The dissociation energy of the N-N bond in hydrazine

The thermal decomposition of hydrazine was investigated over a temperature range of 630 to 780 degrees C and at pressures of a few mm. Hg. The experiments were carried out in a flow system, toluene being used as a carrier gas. This technique makes it possible to discriminate between the heterogeneous decomposition of hydrazine 3N$_{2}$H$_{4}$$\rightarrow $N$_{2}$+4NH$_{3}$, $\quad \quad (1)$ and 2N$_{2}$H$_{4}$$\rightarrow $H$_{2}$+N$_{2}$+2NH$_{3}$ $\quad \quad (2)$ and the homogeneous decomposition to N$_{2}$H$_{4}$$\rightarrow $2NH$_{2}\boldsymbol{\cdot}$. $\quad \quad (3)$ The NH$_{2}$ radicals produced by the latter process were removed, in the presence of excess toluene, by the rapid reaction (4) C$_{6}$H$_{5}$.CH$_{3}$+NH$_{2}$$\rightarrow $C$_{6}$H$_{5}$.CH$_{2}\boldsymbol{\cdot}$+NH$_{3}$. $\quad \quad (4)$ Thus the rate of formation of dibenzyl measures the rate of reaction (3). The study of the stoichiometry of the overall process, of the kinetics of various steps and of the effect of packing the reaction vessel led to the conclusion that (3) is a homogeneous, unimolecular gas reaction, the rate constant being 4 $\times $ 10$^{12}$ exp (60,000/RT). Assuming that the recombination of NH$_{2}$ radicals does not involve any energy of activation, it is found that D(NH$_{2}$-NH$_{2}$) = 60 $\pm $ 3 kcal./mole. This value in conjunction with the relevant thermochemical data leads to the heat of formation of NH$_{2}$ radical as 41 kcal./mole and to D(NH$_{2}$-H) = 104 $\pm $ 2 kcal./mole. The latter is good evidence in support of Gaydon's value of 225 kcal./mole for the heat of dissociation of N$_{2}$.