note H2Oa | Rossini 1939 gives an enthalpy of formation of H2O of -285795 +- 40 int. J/mol for the then current molecular weight of H2o (18.0162). Rossini 1940 restates the exact same result. The reinterpretation detailed below results in a very slightly different value of -285793.8 +- 40 int. J/mol. The value -285825 +- 40 J/mol can be recalculated from the reinterpreted value using the current molecular weight of H2O (18.01528) and converted to current (=absolute) J/mol. The value of Rossini 1939, if corrected for the present molecular weight and converted to abs. J/mol would differ very little, -285826 +- 40 J/mol. Note that the value given by CODATA Key Vals is slightly higher (round-off?): -285830 +- 40 J/mol. The details of the recalculations are as follows. Rossini 1931 reports for H2 + 1/2 O2 -> H2O an enthalpy of formation at 25° C of 285755 +- 40 int. J/mol using m.w. of H2O 18.0156 g and commenting that for small changes in T and p the enthalpy changes -32 J/mol per deg inrease in T (correct) and -5 J/mol per mm Hg increase in p (incorrect). Rossini 1931b reports correctly that the enthalpy is pressure independent and that Rossini 1931 contains incorrect corrections; in particular the Schuller and Warta pressure corrections are wrong. Hence, as per Rossini 1931a, the averages appearing in Rossini 1931 should be changed thus: Set I Table 2 285749 +- 53 -> 285756 +- 43 int. J/mol, and Set II Table 4 285781 +- 28 -> 285788 +- 23 int. J/mol. While Rossini 1931b also claims that the final "best" value of Rossini 1931 does not change, Rossini 1939 tacitly implies that it did change, i.e. reports -285795 +- 40 int. J/mol after a -3.6 J/mol for non-ideality and -9.5 J/mol correction for new m.w. 18.0162 g. Hence, Rossini 1931 must have been taken as ~ 285782 int. J/mol, rather than the originally reported ~ 285775 int. J/mol. Reworking Rossini 1931 nine values in Table 2 (Set I) and leaving out the p correction produces a mean value of 285756.0 (285749.3) int. J/mol and a mean deviation of 43.3 (53.1) int. J/mol where values obtained by repeating the original calculations with the erroneous p correction are given in parentheses. Following further as in the original, the 95.45% error is 36.6 (44.1) int. J/mol or 0.012791 % (0.015425 %). The percentage error of calibration, 0.013% can now be propagated to produce a total error of 0.018323 % (0.020172 %), which results in an error of 52.1 (57.6) int. J/mol. Reworking Rossini 1931 nine values in Table 4 (Set II) in a similar way produces a mean value of 285787.9 (285781.2) int. J/mol and a mean deviation of 23.9 (28.3) int. J/mol, the 95.45% error is 19.8 (24.9) int. J/mol or 0.006927 % (0.008710 %). The percentage error of calibration, 0.007 % can now be propagated to produce a total error of 0.009848 % (0.011174 %), which results in an error of 28.1 (31.9) int. J/mol. Continuing as in Rossini 1931 Table 5, the "best value" by weighted average is now 285780.7 (285773.7, cf. to stated "best" of 285775 +- 34 int. J/mol which is slightly different most likely because of round-off in interim results by Rossini). The origin of the final error bar is not entirely clear, but it was most likely assigned by Rossini by considering the deviations of the means of Set I and Set II from the "best value", -28 and +6 int. J/mol, and taking twice the absolute value of the mean of these two deviations, which is 34 int. J/mol. Repating this same approach with our reworked values leads to deviations of the means of Set I and II from the "best" of -24.7 (-24.4) and 7.2 (7.5) int. J/mol, leading to twice the average of 31.9 (31.9). Note that the actual std. dev. is 13.3 (13.5) int. J/mol. Rossini 1931 in the end uses a slightly larger ("liberal", in his own words) uncertainty of +- 40 int. J/mol, which we also adopt here. Hence, the enthalpy of formation as per corrected Rossini 1931 is 285780.7 +- 40 int. J/mol for 18.0156 g water. Rossini 1939 in Table 1 gives corrections for real gas at p=0 for H2 of -0.48 J/mol and O2 of 8.06 J/mol, resulting in a correction for H2 + 1/2 O2 of 3.55 J/mol, which brings the enthalpy to 285784.3 +- 40 int. J/mol. Further correction to m.w. 18.0162 of Rossini 1939 produces 285793.8 +- 40 int. J/mol, to be compared to explicitly given value in Rossini 1939 of 285795 +- 40 int. J/mol. Using instead the present m.w. water of 18.01528 produces 285779.2 +- 40 int. J/mol, or 285824.9 +- 40 abs. J/mol (using 1 int. J = 1.00016 abs J). Using the conversion factor 1.000165 would result in 285826.3 +- 40 abs. J/mol. Finally, using the latter factor and SMOW m.w. of 18.015268 produces 285826.1 +- 40 abs. J/mol, which is the preferred value to be entered into the network. |