Nitrogen pentoxide decomposition activation energy

In a purely photochemical reaction the absorption of radiant energy is plainly responsible for the activation. This suggested the possibility that thermal reactions are also due to activation by the thermal radiation which is present at every temperature. The argument was very forcibly presented by Perrin who showed that if the specific rate of a imimolecular gas reaction remains constant, with indefinite diminution in pressure, activation must be by radiation since the number of opportunities for activation by collision also diminishes without limit. In fact, the decomposition of nitrogen pentoxide, the first gas reaction shown to be unquestionably unimolecular, was found to have a specific reaction rate constant over a wide range of pressure, and apparently increasing at very low pressures. ... 

In spite of the large amount of work which has been done on nitrogen pentoxide, it is planned to carry out a still more precise measurement of the decomposition rate in the gas phase. The constancy of the energy of activation at different temperatures is a matter of great theoretical importance. Although few gas reactions in chemical kinetics are more accurately known, the present meas-... 

The simplest reaction which has been studied directly in the gas phase and in solution is the decomposition of nitrogen pentoxide.11 It is not a chain reaction and it is free from wall effects. The gas phase reaction seems to be free from complications and it has been checked in many laboratories. It is an excellent unimolec-ular reaction, the decomposition rate being exactly proportional to the concentration. This proportionality constant is nearly the same from 0.05 mm. to 1,000 mm. in the gas phase and up to an osmotic pressure of fifty atmospheres in solution, and the energy of activation is practically the same in the gas phase and in a group of chemically inactive solvents. 

In other systems, the solvent only acts as an inert medium where reaction occurs without affecting the mechanism. This appears to be the case of the decomposition of nitrogen pentoxide, in which the pre-exponential factor and the activation energy are very similar in the gas phase and in solvents of various types, as shown in Table 9.1. 

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