SULPHURIC ACID.] CHEMISTRY 505 No. of mole cules of vrater added. Units of heat developed. No. of mole cules of water added. Units of heat developed. 1 6,272 99 16,850 2 9,364 199 17,056 3 11,108 499 17,304 5 13,082 799 17,632 9 14,940 1599 17,848 19 16,248 00 17,994 49 16,676 The last number in the table represents the amount of heat which, according to calculation, would be developed on mixing a molecule of sulphuric acid with an infinitely large quantity of water. It will be seen that the addition of the first two molecules is accompanied with the develop ment of an amount of heat equal to half the entire amount developed on adding an excess of water ; this appears to be accounted for by the formation of definite compounds of sulphuric acid with water. The first of these hydrates, H 2 SO 4 + H 2 O, may be obtained crystallized in colourless six-sided prisms, by cooling a mixture of the acid and water in these proportions to about 8 C. ; it boils at 205-210 C., and may be produced by evaporating any more dilute sulphuric acid at 205 C. until it ceases to lose water. The second hydrate, H 2 S0 4 + 2H 2 0, may, in a similar manner, be obtained by evaporating any more dilute acid at 100 C. until it ceases to lose water ; it boils at 193 C. The specific gravity of the first hydrate is 178, and that of the second 1 -62 ; when water is added to the acid in the proportions to form the latter, the maximum condensation (about 8 per cent.) is observed which occurs on mixing these substances. Further evidence in support of the view that sulphuric acid forms definite compounds with water which may be regarded as distinct acids is afforded by its behaviour on neutralization, by the persistent manner in which many sulphates retain one or two molecules of water, and by the existence of so-called basic sulphates. Thus, when it is neutralized by sodium hydroxide, considerably less heat is developed by the first than by the second molecule of hydroxide ; this behaviour does not appear remarkable if it be supposed that it exists in solution in combination with water, and that, therefore, as in the case of periodic acid, not only is hydrogen displaced by sodium on the addition of the hydroxide, but the elements of one or more mole cules of water are also separated, the latter being an opera tion which necessarily would involve the expenditure of energy, as the combination of the acid with water is attended with the development of heat (see page 488). By displacing one-half the hydrogen in sulphuric acid by metals acid sulphates are formed, and normal sulphates are produced by displacing the whole of the hydrogen. Many of the normal sulphates crystallize with the same number of molecules of water, and are isomorphous, as, for ex ample Magnesium sulphate MgS0 4 ,7H 2 Zinc ZnS0 4 ,7H 2 Iron FeS0 4 ,7H 2 O Nickel NiSO 4 ,7H,0 Cobalt CoS0 4 ,7H 2 Five of the seven molecules of water are removed with facility from these salts, but the sixth is less readily re moved, especially from magnesium sulphate, and they are not deprived of the seventh unless heated to 200-300 C. Copper, cadmium, and manganese sulphates bshave simi larly, and are only with difficulty deprived of the last molecule of water. The last molecule retained with such persistency by these salts was termed, by Graham, water of constitution, to distinguish it from water of crystallization. In the present state of our knowledge it is impossible absolutely to define the meaning of these terms, or even to say that there is an absolute difference between the so- called water of constitution and water of crystallization, and not merely one of degree ; but since we have inde pendent evidence tending to prove the existence of distinct acids formed by the combination of sulphuric acid with water, we may conveniently regard the above-mentioned salts as derived from these acids. Magnesium sulphate, for example, we may regard as the normal magnesium salt of the acid H 6 SO 6 , crystallized with five molecules of water. It will be evident that such salts as this bear a relation to the acid from which they are regarded as derived similar to that which the normal periodates bear to crystalline periodic acid, H 5 I0 6 ; indeed, a very consider able general resemblance may be traced between the periodates and the sulphates. Thus, the sulphates of potassium, K 2 S0 4 , and of silver, Ag 2 S0 4 , separate from aqueous solution in anhydrous crystals ; and sodium sul- phate> Na 2 S0 4 , is also deposited in the anhydrous state at temperatures above 34 C. ; it is only with metals such as magnesium, zinc, and copper, that salts are obtained which may be regarded as derived from the acids H 4 S0 5 or H 6 S0 6 . Similarly, the periodates KI0 4 , NaIO 4 , and AgI0 4 , derived from the at present hypothetical acid HI0 4 , are readily produced, but with other metals chiefly salts derived from the acid H 5 I0 6 are obtained. But that these hydrates of sulphuric acid are distinct acids derives its most important confirmation from the existence of the so-called basic sulphates, such as Zn 2 SO 5 or ZnS0 4 + ZnO, Hg 3 SO 6 or HgSO 4 + 2HgO, and Cu 3 SO d + 3H 2 O or CuSO 4 + 2CuO + 3H 2 O ; these salts are strictly comparable with the basic periodates. A large number of double and mixed salts formed by the union of two or more sulphates are known. Thus, magnesium sulphate and the isomorphous sulphates form isomorphous double salts with potassium, sodium and ammonium sulphates, which crystallize with six molecules of water, and which usually are therefore regarded as formed by the displacement of the molecule of so-called water of constitution by the alkaline sulphate Magnesium potassium sulphate MgK<j(SO,) 2 6H 2 Zinc ZnK 2 (SO<) 2 6H 2 O Iron FeK 2 (S0 4 ) s 6H 2 Nickel NiK 2 (S0 4 ) 2 6H 2 Cobalt ,, CoE s (SO^ l 6H 2 It has -already been pointed out that sulphuric acid may be regarded as a compound of the dyad radicle S0 2 with the monad radicle OH, or as (S0 2 )"(OH) 2 , this view of its " constitution " being chiefly founded on its behaviour with the chlorides of phosphorus. Thus, by the action of phosphorus pentachloride on sulphuric acid, an atom of oxygen and an atom of hydrogen, in other words, the group OH, are displaced by a single atom of chlorine Ho.so 2 . OH + pci 3 = HO . so 2 . ci + roci 3 + HCI . Although evidence of the existence of a second OH group in sulphuric acid cannot be obtained in a similar manner by the continued action of the phosphorus pentachloride, it is afforded by the behaviour of the compound S0 2 C1 2 , produced by the direct union of chlorine and sulphur dioxide with water, which successively converts it into the compound HO . S0 2 . Cl and sulphuric acid = HO.S0.C1 HCI; C1.SO Sulphuric chloride, S0 2 C1 2 , and sulphuric chlorhydrate, S0 9 (OH)C1, are both colourless liquids ; the former boils at about 70 C. , and the latter at 158 C. Sulphuric chlor- hydrate may be formed by the direct combination of hydrogen chloride with sulphuric anhydride, and when heated in closed vessels to 170 C., it furnishes sulphuric chloride and sulphuric acid : 2Cl.SOo.OH = C1.S0 2 .C1 + HO.S0 2 .OH.
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