Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/500

Rh 488 To facilitate comparison between the various acids they are arranged in the following table almost in accordance vyith the amounts of heat developed on the addition of sodium hydroxide to equivalent quantities. CHEMISTRY [ACIOB AND &amp;lt;J Units of heat developed in the reaction (2NaOHAq, Name of Acid. Formula. QAQ). 2H.F1 32,540 Sulphuric H 2. S0 4 31,380 H s .Se0 4 30,390 Hypophosphorous. . . Sulphurous 2(H . PH.O,) H 2. S0 3 30,320 28,970 Metaphosphoric 2(H . P0.j&amp;gt; H 2 . PHO. 28,750 28,370 Oxalic H 2 . C 2 O 4 28,280 Hydrochloric Hydrobroiiiic 2H.C1 2H . Br 27,480 27,500 Tivdriodic 2H.I 27,350 Chlorio 2H.C10 3 27,520 j^itric 2H. NO 3 27,360 Dithionic H. 2 .S 2 O 6 27,070 Selenious H 2 .Se0 3 27,020 Chloroplatinic H 2 . PtCl 6 27,220 Vluosilicic H..SiF 6 26,620 Sulphovinic 2(H. S0 4 C 2 H 5 ) 26,930 Formic 2(H.CH0 2 ) 26,400 Acetic 2(H. C 2 H,0 2 ) 26,310 Pyrophosphoric Phosphoric i(H 4 .P 8 &amp;lt;W H 2. PO 4 H 26,370 27,080 Arsenic. . .. . ll. 2 . As0 4 H 27,580 Citric 3(H,. CJI-O-I 25,470 Tartaric H Q . C,H,O 25,310 Succinic H 9 . C.H.O. 24,160 Chromic H 2 . CrO 4 24,720 Carbonic H. 2 . CO, 20,180 Boric . H 2 . B 2 O 4 20,010 Hypochlorous Hydrosulphuric Hydrocyanic 2(H. OC1) 2(H. SH) 2(H. CN) 19,370 15,480 5,530 Stannic (H 4. Sn0 4 ) 4,780 Silicic 4(H,.SiO,) 2,710 From these tables it will be evident (1), that when a molecule of sodium hydroxide in aqueous solution enters into reaction with an acid, the heat developed is very nearly proportional to the quantity of acid present until this amounts to 1, J, J, or ^ molecule, according as the acid is mono-, di-, tri-, or tetrabasic ; but that when the amount of acid added exceeds that requisite to form the normal salt, the different acids behave differently, heat being in some cases developed, and in others absorbed, according to the constitution of the acid ; and (2), that mostly when a molecule of an acid in aqueous solution enters into reaction with sodium hydroxide, the amount of heat developed increases almost in proportion to the amount of the latter, and until 1, 2, 3, or 4 molecules are added, according as the acid is mono-, di-, tri-, or tetrabasic ; the further adlition of sodium hydroxide is not then attended with any considerable development of heat. Very different amounts of heat, it will be observed, are develop?! on neutralizing the different acids, but there is mostly a remarkable similarity in the results obtained in the case of acids which from chemical evidence are known to be closely allied. Thus, hydrochloric, hydrobromic, and hydriodic acids exhibit the same heat of neutralization; and the numbers for sulphuric and selenic acids, and for phosphoric and arsenic acids, are very similar. Hydro fluoric acid, it will be seen, differs considerably from the allied monobasic acids since the addition of the first half molecule of sodium hydroxide produces less heat than the second, which is not improbably owing to the formation of the acid salt HF 2 Na; it differs also by its high heat of neutralization. The tables show also that the dibasic acids may be divided into several groups, according to the amounts of heat developed on the addition of the first and second molecules of sodium hydroxide. The first group includes hydrofluosilicic and hydrochloroplatinic acids, the amount of heat developed by the second molecule of hydroxide being equal to that developed on the addition of the first molecule. The second group includes sulphuric, selenic, oxalic, and tartaric acids ; with these acids less heat is de veloped by the first than by the second molecule ; thus : Sodium hydroxide. Sulphuric acid. Selenic acid. Oxalic acid. Tartnrlc acid. 1st Molecule 146 148 138 124 2d 164 156 145 129 111 the third group, which includes sulphurous, selenious, carbonic, and boric acids, and probably also chromic, phosphorous, and succinic acids, the contrary is the case : Sodium hydroxide. Sulphurous acid. Selenious acid. Carbonic acid. Boric acid. 1st Molecule 159 148 110 Ill 2d ,, 131 122 92 89 The tribasic acids exhibit similar differences ; thus : Sodium hydroxide. rltric acid. Arsenic acid. Phosphoric acid 1st Molecule 124 150 148 2d ,, 120 126 123 3d 132 83 69 These differences which acids of the same basicity ex hibit when submitted to thermochemical investigation correspond to differences in their chemical behaviour. For example, when a solution of citric acid is neutralized with sodium carbonate and evaporated to crystallization, the trisodium salt Na 3 C 6 H 6 7 is readily obtained, but when a solution of phosphoric or arsenic acid is similarly treated, the disodium salt Na 2 HP0 4 or Na 2 HAs0 4 is formed ; the trisodium salts of these acids can only be procured by adding sodium hydroxide. Apparently the trisodium salts of phosphoric and arsenic acids are par tially decomposed by water, as their solutions are strongly alkaline; hence the third molecule of hydroxide does not effect the complete conversion of the di-into the tri-sodium salt. The behaviour of sulphuric acid will be discussed later on. All soluble hydroxides when in solution appear to have nearly the same heat of neutralization, as will be seen from the following table, which exhibits the number of units of heat developed on neutralizing solutions of equivalent quantities of various hydroxides with a solution of one molecule (in grammes) of sulphuric acid, or of the equivalent quantities (2 molecules) of hydrochloric or nitric acid at 18 C. : Name of hydroxide. Sulphuric acid. Hydrochloric acid. Nitric acid. Lithium hydroxide 31,290 27,700 Sodium 31,380 27,490 27,360 Potassium Thallium 31,290 31,130 27,500 27,520 27,540 27,380 Barium Strontium Calcium 30 ,710 31,140 27,780 27,630 27,900 28,260 But very different amounts of heat are developed on dissolving the hydroxides which are insoluble in water in acids. The following numbers represent the heat of