Kinetics and Equilibria of the Difluorocarbene Radical Decomposition Behind Shock Waves

Abstract

The thermal decomposition of the difluorocarbene radical diluted in argon has been studied behind incident shock waves over the temperature range from 2600° to 3700°K at a total pressure around 0.5 atm. The progress of the reaction has been followed by monitoring the time rate of change of CF₂ molecules in absorption at 2536 Å. The observed rate data for the disappearance of CF₂ are interpreted by the dissociation—recombination mechanism, $$\mathrm{-d[}{\mathrm{CF}}_2{\mathrm{]/dt=k}}_f[\mathrm{Ar}\mathrm{][}{\mathrm{CF}}_2{\mathrm{]-k}}_r[\mathrm{CF}\mathrm{][}\mathrm{F}\mathrm{][}\mathrm{Ar}\mathrm{],}$$ with $$\begin{array}{c}{k}_f{\text{=(4.20±0.67)10}}^{26}{T}^{\text{−2.85±0.62}}\exp \text{\hspace{0.17em}(−106\hspace{0.17em}000±5700/RT)\hspace{0.17em}}\mathrm{cc}/\mathrm{mole}·\sec ,\\ k_r{\text{=(6.57±1.79)10}}^{26}{T}^{\text{−2.85±0.62}}{\mathrm{cc}}^2/{\mathrm{mole}}^2·\sec .\end{array}$$ Experimental equilibrium constants based on the reaction, CF₂=CF+F, are calculated and fitted by the van't Hoff equation, yielding $$\log K_c=\frac{\text{−103\hspace{0.17em}000±5700}}{\text{2.303\hspace{0.17em}RT}}{\text{−0.41±0.11\hspace{0.17em}(K}}_c\hspace{0.17em}\mathrm{in\; mole}/\mathrm{cc}\mathrm{),}$$ for the temperature interval between 2600° and 3500°K. From the heat of reaction (107.3±5.7 kcal/mole) and the heat of formation of CF₂ (−39.6±3 kcal/mole) at 298°K, the heat of formation of CF at 298°K is estimated to be 49.2±8.7 kcal/mole.