Nitrogen Pentoxide, N₂O₅

Deville first isolated this oxide by decomposing silver nitrate with dry chlorine. Meyer obtained it later from nitric acid by dehydrating with phosphorus pentoxide.

1. Dry chlorine reacts with silver nitrate at 95° C., and as soon as the action has started the mixture is cooled to 50°-60° C. The nitrogen pentoxide evolved is separated from the oxygen by condensing in a U-tube immersed in a freezing mixture. No corks or rubber joints may be used owing to the corrosive action of the gas:
   
   4AgNO₃ + 2Cl₂ = 4AgCl + 2N₂O₅ + O₂.
   
   It is also produced by the reaction between nitryl chloride and silver nitrate:
   
   NO₂Cl + AgNO₃ = AgCl + N₂O₅.
2. The most convenient method is by the dehydration of nitric acid. This is first obtained pure by repeated distillation with concentrated sulphuric acid, and bubbling dry air through the final distillate in order to remove oxides of nitrogen. 200 grams of this "white fuming acid " are put into a 2-litre flask with a side arm at right angles to the neck, and 400 grams of phosphorus pentoxide are slowly added during cooling until pasty. The flask is then attached by means of sealing-wax to three wash-bottles, the first containing glass-wool and phosphorus pentoxide, and the others being empty, and immersed in a freezing mixture of ice and salt. The flask is heated on a water-bath to 60°-70° C. and a slow stream of air (dried by concentrated sulphuric acid) is passed through the paste. The nitrogen pentoxide collects in the wash-bottles as a slightly yellow brittle solid:
   
   2HNO₃ = N₂O₅ + H₂O.
3. Nitrogen trioxide and nitrogen tetroxide are both oxidised by ozone to nitrogen pentoxide.
4. A mixture of nitrogen and oxygen can be converted into nitrogen pentoxide by means of the silent electric discharge in the presence of ozone.

Solid nitrogen pentoxide exists in colourless, transparent rhombic plates, melting-point 29.5° C., and specific gravity 1.63. The yellow colour which develops at the melting-point indicates decomposition, and this latter is increased on further heating: the orange liquid turns dark brown, with considerable evolution of brown fumes. Rapid heating causes explosive decomposition. Nitrogen pentoxide volatilises in dry air, but deliquesces in moist air with the formation of nitric acid.

The vapour pressures at various temperatures are as follows:

Vapour pressure, mm. Hg	13	32	79	183	420	760
Temperature, ° C	-15	-5	+5	+15	+25	+32.5

These values are expressed in the equation



The heats of formation are as follows:

N₂ + 5O = N₂O₅ (gas) - 1,200 calories.
N₂ + 5O = N₂O₅ (liquid)+3,600 calories
N₂ + 5O = N₂O₅ (solid) +11,800 calories
N₂ + 5O = N₂O₅ (aq.) +28,600 calories

The heat of combination with 1 molecule of water is 2.320 Cals., and with excess of water 16.200 Cals. The calculated heat of evapora tion is -4.840 Cals., and heat of fusion -8.280 Cals.

The heats of sublimation in a closed vessel below 30° C. are as follows:

Temperature, ° C.	-10	0	+10	+20
Heat of sublimation (cals.)	12,360	12,760	13,360	14,140

The decomposition of nitrogen pentoxide is a homogeneous reaction, i.e. it proceeds in the gas phase and is independent of catalysis or wall effect. The decomposition proceeds according to the equation -

2N₂O₅ = 2N₂O₄ + O₂,

and is a unimolecular reaction at all temperatures:

t°	65	55	45	35	25	0
K	0.292	0.09	0.0299	0.00808	0.00203	0.0000472
Q (cals.)	-	25,830	22,750	25,370	25,100	24,240

The "heat of activation," Q, is derived from the Arrhenius equation



It is equal to the quantum of energy required to activate each molecule, multiplied by the Avogadro number N:

Q = Nhν

The reaction constant remains that of a unimolecular reaction down to low pressures:

p(N2O5)	278	281	114	51.6	9.6	0.0264	0.00185
K	0.027	0.027	0.026	0.026	0.027	0.020	0.021

The rate is much the same in inert solvents such as chloroform and carbon tetrachloride.

The decomposition of nitrogen pentoxide is retarded by the presence of ozone.

Nitrogen pentoxide is a powerful oxidising agent owing to its decomposition with the liberation of oxygen. Many substances when heated in it burn, such as carbon, phosphorus, and sulphur. The latter element gives rise to white vapours which form a sublimate of the composition S₂O₅(NO₂)₂ - nitrosulphonic anhydride.

The pentoxide is a useful nitrating agent for organic compounds, the reaction being similar to that of a mixture of nitric and sulphuric acids.

Nitric acid dissolves nitrogen pentoxide, and a definite compound, 2HNO₃.N₂O₅, has been obtained which is liquid at ordinary temperatures but solidifies at 5° C.

Sulphur trioxide reacts with nitrogen pentoxide in carbon-tetrachloride solution, with the formation of a crystalline precipitate melting at 124° to 128° C., which is probably (SO₃)₄.N₂O₅: