Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/520

Rh 508 CHEMISTRY [SULPHUR GROUP. the solution may be concentrated by a gentle heat till it attains a specific gravity of 1 25 -1*3, and may then be further concentrated in a vacuum to the specific gravity of about 1 6. The solution is colourless and inodorous, and has a strongly acid taste; it may be preserved un changed at ordinary temperatures, but on heating a con centrated solution of the acid, hydrogen sulphide and sulphur dioxide are evolved, sulphur is deposited, and sulphuric acid remains. The salts of pentathionic acid are so unstable that it is difficult to obtain them in the solid state. In their forma tion from the acid there is a great tendency for the fifth atom of sulphur to be separated, tetrathionates being pro duced, which have greater stability, and sometimes two atoms of sulphur are given up and trithionates are formed. A number of the reactions involved in the formation of various sulphur compounds, and also of a few selenium and tellurium compounds, have been submitted to thermo- chemical investigation by Thomsen. The results deduced from his experiments are given in the following table : Reaction. Units of heat developed or absorbed. Remarks. Sulphur. (I 2) Aq, SH 2 ) 21 830 Hydrogen (S, H s ) 4 510 sulphide (SH 2, Aq) 4 750 (S, II,, Aq).... 9 260 f(S0 9 , Aq). . 7 700 Gaseous acid. (S0 2 , Aq) 1,500 Condensed acid. Sulphur (S, 0,)... 71 070 ( Determined by &amp;lt; Favre and Sil- ous acid (S, 2, Aq) 78,770 (_ bermann. (S0 2 Aq, 2NaOHAq) ((SO,, 0)..., 28,970 32,160 SO. becomes (SO Z, 0, Aq) 71,330 liquid. (S0 2, Aq, 0) 63,630 (S0 2, O,, H s ) 121,840 Sulphuric (S0 3, ILO) 21,320 SO-H, becomes acid (S0 4 H 2, Aq) 17,850 liquid. (S0 3, Aq) 39,170 Liquid anhydride. (S, 3 ) 103,230 (S, 4, H S) ) 192,910 Supposing that Dithionic acid (S, 4 , H 5 , Aq).... KSOoAq, 2NaOHAq) f(2S0 1, 0, Aq) (2S0 2 Aq, 0) (S0 3 Aq, S0 2 Aq; ... (S a, 5 , Aq)... 210,760 31,380 68,950 53,550 -10,080 211,090 (S,0,=71,070). (S 8 ,0 6 ,H 2 , Aq).... JS s O,Aq, 2NaOHAq) C SO,, S, Aq) ... 279,450 27,070 -1,570 (S0 2 Aq, S) -9,270 Thiosulph- S.O.Aq, O,)..., 225 300 uric acid (S,,, An).... 69,500 Tetrathi- onic acid l(S 2) 0,,H 2, Aq).... (2S0 2 , 0,8,, Aq).. (2SO a Aq, 0, S 2 S,, 0,, Aq)...! 137,860 62,820 47,420 204,960 (S 4 ,0 6 ,H if Aq).... Selenium. (Se, 0,) 273,320 57,710 Cryst. anhydride. Selenious (Se0 2, Aq) -920 acid (Se, 2 , Aq) 56,790 (SeO.Aq, 2NaOHAq) Se. 0,, Aq) 27,020 77,240 Selenic acid (Se0 2, 0, Aq) (SeO,Aq, 0)... 19,530 20,450 j Tellurous acid Telluric (SeO,Aq,2NaOHAq) rdlurium. (Te, 2, H 2 0) i (Te0 9 Aq, 0).... 30,390 81,190 25,850 acid (Te, 0,, Aq) 107,040 The number 4510 for the reaction S, H 2 applies strictly only to sulphur in the state in which it separates when hydrogen sulphide is decomposed by iodine, and would require a small correction to make it applicable to rhombic sulphur. On comparing the numbers representing the amounts of heat developed in the formation of sulphurous and sul phuric acids and of the corresponding selenium and tellu rium compounds, thus Sulphur. Selenium. Tellurium. R-S K-Se R = Te R,0 2, Aq 78,770 56,790 81,190 R, 3, Aq 142,400 77,240 107,040 R0 2 Aq,0 63,630 20,450 25,850 it will be evident that the affinity of selenium to oxygen, as measured by the heat developed, is less than that of sulphur to oxygen, and also less than that of tellurium to oxygen. In this respect, therefore, sulphur, selenium, and tellurium form a series corresponding to that in which chlorine, bromine, and iodine may be arranged. Bromine, we have seen, is intermediate in its properties bet-ween chlorine and iodine, and its atomic weight is also almost the mean of the atomic weights of these two elements, and its affinity to oxygen is less than that of either chlorine or iodine ; the atomic weight of selenium, which is inter mediate in its properties between sulphur and tellurium, is also nearly the mean of the atomic weights of these elements. Constitution of the Sulphur Compounds, The constitution of the compounds of sulphur with monad elements may be readily deduced ; thus, the only formulae by which the chlorides of sulphur can be repre sented graphically are as follows : Cl C1_S S Cl Cl S Cl Cl SC1 Cl But the constitution of each of the oxides of sulphur may be expressed in two ways, thus S O S O / / O O o Sulphur dioxide. Sulphur trioxide. In like manner, two corresponding formula may be assigned to each of the acids formed by combining these oxides with the elements of a molecule of water. Sulphuric acid, for instance, may be represented by either of the following formulae O II H S H H O S O H II O But as compounds in which two or more atoms of oxygen are directly united together (for example, hydrogen dioxide, the higher oxides of chlorine, and chloric acid, as to the constitution of which there can be little doubt as they contain only monad elements associated with oxygen) readily decompose with separation of oxygen, the first of these formulae for sulphuric acid appears improbable on account of the stability of the acid and of most of the