Lavoisier and Laplace

Laws of Thermochemistry. Lavoisier and Laplace (1780) found that the heat required to decompose a chemical compound into its elements was numerically equal to the heat generated in its formation under the same conditions of T and P. That is, AHj = -AHp where the subscript d refers to decomposition reaction [52, p. 24 61, p. 303]. 

It is not essential, however, that the unit of heat should be defined in terms of the rise of temperature produced when heat is absorbed by a standard body, say unit mass of water. Any effect of heat absorption which is capable of measurement and numerical expression might be used, and the method of measurement would in all cases be consistent with the axiom that if two identical systems are acted upon by heat in the same way so as to produce two other identical systems, the quantities of heat supplied to the systems are equal. Lavoisier and Laplace (1780-84) took as unit that quantity of heat which must be absorbed by unit mass of ice in order to convert it completely into water. This unit is of course different from the one we adopted, but if a quantity of heat A has been found to raise from lo ° to 16 ° twice as much water as another quantity of heat B, then A will also melt twice as much ice as B. 

Thermochemistry has been defined in one of the most popular physical chemistry textbooks as the study of the heat produced or required by chemical reactions [1], The use of heat, instead of the more general word energy, immediately suggests a close association between thermochemistry and calorimetry—the oldest experimental technique for investigating the thermodynamics of chemical reactions. This view is, in fact, shared by many of our students and some of their teachers, together with the belief that thermochemistry, founded in the eighteenth century by Black, Lavoisier, and Laplace, has seen few major developments since the days of Berthelot and Thomsen, over 100 years ago [2],... 

In the experiments of Lavoisier and Laplace, the body employed for this purpose was pounded ice, which was so applied around a balloon as to intercept all the heat, Fig. 83 shows a section of their apparatus, which they termed a calorimeter. The heating powei... 

Calorimeters of Historical and Special Interest Around 1760 Black realized that heat applied to melting ice facilitates the transition from the solid to the liquid stale at a constant temperature. For the first time, the distinction between the concepts of temperature and heat was made. The mass of ice that melted, multiplied by the heal of fusion, gives the quantity of heal. Others, including Bunsen, Lavoisier, and Laplace, devised calorimeters based upon this principle involving a phase transition. The heat capacity of solids and liquids, as well as combustion heats and the production of heat by animals were measured with these caloritnelers. 

The measurements that Lavoisier and Laplace obtained with their instrument,... 

Phase-change adsorption calorimetry. This was the earliest type of diathermal-conduction calorimetry and was originally developed in the form of ice calorimetry by Lavoisier and Laplace (1783), who weighed the liquid water, and by Bunsen (1870), who measured the change of volume. Dewar (1904) devised an elegant adsorption calorimeter at liquid air temperature the heat was evaluated from the volume of air vaporized. Of course, the temperature of the calorimeter is imposed by the temperature of the phase change. Because these calorimeters lack adaptability and cannot be readily automated, they are mainly of historical interest. 

A few years later, when Lavoisier and Laplace s memoir on heat was published, Watt ensured that Black received a copy and told his friend that the piece was... 

In 1781 Cavendish and Watt proved water to be produced by the combustion of hydrogen. A repetition of their experiments in 1783 by Lavoisier and Laplace indicated that water contains 1 volume of oxygen and 1 91 volume of hydrogen. The interaction of red-hot iron and steam, with liberation of free hydrogen and production of a calx of iron, was also observed by Lavoisier. 

The following Rule of Lavoisier and Laplace is really a corollary of the Hess s Law. 

The third section of the memoir, Reflections on the theory of heat, summarized lucidly what Lavoisier and Laplace sought to accomplish with their machine exact quantitative control of the distribution and the flow of heat in a system of bodies. In order to frame a complete theory of heat, four different kinds of measurement were necessary a linear thermometer, the specific heats of bodies as a function of temperature, the absolute quantities of heat contained in bodies at a given temperature, and the quantities of heat evolved or absorbed in chemical combinations or decompositions. This is in fact an excellent summary of the directions in which the thermometric investigation of heat had proceeded until then, except for the last item, which Lavoisier and Laplace added. They could not measure all these quantities directly, however, as they readily admitted. Particularly problematic was the relationship between the thermometer readings and the absolute quantities of heat. The assumption that the ratio of absolute heats was proportional to the ratio of specific heats was very uncertain and would require many experiments for confirmation. Specific heats only indicated the difference... 

Lavoisier and Laplace, Memoire sur la chaleur (1783), Oeuvres, volume 2, 283-333 translated by Henry Guerlac as Memoir on Heat (Neale Watson, 1982). 

Ibid., Guerlac traced Lavoisier and Laplace s knowledge of this theory to J. A. Deluc, Recherches sur les modifications de Vatmosphere (1772). De Luc was known personally to Lavoisier since 1768 as a corresponding associate member of the Academy. He visited Paris in October 1781 and was in touch with Guettard, Lavoisier s old mentor Guerlac, Chemistry as a Branch of Physics, 246-249. 

Lavoisier and Laplace found that solutions of nitric acid had a smaller heat capacity than predicted by the mixture rule, and this result was extended to solutions of six salts by Person. Schuller (with NaCl, KCl, NH4CI, Nal, Na2S04, NaNOs, KNO3) also found the heat capacity of the solution mostly smaller than the sum of those of the components the ratio of the observed and calculated value was independent of dilution only with NaCl and Nal. The calculated values for certain concentrated solutions of Na2S04 and NaN03... 

Black 1 about 1764 determined an average value of the latent heat of steam by. a crude method as 450 g.cal./g. Irvine found 430, and Watt, in 1781, found 533, which is a good value. Other experiments were made by Lavoisier and Laplace,3 Ure, Despretz, Brix and Andrews. ... 

The quantity of heat set free by a system which undergoes a transformation does not depend solely upon the initial and final states.—The chemists who occupied themselves with calorimetry, from the time of Lavoisier and Laplace down to the time of the foundation of thermod3mamics, have all admitted and made use of the following law ... 

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