The close thermal analogy between the hydracids in question is exhibited by these, among other, numbers; [HX, Aq] X=Br=19,900 X=I = 19,200. [HXAq, NaOHAq] X=Br=13,700 X=I=13,700. But when we compare the heats of formation of these acids, in aqueous solutions, we find that the value of this constant for each acid decreases as the atomic weight of the halogen The three oxyacids corresponding, in composition, to the three hydracids, are HCIO,, HBrO,, and HIO,. The following numbers shew that, in some respects at any rate, the thermal relations between HClO, and HBrO, are analogous to those between HCl and HBr: [H, X, O3, Aq] X=Cl=23,900 X= Br=12,400: hence the difference, [H, Cl, Aq]-[H, Br, Aq], is approximately equal to the difference [H, Cl, O3, Aq]-[H, Br, O3, Aq]. From this we might provisionally conclude that the difference between the heats of formation, in aqueous solutions, of chloric and iodic acids, would probably be nearly the same as the difference between the heats of formation, under the same conditions, of hydrochloric and hydriodic acids. The value of the second difference is 26,100; hence, on this supposition, the first difference should be about 26,000. Now, [H, Cl, O3, Aq] = 23,900; .. [H, I, O®, Aq]= – 2,100. But experiment shews that [H, I, O3, Aq]= +55,7oo. Hence it is evident that iodic acid differs in the most marked manner from bromic and chloric acids. This difference is accentuated in the numbers expressing the heats of formation of these three acids from the three hydracids: thus, [HXAq, O3] X=Br=15,900 - . X=I = 42,600+. Iodic acid is probably dibasic, and may be represented by the formula H,IO̟1. 128. A comparison of the mutual thermal actions of acids and bases throws considerable light on the classification of the substances which are included under these terms. The first volume of Thomsen's Untersuchungen is devoted to a consideration of this subject. 'Heat of neutralisation of an acid by a base' is defined as, the quantity of heat evolved on mixing equivalent quantities, in grams, of the acid and base, in dilute aqueous solutions, the products of the action being also soluble in water. Thomsen employs a solution of 2NaOH in about 200 H2O (grams), and adds the acid solution diluted to a similar degree, temperature being 18°-19°; in other words he determines the thermal value of the change [2NaOHAq, 2HXAq] in the case of a monobasic acid, Most of the general conclusions drawn by Thomsen, and others, belong more to chemical kinetics than to statics, but some of the generalisations may fitly be introduced here. The commoner acids may be broadly divided into four groups according to the values of their heats of neutralisation, as thus defined. I. Those acids which have a heat of neutralisation approximately equal to 20,000 gram-units :— HNO2, HCIO, H2B2O4, H,CO, &c. 1 See Thomsen, Ber. 7. 112 (or Untersuchungen, 2. 423). 2 See especially for more details Thomsen, loc. cit. 1. 293–309, and 422–449. II. Those acids which have a heat of neutralisation approximately equal to 25,000 gram-units : HọCrO4, C,H,CO,H),, CH,CHOH(CO,H), &c. III. Acids the heat of neutralisation of which is equal to about 27,000 gram-units: HCL, HBr, HI, HCIO,, HBrO3, HIO3, HNO3, H2S2O3, H.SiF, H.CO2H, CH,. CO,H &c. Most of the acids belong to this class. IV. Acids having a heat of neutralisation greater than 27,000 units, and varying from 28,000 to 32,500 units:— CH2CI.CO2H, CHCI.CO2H, CC. CO2H, H2С2O, H ̧PО ̧, H2SO3, H2SO, H.Se04, HF, HPO, &c. A few acids have heats of neutralisation less than 20,000 units. The value of the heat of neutralisation of an acid does not appear to depend on the basicity, nor on the composition of the acid; neither does it depend on what Thomsen calls the ' avidity' of the acid, i.e. the striving of the acid to displace another from combination with a base. The relative 'avidities' of acids will be considered in book II', meanwhile the meaning of the term may be made clear by an example. When equivalent quantities of NaOH, HNO,, and H2SO, àre mixed in dilute aqueous solutions, two-thirds of the NaOH are found to combine with the HNO,, and one-third with the H.SO. Hence HNO, is said to have an avidity' for NaOH twice as great as that of H,SO, for the same base; HNO, in aqueous solution is therefore a 'stronger' acid than H2SO. The basicity of an acid may be determined by thermal methods. One gram-molecule of the acid in dilute aqueous solution is mixed with,,, 1, 2, &c. gram-molecules of NaOH also in dilute solution, and the heat evolved in the reactions is measured. (The ordinary formulæ NaOH, H2SO, &c. are here assumed, for the sake of convenience of nomenclature, to be molecular). Comparing in this way HCl, H,SO,, and C.H.O, (citric acid), we have this result, 1 Chap. III. par. 233. [HCIAq, NaOHAq]=about 6,000 [H2SO1Aq, NaOHAq]= about 7,000 Hence we conclude that HCl is a monobasic, H2SO, a dibasic, and CHO, a tribasic acid. The application of this method to the oxyacids of phosphorus and arsenic leads to interesting results'. The data are presented in the following table : The heat of neutralisation of this acid gradually increases till it becomes equal to about 33,600 units. 1 See Thomsen, loc. cit. 1. 201—205; and Jahn, Die Grundsätze der Thermo chemie, 110-113. = 4 Hypophosphorous (H,PO,), and metaphosphoric (HPO1) acids are evidently monobasic; phosphorous (H,PO), and arsenious (H,AsO,) acids are dibasic; orthophosphoric, and arsenic acids (H,PO, and H,AsO,) are tribasic, and pyrophosphoric acid (H,P,O,) is tetrabasic. The gradual rise in the value of the heat of neutralisation of HPO,Aq is explained by the fact, that an aqueous solution of this acid is slowly decomposed with formation of H,PO,; the final number obtained (33,600) therefore represents the neutralisation of H,PO, and not of HPO,. A comparison of the heats of neutralisation of H,PO, and H,AsO, shews that the former is a much 'stronger' acid than the latter. A similar comparison of H,PO, with H,ASO, however shews that these acids are very analogous. Thus, the heat of neutralisation of H,PO,= 34,000, and of H,ASO, 36,000; moreover about three-fourths of the total heat is evolved, in each case, during the replacement of the first and second atom of hydrogen by sodium; and finally the addition of an excess of soda over that required for neutralisation, causes the evolution of an appreciable quantity of heat. The last fact is explained by the comparative instability in aqueous solutions of Na,PO, and Na,AsO,, which salts are partially separated by water into Na,HPO, and H.PO, and Na,HASO, and H,ASO,, respectively; hence the addition of more soda than is required to form either of these salts evolves a little heat, because it enters into reaction with the small quantity of phosphoric, or arsenic acid, present in the liquid. A similar phenomenon is noticed in the neutralisation of tetrabasic phosphoric acid; more than one-fourth of the total quantity of heat is evolved during the action of the first molecule of NaOH, and more than a half during the action of the first and second molecules. We should thence expect the tetrasodic salt to be less stable than the disodic salt; that this is so is shewn by the evolution of 1800 units of heat on addition of two molecules of soda more than the amount required for complete neutralisation'. The polybasic acids may also be classified in accordance 1 In these, and other similar reactions, for convenience of nomenclature, the formulæ NaOH, H2SO4, &c. are regarded as molecular. |