of barium, strontium, and calcium, obey the same law; each acid acts in accordance with its mass and its specific affinityconstant1. 228. The questions thus partly solved by Ostwald have been attacked by J. Thomsen by thermochemical methods. When two, or more, acids and one base react in equivalent quantities, in a dilute aqueous solution, what are the proportions in which the acids combine with the base ? Measurements of thermal changes must, it would seem, throw light on this question. The process evidently is one wherein an equilibrium is established. Can the distribution of the masses of the acting bodies be deduced from measurements of the thermal gains or losses which accompany this distribution? 229. Thomsen's method of attacking these questions rests upon the following considerations. The various acids, by neutralisation with the same base, develop unequal quantities of heat. Now if one acid replaces another from its combination with a given base, the operation will be attended by a thermal change, which will be positive or negative according as the free acid, or the acid already combined with the base, possesses the greater heat of neutralisation. The extent of the reaction can be deduced from measurements of the thermal values of the different parts of the operation. Thus, take the reaction between nitric acid and sodium sulphate; the thermal values of the following changes must be determined. (1) Neutralisation of sulphuric acid by soda. (2) Neutralisation of nitric acid by soda. (3) Decomposition of sodium sulphate by nitric acid. (6) Action of nitric acid on sodium nitrate. (7) Action of sulphuric acid on nitric acid2. 17. für prakt. Chemie (2) 29. 49. * Thomsen, Thermochemische Untersuchungen 1. 98. M. C. 28 230. Now when nitric acid and sodium sulphate react, in equivalent quantities, in a dilute aqueous solution, heat is absorbed; but when sulphuric acid and sodium nitrate react, under similar conditions, heat is evolved. But the final distribution of the base between the two acids will be the same in both cases; and moreover this distribution will be the same as that which results when equivalent quantities of the two acids (sulphuric and nitric), and the base (soda) mutually react. This statement may be put in a general form thus. Let the three bodies A, B, and A' react in equivalent quantities, in a dilute aqueous solution; then, hence [A, B, A']=[A, B]+[AB, A'] =[A', B]+[A'B, A]; [A'B, A]–[AB, A']=[A, B]–[A', B]. Now if ASO,Aq; B = Na,OAq; and A' = N,O,Aq; it follows that [Na2N2O Aq, SO3Aq] - [Na2SO1Aq, N2O5Aq]=[Na2OAq, SO3Aq] -[Na2OAq, N2O6Aq]. The differences between the thermal values actually observed were 4,144 and 4,080 gram-units respectively'. When an equivalent of nitric acid (A') reacts on one equivalent of sodium sulphate (AB) with decomposition of x equivalents of the latter salt, the final distribution of acids and base may be represented by the expression, (1-x) AB+xA'B+xA+ (1 − x) A'. And the total thermal change accompanying this operation will consist of the following partial changes; (a) that attending the decomposition of r equivalents of AB, i.c. Na,SO4; (b) that attending the formation of r equivalents of A'B, i. e. Na NO; 1 loc. cit. p. 112. (c) that attending the reaction of r equivalents of the acid A, i.e. H.SO4, on (1-x) equivalents of the salt AB; (d) that attending the reaction of (1 − x) equivalents of the acid A', i.e. HNO3, on r equivalents of the salt A'B; and (e) that attending the reaction of r equivalents of the acid A on (1-x) equivalents of the acid A'. The total thermal change may therefore be expressed by the formula, [AB, A']=x[(A', B) − (A, B)]+[(1 − x) AB, xA]+[xA'B, (1 − x) A'] +[(1−x) A', xA]1. 231. Values have been found by Thomsen for all the partial thermal changes, except the last, the value of which is so small that it cannot be accurately determined, and may therefore be omitted from the calculation. The following data, required for determining the values of the reactions (a), (b), (c), are the results of a large series of measurements made by Thomsen 2. 6 The complete decomposition of Na,SO into NaNO̟ is attended with the absorption of 4144 gram-units of heat. [Na2SO1.ySO3Aq, 2N2O5Aq] - 4052 (2) [Na N2O6Aq, N2O5Aq]=-78 (as this is so small it is neglected in the calculations). The following approximate formula is deduced for finding the thermal value of this reaction for any value of n: 232. Substituting the chemical formulæ of the various bodies, and the actually observed thermal values, in the equation in par. 230, p. 435, we have this result: [Na2SO4Aq, NO3Aq]=x[N2O3Aq, Na2OAq] - [SO3Aq, Na2OAq] (the thermal values of the other parts are so small that they are omitted) Then, from the actually observed thermal values of the reaction between Na,SO,Aq, and пSO,Aq (see data for reaction (c)), a value must be sought for x which shall give a result in agreement with the total thermal value of the change, which value is - 3504. If x is taken as equal to }, we get this result: [Na2SO1Aq, N2O5Aq]=-.4144+} [Na2SO4Aq, 2SO3Aq] The difference between the observed and calculated values is only about 125 per million of the heat of neutralisation. The thermal value of the reverse action, that namely between sulphuric acid and sodium nitrate, is found by the equation, x [Na2N2O®Ag, SO3Aq]=(1 − x) 4144+ (1 − x) [ Na2SOaAq, x) [Na2SO1Aq,, ~ SO3Aq] = 598. The observed value was 576; the difference does not amount to more than 7 per million of the heat of neutralisation. 233. From these results, Thomsen draws the following conclusions. (a) When soda, nitric acid, and sulphuric acid mutually react in equivalent quantities, in a dilute aqueous solution, two-thirds of the soda combines with the nitric acid, and one-third with the sulphuric acid. (b) The final division of the base between the two acids is the same whether the soda were originally present as sulphate or nitrate1. (c) The striving of the nitric acid to saturate itself with the base (das Bestreben sich mit der Basis zu sättigen), is twice as great as that of the sulphuric acid. Nitric acid, in aqueous solution, is therefore a stronger acid than sulphuric 2. This striving of the acids towards neutralisation, Thomsen calls the avidity of the acids. The expression evidently conveys exactly the same meaning as the term affinity in Ostwald's nomenclature. 234. Applying the method sketched above to the case of hydrochloric and sulphuric acids reacting on soda, Thomsen gets the following result: [Na2OAq, H2Cl2Aq]=27,480. 1 The truth of this statement has already been assumed (par. 230). |