209.] HEAT EVOLVED DURING METALLIC PRECIPITATIONS. 437 obtained during the precipitation of copper by zinc—the corresponding numbers in both sets of experiments agreeing very closely with each other. In the first set of experiments metallic copper was precipitated from a strong solution of its sulphate by means of zinc, in a small glass vessel, and the heat estimated by the rise of temperature experienced by the water of a calorimeter in which the glass vessel was contained in the second series, a dilute solution of sulphate of copper was employed, and the heat was measured by the rise in temperature experienced by the liquid itself. The mean of 4 experiments by the first plan gave 864 as the number of units of heat evolved by the precipitation of each gramme of copper from its sulphate; whilst the mean of 5 experiments upon the latter method was 868. The foregoing are the numbers given by Andrews, but he has purposely abstained from any attempt to deduce from them the amount of heat developed during the indirect oxidation of the metal which acts as the precipitant. In these experiments a known quantity of finely-divided zinc, iron, lead, or copper, as the case might require, was mixed with a solution of the salt to be decomposed; taking care that the metal employed was always more than sufficient completely to decompose the salt in solution: the rise in temperature which occurred was noted with the usual precautions. Andrews states, as the result of a large number of experiments, that the quantity of heat developed during the mutual action of the same pair of metals is the same, when an equivalent of one metal, A, displaces another metal, B, from any of its salts, whatever may be the acid of the salt employed, provided that B is in the same state of oxidation in each of the compounds submitted 438 HEAT EVOLVED DURING METALLIC PRECIPITATIONS. [209. to experiment. But if a different metal he employed to effect the precipitation, the amount of heat evolved is different. For instance, whether chloride, or sulphate, or acetate, or formiate of copper be precipitated by zinc, the quantity of heat developed in each case for every equivalent of copper is sensibly the same, viz., 27559. But if iron be substituted for zinc in the precipitation of the copper, the amount of heat is different, viz., 18796; though iron evolves the same amount of heat, whether the sulphate or the chloride of copper be employed. The principle, that the quantity of heat developed during the mutual action of the same pair of metals is always the same, whatever be the nature of the acid contained in the salts employed, has since been assumed by Favre and Silbermann in their calculations. If the metals be arranged in a list, beginning with those which develope the largest amount of heat when used as precipitants, the order in which they will stand is the followingzinc, iron, lead, copper, mercury, silver, and platinum. Now, it will be remarked that this is exactly in the electro-chemical order (261), zinc being the most electro-positive, and platinum the most electro-negative. Another interesting point of connexion between the thermal and the electrical phenomena exhibited by the metals is to be observed in the fact, that the nature of the acid contained in the salt which is undergoing decomposi tion does not influence either its thermal equivalent, or the electro-motive force (260) which it exerts when employed in the production of voltaic action. The following remarkable conclusion was deduced by Andrews from these experiments :-If three metals, A, B, and C, be so related that A is capable of displacing B and C from their com binations, and B he also capable of displacing C, the heat developed by the substitution of A for C will be exactly equal to that developed in the substitution of A for B, together with that deve loped in the substitution of B for C:: Heat Units. Thus I equivalent of lead displaced by zinc = 18837 1 equivalent of copper by zinc = 8509 27346 The experimental number for copper by zinc being 27559 An analogous phenomenon is observed in the electrical relations of the metals (260): when three metals such as platinum, zinc, and potassium are arranged two and two in their electrical 210.] CALORIFIC EQUIVALENTS OF THE ELEMENTS. 439 order, the electro-motive force generated between the two extremes, platinum and potassium, is equal to the sum of the electro-motive forces of the pairs platinum and zinc, and zinc and potassium. (210) Calorific Equivalents of the Elements.