrophile" HNO22+.
In the atmosphere, dinitrogen pentoxide is an important reservoir of the NOx species that are responsible for ozone depletion: its formation provides a null cycle with which NO and NO2 are temporarily held in an unreactive state. Mixing ratios of several ppbv have been observed in polluted regions of the night-time troposphere. Dinitrogen pentoxide has also been observed in the stratosphere at similar levels, the reservoir formation having been postulated in considering the puzzling observations of a sudden drop in stratospheric NO2 levels above 50 °N, the so-called 'Noxon cliff'.
Variations in N2O5 reactivity in aerosols can result in significant losses in tropospheric ozone, OH radicals, and NOx concentrations. Two important reactions of N2O5 in atmospheric aerosols are: 1) Hydrolysis to form nitric acid and 2) Reaction with halide ions, particularly Cl-, to form ClNO2 molecules which may serve as precursors to reactive chlorine atoms in the atmosphere.
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