Atomic heat weight

The researches of F. Neumann (1831), Regnault (1840), and H. Kopp (1864), indicated that solid elements preserve unchanged their atomic heats when they unite to form solid compounds. Thus, the product molecular weight) X s )ecific heat) = (molecular heat) is composed additively of the atomic heats MC = niaiCi + h2 2c2 + n t s + (9)... 

ATOMIC HEAT. The product of the gram-atomic weight of an element and its specific heat The result is the atomic heat capacity per gram-atom. For many solid elements, the atomic heal capacity is very nearly the same, especially at higher temperatures and is approximately equal to 3R, where R is the gas constant (Law of Dulong and Petit). 

Estimates of the specific heat of coal have also been made on the assumption that the molecular heat of a solid material is equal to the sum of the atomic heats of the constituents (Kopp s law) the atomic heat so derived is divided by the atomic weight to give the (approximate) specific heat. Thus, from the data for various coals it has been possible to derive a formula that indicates the... 

The specific heat of cobalt is 0-108. Assuming a mean atomic heat of 6-4, the atomic weight, according to Dulong and Petit s Law, is approximately 59-8. 

The specific heat of iridium between 0° and 100° C. is 0-0323. Application of the Law of Dulong and Petit leads to an atomic weight of 198, the atomic heat being taken as 6-4. 

The atomic heats are therefore 5-83, 5-74 and 6-26 over the three ranges of temperature (1), (2), (3). There is a slight deviation from Dulong and Petit s law 9 at the lower temperatures, in the same sense as that met with in the case of the elements carbon, boron and silicon. But although phosphorus has a relatively low atomic weight, it also has a low melting-point, and the atomic heat as usual assumes the normal value at temperatures near the melting-point. 

Specific heat and chemical character. In 1819 Dulong and Petit discovered the following law The product of the atomic weight and the specific heat is a constant, namely 6 4, for all elements in the solid state. The atoms of the elements have all, therefore, the same specific heat (atomic heat). The following table shows that this law only holds approximately, and that several of the elements are exceptions to it, if we take the specific heat at the ordinary temperature as the basis of our calculations. The values of c are taken from the third edition of Landolt and Bornstein. The values of the true specific heat at room temperature were taken wherever possible. When Wied. Ann. 66, 235 (1898). 

G. N. Lewis J has calculated the specific heat at constant volume in this way, and finds that all elements which have a higher atomic weight than potassium have an atomic heat approximately equal to 5-9. The deviations from this mean value are, on the average, 0 1 less than the corresponding deviations for c,. Dulong and PetiUs law, therefore, holds better for than for c, . 

Although the law of constant atomic heats is only very approximately true, it has proved very useful in many branches of chemistry for example, in the determination of atomic weights. Chemical analysis gives us only the equivalents. For example, the analysis of indium chloride shows that 38 parts of indium combine with 35-5 parts of chlorine, and hence the atomic weight of indium must either be 38 3, or an integral multiple of this number. By Dulong and Petit s law, we find the atomic weight... 

The values in column (iii) were calculated on the simple assumptions that the atomic heats of all elements are equal to 64, and that this value is unaltered when the atoms undergo chemical combination, so that the molecular heat is simply the sum of the atomic heats. The table shows that this hypothesis is not in agreement with the facts. The dilTerences between the calculated and observed values of the molecular heats (i) and (iii) are, for the most part, considerable. These deviations show that one or both of our hypotheses must be incorrect. Either the atomic heats of the elements are not aU equal, or, in addition to this, there is an alteration in the specific heat when the atoms combine to form molecules. Joule and, after him, Kopp assumed that the first hypothesis only was at fault, and stated the law as follows the molecular heat of a compound is the sum of the specific heats of the individual atoms composing the molecule. If the atoms A2, As, etc., with the specific heats Cj, Cg, Cg, etc., unite to form the compound, As, 3 of molecular weight,... 

Dulong and Petit s law. The atomic heat capacity (atomic weight times specific heat) of elementary substances is a constant whose average... 

Atomic Weight Mean Specific Heat 0-100° Mean Atomic Heat Melting Point C° Boiling Point C° Density COSPFl CIENT OP Expansion... 

Dulong and Petit s law /doo-long and pe-teez / The molar thermal capacity of a solid element is approximately equal to 3R, where R is the gas constant (25 J moh ). The law applies only to elements with simple crystal structures at normal temperatures. At lower temperatures the molar heat capacity falls with decreasing temperature (it is proportional to T ). Molar thermal capacity was formerly called atomic heat - the product of the atomic weight (relative atomic mass) and the specific thermal capacity. The law is named for the French physicists Pierre-Louis Dulong (1785-1838) and Alexis-Therese Petit (1791-1820). 

Two methods used by Berzelius in fixing the values of atomic weights were the law of atomic heats of Petit and Dulong (1819), and Mitscherlich s law of isomorphism (1819). 

In order to obtain constant values of the atomic heats they changed some of Berzelius s atomic weights (see p. i66) those of sulphur and silver were halved the number for cobalt was further divided by i but the specific heat was in this case incorrect and the value for tellurium involved errors in both factors. Berzelius accepted the halving of the atomic weight of sulphur but refused to change those of silver, tellurium, or cobalt, remarking that a continuation of Dulong and Petit s excellent research would be a real service to science . 

---