Use of the Heat Theorem to control experimental work

Use of the Heat Theorem to Control Experimental Work.—The Second Law has long been used to subject to an independent test the accuracy of the measurements made in investigating chemical equilibria. The new Heat Theorem is frequently suited, perhaps even more highly, to this purpose, a fact which has not been sufficiently noted hitherto. We shall, therefore, show by means of a few examples how the Theorem has already led, or could have led, to the discovery of inaccuracies or of incorrect interpretation of observations. 

In the ammonia equilibrium I found, (1) and (8), a large difference between the value calculated by means of the approximation formula and the measurements carried out by Haber and van Oordt, which, though stated to be preliminary, appeared to claim at least a certain degree of reliability. The redetermination of the constants of the equilibrium which was then made (cf. Jost, 13) showed that the earlier value was considerably in error, and confirmed satisfactorily that calculated by means of the approximation formula. 

The determination of the equilibrium of the reaction C + COfl = 2CO, by Clement, led to values which differed both from the older measurements of Boudouard and from those calculated by means of the approximation formula a later paper by Rhead and Wheeler f confirms the latter values, and renders it probable that Clement s are wrong by a systematic error. 

The case of the two sulphides of copper may also be mentioned. The dissociation pressure of sulphur above a mixture of the two crystalline sulphides should, from a calculation with the approximation formula, be equal to the vapour pressure of solid sulphur at the same temperature, if Thomsen s heats of formation are taken as a basis for the calculation. In contradiction to this, Frl. Wasjuchnow 

From Thomsen s determinations of the heat of formation of silver iodide it may be calculated, by means of the approximation formula, that iodine should have a measurable dissociation pressure at moderately high temperatures. Naumann (n) found, however, that this was too small to be measured even at 6oo°. Analogous differences occurred in the theoretical calculation of the E.M.F. of the silver-iodine electrode. U. Fischer then found the heat of formation by three independent methods as 15,200, 14,800, and 15,000, whereas Thomsen had given 13,800 so the differences between theory and observation are reconciled (cf. further, p. 113). 

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