Nitrogen pentoxide decomposition rate constants

Dinitrogen pentoxide gas decomposes to form nitrogen dioxide and oxygen. The reaction is first-order and has a rate constant of 0.247 h1 at 25°C. If a 2.50-L flask originally contains N205 at a pressure of 756 mm Hg at 25°C, then how many moles of 02 are formed after 135 minutes (Hint. First write a balanced equation for the decomposition.)... 

Rate constants for the first-order decomposition of nitrogen pentoxide (N2O5) at various temperatures are as follows (Alberty and Silbey, 1992, p. 635) ... 

Let s apply the preceding two paragraphs to an example problem. The first-order decomposition of gaseous dinitrogen pentoxide, N205, to nitrogen dioxide, NO2, and oxygen, 02, has a rate constant of 4.9 X 10 4 s 2 at a certain temperature. Calculate the half-life of this reaction. 

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. ... 

It is to be noted that this is like the equation for a zero order reaction (11-12), but the rate is constant, not because it is independent of concentration but because the concentration is kept constant artificially. It was gratifying to find, as expected, that the quantity of crystals of nitrogen pentoxide in the vessel had no influence on the rate. This fact proves that the reaction being measured, is strictly a gas phase reaction and that there is no decomposition in the crystal itself and no catalysis by the crystal surface. 

The time in which a given molecule remained in the tube was estimated by dividing the volume of the tube by the rate of gas flow as determined with a flowmeter. The ratio of nitrites to total acidity gave the ratio N()2/(N2O5+NO2) and hence a measure of the percentage decomposition. The specific decomposition rate k was then calculated for various temperatures and for different pressures. Within the limit of experimental accuracy the specific decomposition rate of nitrogen pentoxide was the same as that determined by the static method, already described, and furthermore, this same constant was maintained down to a few millimeters and even to tenths of millimeters. 

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. 

Another way to determine the instantaneous rate for a chemical reaction is to use the experimentally determined rate law, given the reactant concentrations and the specific rate constant for the temperature at which the reaction occurs. For example, the decomposition of dinitrogen pentoxide (N2O5) into nitrogen dioxide (NO2) and oxygen (O2) is given by the following equation. 

Now let us determine graphically the order and rate constant of the decomposition of nitrogen pentoxide in carbon tetrachloride (CCI4) solvent at 45°C ... 

In the decomposition of nitrogen pentoxide the situation w as even more inexplicable until it w as suggested that the reaction proceeds in steps and that the first-order rate constant must not be identified with a single elementary reaction (Ogg, J. Chem. Phys. 1947,15, 337). 

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