The heat of formation of water

69464CHEMISTR Y: F. D. ROSSINI

PROc. N. A. S.

THE HEAT OF FORMATION OF WATER1'2 By FREDERICK D. RossINI NATIONAL BUREAU OF STANDARDS, WASINGTON, D. C. Read before the Academy September 19, 1930

As part of the program of thermochemical research now being carried on at the National Bureau of Standards, the heat of formation of water has been redetermined. The importance of this thermal quantity lies in its direct use in the calculation of the heat of formation of practically all organic and of many inorganic compounds, and in its indirect use in thermodynamic calculations for chemical reactions involving H20. During the past century there have appeared fourteen papers reporting measurements of the heat of formation of water. Up to the present time the usually accepted value3 has been based upon the measurements of Thomsen4 in 1873, Schuller and Warthal in 1877, and Mixter6 in 1903. Of these, the data of Schuller and Wartha are the most reliable, yet even their average value has attached to it an uncertainty of some 8 parts per 10,000. The method employed in this investigation was as follows: The apparatus consisted of a calorimeter surrounded by a constant temperature jacket. The calorimeter was composed of a calorimeter can containing a measured mass of water, a device for measuring changes of temperature, a stirring mechanism, a reaction vessel for burning the gases at constant pressure and a heating coil. The problem was to determine the quantitative correspondence between the thermal energy liberated (1) by the reaction between hydrogen and oxygen to form a measured mass of water and (2) by a measured quantity of electrical energy, using the calorimeter system as the absorber of the two quantities of energy and its temperature rise as the comparator. A combustion experiment, in which m grams of water were formed in the reaction vessel, produced a temperature rise of At degrees in the calorimeter at some average temperature ti. In a calibration experiment, the same temperature rise of At degrees, at the same average temperature ti, was brought about in the same calorimeter by a measured quantity of electrical energy, E joules. In this manner, the formation of m grams of water was found to be equivalent, in energy evolved, to E joules of electrical energy. The supreme advantage of this method lay in the fact that the accuracy of the value obtained for the heat of formation of water depended only upon the determination of the mass of water formed and of the quantity of electrical energy. This required accurate standards of mass, and of the ohm, the volt and the second, all of which were available at this bureau. High precision was obtained by the use of proper calorimetric technic,

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VOL. VO.6,190CHEMISTRY: F.D. ROSSINI 16, 1930

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a sensitive platinum resistance thermometer for measuring changes of temperature, a sensitive potentiometric system for measuring the electrical power input, a precision timing device and a suitable balance for determining the mass of water formed. Figure 1 shows the assembly of the apparatus. A, B, C and D comprise the gas purifying system; E, F are flow meters; G shows a group of stopcocks; J, H are inlet tubes; K is the reaction vessel and support; L, the ~~~~~~~~~A

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