Ethane high-pressure rate constants

The unimolecular decomposition of azomethane, CH3—N=N—CH3, to ethane and nitrogen has been extensively studied as a function of temperature and pressure. The high-pressure rate constant is... 

In pure ethane the initial first-order rate constants fall off with decreasing pressure. Sacchse, loc, city found that at about 38 mm Hg the rates were about half their high-pressure limiting values. This is of course quite reasonable in terms of the proposed mechanism. 

The pressure dependence of kg/ku at X = 2288 A. can be accounted for by the observation that at this wavelength small amounts of disulfides are formed in the ethane and propane reactions, which were not taken into consideration evaluating the rate constant values. The disulfide can arise (1) from the secondary photolysis of the mercaptan product when the COS pressure is low, and 2) by cracking of hot RSH molecules at low total pressures. From Table III there appears to be a twofold variation in the value of h/kn for the ethane reaction, in going from low COS and total pressure to high COS and total pressure. 

Many statistical models have been applied to reaction (3.1), and it might be considered a test case for theoretical treatments of the rate constant. The process inverse to (3.1), the dissociation of ethane, has also been extensively studied experimentally25,26 and theoretically.116,2 2,27 The theoretical predictions for the rate of dissociation are, of course, quite sensitive to the value of the bond dissociation energy. On the other hand, recombination rates depend only weakly on that quantity. In the present review, attention is focused on the prediction of the recombination rate using the transition state theory outlined in Section IIC. First, the high-pressure limit of kr, denoted by kK, is considered, particularly its temperature dependence. This is followed by a brief description of some results for the pressure dependence of kr and for the dissociation of a vibrationally excited C2H6 molecule. 

G. Transition state theory for unimolecular reactions. In the high-pressure limit one can assume that the energy-rich species A has reached thermal equi-libriiun. (a) Verify the TST result for the rate of unimolecular dissociation k(T) = (ytBr/A)(gV0exp(— o)wheregis the partition function forAand 0 is the partition function for the transition state, (b) This result looks just like the TST expression for the bimolecular thermal reaction rate constant. But this cannot be. A imimolecular reaction rate constant has different dimensions from a bimolecidar one. Resolve this dilemma, (c) The thermal dissociation of ethane. 

---