Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/512

500 CHEMIST II Y [COMPOUNDS OF THE Hydrogen sulphide, hydrosulphuric acid, or sulphuretted hydrogen gas, is transparent and colourless ; it possesses a most offensive odour, and cannot be breathed with im punity, frequently giving rise to nausea and vertigo even when much diluted. It may be condensed by powerful pressure to an extremely mobile liquid, which solidifies when cooled in a bath of solid carbon dioxide and ether to a white transparent mass, which melts at - 85 C. Hydrogen sulphide is readily inflammable, burning in air with a blue flame, and forming water and sulphur dioxide 2SH 2 + 30 2 = 2S0 2 + 2H 2 0. Hydrogen sulphide. Oxygen. Sulphur dioxide. Water. Most metals when heated in the gas decompose it, a metallic sulphide being produced, and hydrogen liberated. Hydrogen sulphide dissolves in water, a saturated solution containing about three times its volume of the gas ; the solution has the odour and taste of the gas, and a slight acid reaction. It is gradually decomposed on exposure to the air, sulphur being deposited It is decomposed in a similar manner by nearly all oxidiz ing agents, and by the action of chlorine, bromine, and iodine Iodine, however, cannot decompose the gas at ordinary temperatures except in presence of water. This is because the reaction requires an absorption of heat. Thus, in the production of a molecule of hydrogen sulphide from hydro gen and sulphur in the state in which it separates when hydrogen sulphide is decomposed by iodine, 4500 units of heat are developed ; but in the production of a molecule of hydriodic acid from its elements, 6000 units of heat are absorbed, so that the reaction indicated by the equation H 2 S + 1 2 = 2HI + S would involve the absorption of 4500 + 2 x 6000 = 1 6,500 units of heat. That it takes place in presence of water is due to the fact that the dissolution of the hydriodic acid produced is attended with the develop ment of 2 x 19200 = 38,400 units of heat ; hence, when the reaction is effected in a dilute solution, heat is developed to the extent of 38,400-16,500 = 21,900 units of heat. But it is found that the reaction takes place the less readily as the concentration of the solution increases, and that it ceases when the solution has attained a specific gravity of T56 at ordinary temperatures; in more concentrated solu tions sulphur even dissolves with production of hydrogen sulphide and liberation of iodine. A simple explanation of this apparently anomalous result, however, is afforded by the observation that the heat developed by the absorp tion of equal quantities of hydriodic acid is less as the quantity of acid already dissolved in the water is greater. The amount of heat developed, therefore, diminishing as tke. quantity of hydriodic formed by the reaction in the liquid increases, at a certain point becomes equal to that absorbed in the decomposition of the hydrogen sulphide by the iodine, and the reaction ceases since it can no longer be attended with a development of heat. This behaviour of iodine with hydrogen sulphide alone, or in presence of water, is one of the most striking illustra tions of the fact that reactions involving the expenditure of energy cannot take place directly, and are only possible when the conditions are such that one or more of the pro ducts of the reactions enter into secondary reactions, so as to cause the development of more heat than is absorbed in the primary reaction. The hydrogen in hydrogen sulphide may be displaced by metals, the compounds formed by displacing one-half the hydrogen being termed sulphydrates or hydrosulphides, whilst those in which the whole of the hydrogen is dis placed are termed sulphides. These two classes of com- pounds correspond to the metallic hydroxides and the metallic oxides respectively, and in many respects closely resemble them ; the sulphur compounds, however, are, with few exceptions, far less stable than the corresponding oxygen compounds. Hydrogen sulphide enters directly into reaction with the metallic hydroxides, exchanging its hydrogen for the metal ; for example NaOH + H 9 S = NaSH + HOH. H 2 S = Sodium Hydrogen Sodium hydroxide. sulphide. hydrosulphide. Water. It therefore exhibits the behaviour of an acid. From Thomsen s experiments it appears that 7740 units of heat are developed on the addition of a solution of one molecule of sodium hydroxide to a solution of one molecule of hydrogen sulphide, and that the further addition of the hydroxide is without effect. Hydrogen sulphide is thus proved to be a monobasic acid, and this result also shows that when soluble sulphides, such as sodium sulphide, are dissolved in water, double decomposition occurs, thus : Na 2 S + OH 2 = NaSH + NaOH, just as when sodium oxide, for example, is added to water : Na 2 + OH 2 = 2NaOH. It is uncertain, however, whether the decomposition of the sulphides by water in this manner is complete, or whether it is only partial, and the more complete the greater the quantity of water present. The highly positive metals lithium, sodium, potassium, calcium, strontium, barium, and magnesium form sohible sulphides and hydrosulphides, but most of the sulphides of other metals are insoluble. The nature of many of the compounds precipitated from metallic solutions by hydro gen sulphide or an alkaline hydrosulphide is not well established ; but in many cases apparently they are inter mediate in composition between the hydroxides and hydro- sulphides ; the precipitate formed on the addition of an alkaline hydrosulphide to a solution of a zinc salt, for example, is probably a compound of this kind, and may be represented by the formula HO. Zn. SH, zinc hydroxide being HO.Zn.OH, and zinc hydrosulphide HS.Zn.SH. The solutions of salts of heavy metals, such as mercury and lead, furnish precipitates of the corresponding sulphides with hydrogen sulphide or alkaline hydrosulphides. The hydrosulphides of certain elements, such as alumi nium and chromium, cannot exist in presence of water, but enter into reaction with it with evolution of hydrogen sulphide ; hence, on the addition of an alkaline hydro- sulphide to solution of their salts, the corresponding hydroxides are precipitated : GNaSH = A1 2 (SH) 6 + 6NaCl Sodium Aluminium Sodium liydrosulphide. hydrosulphide. chloride. A1 2 (SH) G + 60H 2 = A1 2 (OH) G + 6SH 2 . Water Aluminium Hydrogen A1 2 C1 6 Aluminium chloride. Aluminium hydrosulphide. Aluminium hydroxide. sulphide. Sulphur unites with all the metals and with most of the non-metallic elements ; the sulphides are therefore usually prepared directly from their elements. Two classes of sulphides corresponding to the basic and acid oxides may be distinguished, but the distinction between them is much less marked than that between the two classes of oxides. The sulphides of the non-metallic elements and the sulphides of arsenic, antimony, tin, molybdenum, tungsten, vana dium, gold, and platinum, which are soluble in solutions of alkaline hydrosulphides, belong to the class of acid sul phides, and the remaining sulphides are basic. These two classes of sulphides are capable of uniting together to form sulphur salts, just as the basic and acid oxides combine forming oxy-salts. As a rule, the sulphides and oxides of the same element have similar formulae and correspond in their general behaviour. Occasionally there are oxides to

i which there are no corresponding sulphides, but more fre-