Decomposition and Oxidation of C2F4 Behind Shock Waves

Abstract

Ultraviolet absorption and mass‐spectrometric observations of shocked C₂F₄–Ar gas mixtures have indicated that the C₂F₄ monomer exists in thermal equilibrium with CF₂ radicals and that the initial C₂F₄ oxidation mechanism involves two steps $$\begin{array}{c}{\mathrm{C}}_2{\mathrm{F}}_4\to {\displaystyle {lim}_{\mathrm{Ar}}}2{\mathrm{CF}}_2\\ {\mathrm{CF}}_2+{\mathrm{O}}_2\to \mathrm{CO}\text{+2}\mathrm{F}+\mathrm{O}.\end{array}$$ Dissociation and oxidation rates were determined from uv absorption measurements of CF₂ at 2536 Å behind incident shock waves. The C₂F₄ dissociation reaction appeared to be second order, $\frac{1}{2}\mathrm{d[}{\mathrm{CF}}_2{\mathrm{]/dt=k}}_{\mathrm{Ar}}[{\mathrm{C}}_2{\mathrm{F}}_4\mathrm{][}\mathrm{Ar}]$ , with $$k_{\mathrm{Ar}}{\text{=7.82×10}}^{15}{T}^{\frac{1}{2}}\hspace{0.17em}\exp \text{(−55\hspace{0.17em}690/RT)\hspace{0.17em}}\mathrm{cc}/\mathrm{mole}·\sec$$ over the temperature interval from 1200° to 1800°K at total concentrations between 0.9×10⁻⁵ and 1.7×10⁻⁵. A least‐squares fit to the experimental equilibrium constants for the reaction C₂F₄=2CF₂, using the integrated form of the van't Hoff equation, yielded $$\log K_c{\text{=−69\hspace{0.17em}432/2.303RT+4.62\hspace{0.17em}(K}}_c\hspace{0.17em}\mathrm{in}\hspace{0.17em}\mathrm{mole}/\mathrm{cc}\mathrm{).}$$ From the measured heat of reaction, the heat of formation of CF₂ at 298°K was calculated to be −39.7±3.0 kcal/mole. Mixtures of C₂F₄ and O₂ in varying mole ratios were allowed to react at temperatures between 1500° to 2500°K with molar concentrations of reactants ranging from 4×10⁻⁸ to 6×10⁻⁷ mole/cc. The oxidation reaction was observed to be first order in both CF₂ and O₂, $\mathrm{d[}{\mathrm{CF}}_2{\mathrm{]/dt=-k}}_{\mathrm{ox}}[{\mathrm{CF}}_2\mathrm{][}{\mathrm{O}}_2]$ , with $$k_{\mathrm{ox}}{\text{=2.92×10}}^{10}{T}^{\frac{1}{2}}\hspace{0.17em}\exp \text{(−13\hspace{0.17em}280/RT)\hspace{0.17em}}\mathrm{cc}/\mathrm{mole}·\sec .$$