Mass-Spectrometer and uv Absorption Study of CHF3 Decomposition behind Shock Waves

Abstract

The thermal decomposition of CHF₃ has been studied in excess argon behind incident and reflected shock waves over a temperature range from 1600° to 2200°K at six total pressures between 0.29±0.03 and 28.2±1.6 atm. Direct mass‐spectrometer analysis of the reaction mixture showed HF and CF₂ to be the major decomposition products. Kinetics for the reaction $${\mathrm{CHF}}_3\to {\displaystyle {lim}_{\mathrm{Ar}}}{\mathrm{CF}}_2+\mathrm{HF}$$ were determined spectrophotometrically by observing the rate of formation of CF₂ in absorption at 2536 Å. The decomposition reaction was first order in CHF₃ concentration and dependent on total concentration. The high‐pressure limiting rate constant is given by $$k_{\infty }^{\mathrm{Ar}}{\text{=(7.03±0.70)10}}^{11}\exp \text{[(−58\hspace{0.17em}400±2200)/RT]}{\sec }^{\text{−1}},$$ where 58.4±2.2 kcal/mole is considered to be the minimum energy E₀ for dissociation. For the lowest pressure 0.29 atm, the reaction is interpreted by a second‐order mechanism, $\mathrm{d[}{\mathrm{CF}}_2{\mathrm{]/dt=k}}_2[{\mathrm{CHF}}_3\mathrm{][}\mathrm{Ar}]$ , with the rate constant expressed as $$k_2^{\mathrm{Ar}}{\text{=(2.05±0.14)10}}^{35}{T}^{\text{−5.75}}{\pm }^{\text{0.23}}\exp \text{[(−58\hspace{0.17em}400±2200)/RT]\hspace{0.17em}\hspace{0.17em}}{\mathrm{cc\; mole}}^{\text{−1}}·{\sec }^{\text{−1}},$$ indicating in terms of the classical Kassel theory the participation of 7.25 effective oscillators for the dissociation of a critically energized CHF₃ molecule. Taking E₀=58.4±2.2 kcal/mole as the heat of reaction at 0°K, the heat of formation of the CF₂ radical is −40.2±4.0 kcal/mole.