Horstmann, August Friedrich

(b. Mannheim, Germany, 20 November 1842; d. Heidelberg, Germany, 8 October 1929)

physical chemistry.

Horstmann stimulated the application of thermodynamics to chemical reactions when he showed that the Clausius-Clapeyron equation adequately explained the heats of dissociation of ammonium chloride upon sublimation.

He studied at the universities of Heidelberg, Zurich, and Bonn, receiving a doctorate from the university of Heidelberg in 1865. He became professor of theoretical chemistry at Heidelberg, where he remained until his death.

In 1869 Horstmann published “Dampfspannung and Verdampfungswarme des Salmiaks,” which laid the basis for his contribution to theoretical chemistry. For most substances the three transitions—from solid to liquid, liquid to gas, and decomposition—occur at three different temperatures. But for sal ammoniac (ammonium chloride) the three transitions occur at the same temperature. Ammonium chloride sublimes and its molecules break up into ammonia and hydrogen chloride at the same time. Horstmann studied the vapor pressure and heat of disintegration of ammonium chloride. He attempted to determine experimentally if the vaporization is in any way dependent on pressure as is the evaporation of liquids. He found that the vapor pressure of ammonium chloride increases with temperature in the same manner as the vapor pressure of other liquids.

Horstmann then assumed that if ammonium chloride reacted to changes of pressure as did other liquids, he should be able to apply the Clausius-Clapeyron equation and calculate the heat of vaporization for ammonium chloride. He did this and found that, within experimental error, his calculated values were less than the experimental values by an amount equal to the heat of combination of ammonia and hydrogen chloride, showing that the two changes can be treated as the sum of two separate transitions.

He later extended his examination to include heats of dissociation of hydrates and carbonates.

BIBLIOGRAPHY

There is no collected works of Horstmann: his paper on the sublimation of ammonium chloride was published in Bericht der Deutschen chemischen Gesellschaft, 2 (1869), 137–140, repr. in no. 137 of Ostwald’s Klassiker der Exakten Wissenschaften (Leipzig, 1903). For discussions of Horstmann’s work, see Harry C. Jones, The Theory of Electrolytic Dissociation and Some of Its Applications (New York, 1900); and Wilhelm Ostwald, Elektrochemie, ihre Geschichte und Lehre (Leipzig, 1896).

Ollin J. Drennan