The Heat of Formation of Boron Trifluoride by Direct Combination of the Elements

An accurate value for the heat of formation of boron trifluoride is of significant importance because this value is involved in the thermochemistry of many boron compounds. The study of the thermochemistry of boron compounds was for a long time hampered by difficulties in measuring a suitable reaction involving elemental boron. The heat of formation of boric oxide, for instance, was uncertain to several kilocalories per mole because of the difficulty of getting comple~e. combustion of the element in oxygen, or of determmmg the amount of reaction, in the absence of complete combustion. The difficulty was apparently due to the glassy and nonvolatile character of the boric oxide formed. which tended to terminate the reaction before completion, and made the analysis ot the Dfoduct a complex problem. The thermochemistry of boron was placed on a firm basis by the work of Prosen, Johnson, and Pergiel [1,2] 2 on the decomposition and hydrolysis of oiborane. and of Johnson. IV1iher. and Prosen [31 on the heat of formation of boron trichloride froHl the dements. With the aid of the heats of these reactions and other data, they obtained reasonably consistent values for B20 3(C), H3B03(c), B2H6(g), and BCla(g). While more recent work has suggested changes in some of the values, these changes have been small.