The Heat of Dissociation of Fluorine

Equilibrium pressure measurements of the dissociation of F2 have been completed with the use of (1) a nickel vessel coated stepwise by heating in contact with fluorine at temperatures up to 850°C and (2) a differentially applied manometer containing a fluorinated oil. This latter technique made it possible to carry out the measurements in the comparatively low temperature range 759° to 1115°K. The method consisted essentially in a direct comparison of the pressure variation with temperature of a quantity of fluorine with that of a quantity of nitrogen. Twenty-four determinations of the equilibrium constant were made, and in a plot of lnK as a function of 1/T, the points gave a straight line. The slope gave ΔH=37.7 kcal/mole as the mean heat of dissociation in the temperature range of the experiments for the reaction F2 (g)→2F (g). This ΔH value is in good agreement with values calculated from entropy quantities and values of lnK obtained from the line.