Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/487

CHEMICAL ACTION.] developed ; in the production of hydrobromic acid from hydrogen and bromine, however, only 8-440 units of heat are developed ; and in the formation of hydriodic acid from hydrogen and iodine 6040 units of heat are absorbed. This dilference in behaviour of the three elements, chlorine, bromine, and iodine, which in many respects ex hibit considerable resemblance, may be explained in the following manner. We may suppose that in the forma tion of gaseous hydrochloric acid from gaseous chlorine and hydrogen, according to the equation a certain amount of energy is expended in separating the atoms of hydrogen in the hydrogen molecule, and the atoms of chlorine in the chlorine molecule, from each other ; but that heat is developed by the combination of the hydrogen atoms with the chlorine atoms, and that, as more energy is developed by the union of the atoms of hydrogen and chlorine than is expended in separating the hydrogen atoms from each other and the chlorine atoms from one another, the result of the action of the two ele ments upon each other is the development of heat, the amount finally developed in the reaction being the differ ence between that absorbed in decomposing the elementary molecules and that developed by the combination of the atoms of chlorine and hydrogen. In the formation of gaseous hydrobromic acid from liquid bromine and gaseous hydrogen- H , + Br 2 = HBr + HBr , in addition to the energy expended in decomposing the hydrogen and bromine molecules, energy is also expended in converting the liquid bromine into the gaseous condi tion, and probably less heat is developed by the combina tion of bromine and hydrogen than by the combination of chlorine and hydrogen, so that the amount of heat finally developed is much less than is developed in the formation of hydrochloric acid. Lastly, in the production of gaseous hydriodic acid from hydrogen and solid iodine so much energy is expended in the decomposition of the hydrogen and iodine molecules and in the conversion of the iodine into the gaseous condition, that the heat which it may be supposed is developed by the combination of the hydrogen and iodine atoms is insufficient to balance the expenditure, and the final result is therefore negative ; hence it is necessary in forming hydriodic acid from its elements to apply heat continuously. These compounds also afford examples of the fact that, generally speaking, those compounds are most readily formed, and are most stable, in the formation of which the most heat is developed. Thus, chlorine enters into re action with hydrogen, and removes hydrogen from hydro- genized bodies, far more readily than bromine ; and hydro chloric acid is a far more stable substance than hydrobromic acid, hydriodic acid being greatly inferior even to hydro bromic acid in stability. When two substances which by their action upon each other develop much heat enter into reaction, the reaction is usually complete without the employment of an excess of either ; for example, when a mixture of hydrogen and oxygen, in the proportions to form water is exploded, it is entirely converted into water. This is stated that a certain amount of heat is developed in the production of a certain body, the production of a quantity of it equal to its mole cular weight in grammes is to be understood. Thus, in the above case, the production of hydrochloric, hydrobromic, and hydriodic acids means the production respectively of 36 36, 81 75, and 127 53 grammes of these bodies. 475 also the case if two substances are brought together in solution, by the action of which upon each other a third body is formed which is insoluble in the solvent employed, and which also does not tend to react upon any of the sub stances present ; for instance, when a solution of a chlo ride is added to a solution of a silver salt, insoluble silver chloride is precipitated, and almost the whole of the silver is removed from solution, even if the amount of the chloride employed be not in excess of that theoretically re quired. But if there be no tendency to form an insoluble com pound, or one which is not liable to react upon any of the other substances present, this is no longer the case. For example, when a solution of a per-salt of iron is added to a solution of potassium thiocyanate, a deep red coloration is produced, owing to the formation of thiocyanate of iron. Theoretically the reaction takes place in the case of the per- nitrate of iron in the manner represented by the equation Fe 2 (NO 3 ) 6 + 6KCXS = Fe. 2 (CXS) 6 + 6KN0 3 ; Ferric nitrate. Potassium thiocyanate. Ferric thiocyanate. Potassium nitrate. but it is found that even when more than sixty times the amount of potassium thiocyanate required by this equation is added, a portion of the ferric nitrate still remains uncon verted, doubtless owing to the occurrence of the reverse change Fe 2 (CNS) 6 + 6KN0 3 = Fe 2 (X0 3 ) 6 + 6KCXS . In this, as in most other cases in which substances act upon one another under such circumstances that the resulting compounds are free to react, the extent to which the dif ferent kinds of action which may occur take place is de pendent upon the mass of the substances present in the mixture. As another instance of this kind, the decomposi tion of bismuth chloride by water may be cited. If a very large quantity of water be added, the chloride is entirely decomposed in the manner represented by the equation BiO, + OH 2 = BiOCl + 2HC1 , Bismuth chloride. Bismuth oxychloride. the oxychloride being precipitated ; but if smaller quanti ties of water be added the decomposition is incomplete, and it is found that the extent to which decomposition takes place is proportional to the quantity of water em ployed, the decomposition being incomplete, except in pre sence of large quantities of water, because of the occur rence of the reverse action BiOCl + 2HC1 = BiCl 3 + OH 2 . Chemical change which merely involves simple decom position is also influenced by the presence of the products of decomposition. Thus when calcium carbonate is strongly heated in an open vessel, it is entirely decomposed into carbon dioxide gas and calcium oxide CaC0 3 = C0 CaO. "i + Calcium carbonate. Carbon dioxide. Calcium oxide. When it is heated in a confined space, the decomposition only goes on until the liberated gas has attained a certain tension, and as long as the temperature does not vary, the tension remains the same, and is independent of the pro-, portion of the compound decomposed, that is to say, of the amount of calcium oxide present ; but the tension increases if the temperature is raised, and diminishes if it is lowered, owing to the recombination of a portion of the carbon dioxide with the calcium oxide; for example, at 800 C. the tension is equivalent to a column of mercury 85 millimetres high, but at 1040 C. it is equivalent to a column of 520 mm. Deville applies the term dissociation to changes which occur in this manner; the term only applies to those cases of decomposition in which products are obtained which under the conditions of the experi

ment are capable of reuniting to form the original sub-