Density ratios across shock waves in a 0.85 Kr+0.15 C2N2mixture at an initial pressure of 50 mm Hg and room temperature, have been determined with an xhyphen;ray densitometer as a function of shock velocity. The heat required to dissociate cyanogen into two CN radicalsD(C2N2) has been determined to be 145plusmn;6 kcal/mole by comparing the experimental data with curves of density ratio vs shock velocity calculated as a function ofD(C2N2). Dissociation energies of 174plusmn;3 kcal/mole for CN and 129plusmn;3 kcal/mole for HCN forming H and CN, and a heat of formation of 109plusmn;3 kcal/mole for CN, were obtained by the application of Hess's law to the appropriate chemical reactions using this value ofD(C2N2) and the currently accepted values for the dissociation energy of nitrogen (225 kcal/mole) and the heat of sublimation of graphite (170 kcal/mole). The value ofD(HCN) was confirmed by analogous densityhyphen;velocity measurements on shock waves in a 0.85 Kr+0.15 HCN mixture. A rate constant for the recombination of CN to form C2N2at 2900deg;K was deduced from the variation of density with time behind the shock. The value obtained was of the order of 1times;109(mole/liter)mdash;2secmdash;1.
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