Bond dissociation energies from equilibrium studies. Part 2.—D(CF3—CF3) and enthalpy of formation of C2F6

Abstract

Equilibrium constants have been measured for the system C₂F₆+Br₂⇌2CF₃Br (2) over the range 621–722°C. Equilibrium was approached from both sides. Using a third-law method ΔH^°₂= 3.66 ± 0.14 kcal mole^–1 at 298°K. This is combined with results from our previous work to give D(CF₃—CF₃)= 96.5 ± 1.0 kcal mole^–1. We also obtain ΔH^°_f(C₂F₆)–2 ΔH^°_f(CF₃H)= 12.0 ± 0.6 kcal mole^–1(10) and the relevance of this result to published enthalpies of formation of C₂F₆, CF₃H and HF,aq. is discussed. There are now six determinations of ΔH^°_f(HF,aq.) which indicate that the value in N.B.S. circ. 500 needs revision. A revised value is proposed and is used to obtain ΔH^°_f(CF₃H). When the latter figure is combined with ΔH^°_f(C₂F₆), the result is in excellent agreement with that in eqn. (10). Our value of D(CF₃—H) is combined with enthalpies of formation to give ΔH^°_f(CF₃)=–112.6 ± 1.2, D(CF₃—F)= 129.3 ± 2.0 and D(CF₃—CH₃)= 99.7 ± 2.0 kcal mole^–1.