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.