-The results obtained by the direct action of oxygen, chlorine, iodine, and bromine upon various elementary bodies, are summed up in the following Table, in which the numbers given indicate the quantity Calorific Equivalents of various Elements (0-8). Observers. Oxygen. Chlorine. Bromine. Iodine. Sulphur. *9322-3606 #2741 *100960 *90188 77268 *45638 F.S. ... ... ... F.S. 94847 ... *42451 *50296 *20940 ... *27675 44730 32802 #23208 *9556 *6113 *34800 *25618 | *18651 $5524 Silver of heat evolved by the union of equivalent quantities of oxygen, chlorine, iodine, and bromine, with each element, taking as the standard of comparison the number of grammes of water at o° C., which would be raised to 1° C. by the combustion of 1 gramme of hydrogen in oxygen. In this case the numbers for the different elements are all calculated from their equivalent numbers, not from the atomic weights. The quantities of heat thus given out are termed by Favre and Silbermann the calorific equivalents of the different elements. The numbers to which an A. is prefixed are those of Andrews: F. S. indicate those of Favre and Silbermann: when an asterisk is prefixed to any number, the result has been calculated by indirect methods, upon the principle already explained (207). From an examination of the foregoing Table of calorific equivalents, it will be obvious that the quantity of heat evolved in the act of combination is greatest in those cases in which the chemical attraction between the two elements is the strongest, and where 440 the compound possesses the greatest stability. No definite quantitative expression of the law which regulates the evolution of heat during combination, can, however, be deduced from these numbers, owing to the variety of disturbing causes when bodies are compared in the solid state. HEAT EVOLVED DURING ACTION OF ACIDS ON BASES. [210. (211) On the Heat developed during the Reaction of Acids upon Bases.-A careful and extensive series of experiments upon the heat developed during the saturation of dilute solutions of different acids, by each of the more important bases with which they form soluble compounds, was published by Andrews in 1841 (Trans. Roy. Irish Acad., xix. 228). In these experiments a slight excess of acid was purposely employed; the bases, where it was possible, being in a state of solution. When the bases are in the insoluble form, the heat observed is of course lower than that due to the chemical action; a portion being absorbed in the passage of the base from the solid to the liquid condition; but although the quantity of heat so absorbed is unknown, this amount is constant for the same base, and therefore the observed results obtained for the combination of equal quantities of this base with different acids are mutually comparable. Heat Units evolved by the Action of 1 Equivalent of Sulphurous 16300 ... ... ... Nitric.... 14800 14500 15300 12700 17600 15800 10600 9300 7200 15510 Phosphoric 14700 14300 15200 : : 14700 18400 12000 16083 15810 17766 Arsenic 14700 14400 Hydrochloric 14900 14700 Hydrobromic 12900 17600 15600 10800 ... ... 13100 14156 13751 13973 13600 | 13308 13425 12051 13658 13178 * In the Table, showing the heat units evolved in the action of the acids on bases, the results obtained by Dr. Andrews from a later series of ex periments, which were communicated in 1870 to the Royal Society of Edinburgh, are substituted for those given formerly in this work. The new results comprise the heat evolved in the action of potash, soda, and ammonis on sulphuric, nitric, hydrochloric, oxalic, acetic, and tartaric acids. In the www 211.] HEAT EVOLVED IN FORMATION OF SALTS. 441 A résumé of this subject is given by the same author in a "Report on the Heat of Combination," published in the Report of the British Association for 1849, 69. From these experiments it appears : 1. "That an equivalent of the same base, combined with different acids, produces nearly the same quantity of heat. 2. "An equivalent of the same acid, combined with different bases, produces different amounts of heat. 3. "When a neutral salt is converted into an acid salt by combining with one or more equivalents of an acid, no disengagement of heat occurs. 4. "When a double salt is formed by the union of two neutral salts, no disengagement of heat occurs. 5. "When a neutral salt is converted into a basic salt, the combination is accompanied by the disengagement of heat. 6. "When one and the same base displaces another from any of its neutral combinations, the heat evolved or absorbed is always [nearly] the same, whatever the acid element may be."* The results of Favre and Silbermann lead to conclusions sub the Undermentioned Bases on 1 Equivalent of certain Acids. 15360 13676 12840 16943 8323 10850 10450 9956 9648 8116 6400 9240 6206 ... 7720 9982 9245 9272 8590 7546 5264 ... ... ... ... ... ... ... ... other cases, the numbers have been calculated from his earlier experiments, which were communicated in 1841 to the Royal Irish Academy, no correction has been made, as was attempted in former editions, for the specific heat of the solutions employed, as the data for making these corrections are very imperfect, and they have not been made by Dr. Andrews himself in either of his original papers. |