### Recherches thermochimiques

Germain Henri Hess

Bulletin scientifique

Volume 8, p. 257-272
St. Petersburg, 1840

The following are the experiments which serve to find the amount of heat developed by the sulfuric acid.

7.
 The apparatus full of water 7,809.7 Glass vessels 100g. x 0.19 19 Acid 266.4 Water 48 314.4 () x 0.474 = 149.9 Total mass = 7,978.6 Increase in temperature 2.1° Which gives 77.17

Readers Note: the symbolism is that of Berzelius, a barred letter referring to a double atom and a dot to an atom of oxygen. Thus is H2SO4 in modern symbols.

8.
 The apparatus full of water 7,809.7 Glass vessels 100g. x 0.19 19 Acid 370 Water 71 441 x 0.474 = 210.3 Total mass = 8,039.0 Increase in temperature 2.9° Which gives 77.33

N.B. The heat capacity was determined by direct experiment in all cases where the contrary is not indicated.

9.
 The apparatus full of water 7,809.7 Glass vessels 150g. x 0.19 27.5 Acid 185 Water 71 256 () x 0.5 = 128.0 Total mass = 7,965.2 Increase in temperature 2.2° Which gives 116.7

N.B. The heat capacity of the acid is not the result of direct experiment, but an approximate estimate.

10.
 The apparatus full of water 7,809.7 Glass vessels 150g. x 0.19 27.5 Acid 528g. Water 85 613.2 [sic] x 0.5 = 306.6 Total mass = 8,143.8 Increase in temperature 1.7° Which gives 38.56

11. I also attempted to determine the quantity of heat evolved by the anhydrous acid [SO3]. For that it was collected in a tube and weighed with the glass. Being unable to use the whole of the calorimeter because of the small quantity of acid which I had at my disposal, I took only the interior cylinder, which was carefully covered with a poor conductor. In this, well closed and shaken, combination took place, and immediately afterward the cylinder was opened to introduce the thermometer and observe the temperature. The heat which was produced at the point of contact was so great that the glass tube was completely shattered. It was then a matter of the greatest care that not the least particle of glass be lost. All the fragments being collected, washed, and weighed, the difference from the preceding weight, 15.92 grams, indicated the quantity of anhydrous acid used. As this manner of determining the quantity of the acid appeared too easily susceptible to error, and as the acid obtained by the mixture was too dilute to be determined by the aerometer, I decomposed a certain quantity of the acid from the mixing with a perfectly neutral solution of barium chloride and dipped therein a piece of weighed marble as recommended by M. Runge for muriatic acid. This gave me 16 grams of anhydrous acid.

 Glass 5.26 } both corrected for their heat capacity Cylinder 93.47 Acid 15.97 Water 700.00 Increase in temperature 10°

The result was not otherwise corrected, in view of the fact that the heat capacity of the mixture was not found to differ sensibly from that of water.

 These values give . . . . . . . . . . . . . . . . . . . . . . . . . 510.1

12. From section 7 and 8 it follows that an atom of water added to evolved 77.17 and 77.33 of heat. Section 9 gives 116.7 for two atoms of water, of which 2/3 (=77.8) corresponds to the first atom and 38.9 to the second. Finally, section 10 gives us directly for this same atom of water 38.56.* If we add to that the result of section 11 and those which were cited above (section 4), we have the following series:

[*Editor's Note: These results in themselves are evidence for Hess's Law, although he choose to prove the law by data from another reaction later on.]

 Composition Heat Evolved . . . . . . . . . 310.4     8 . . . . . . . . . 77.86     2 . . . . . . . . . 38.9        1 . . . . . . . . . 38.9 . . . . . . . . . 38.9 ------- 504.96

From these figures we should obtain by mixing with an excess of water

 . . . . . . . . . 504.96 . . . . . . . . . 194.5 . . . . . . . . . 116.7 . . . . . . . . . 77.8 . . . . . . . . . 38.9

The agreement among these numbers is such that they prove perfectly the law of multiple proportions for the quantities of caloric evolved. As to the absolute value of these quantities, it is certain that they have not attained the rigor which we could want later, but for the moment I believed that it was more important to attempt to establish the fundamental laws of this part of the science rather than stop to discuss whether for agreement the value 38.9 or 39 ought to be admitted.

13. A combination having taken place, the quantity of heat evolved is always constant whether the combination is performed directly or whether it takes place indirectly and in different steps.

14.
 The apparatus full of water 7,809.7 Glass vessels 100g. x 0.19 19 Acid 92.5 Ammonia (density 0.936) 280.5 373.0 x 0.828 = 308.0 8137.5 Increase in temperature 5.44° Another experiment 5.6 ---- mean 5.52 These figures give 595.8

15.
 The apparatus full of water 7,809.7 Glass vessels 100g. x 0.19 19 Acid 88 Ammonia 233.75 321.7 x 0.76 = 244.5 8073.2 Elevation of temperature 3.92° These figures give 518.9

16.
 Acid 81.5 Ammonia 187.0 268.5 x 0.77 = 206.7 Glass 19 The apparatus full of water 7,809.7 8035.4 Increase in temperature 2.9° These numbers give 480.5

17.
 Acid 70.5 Ammonia 93.5 164.0x 0.786 = 128.9 Glass 19 The apparatus full of water 7,809.7 7957.6 Increase in temperature 1.7° These numbers give 446.2

18. The experiment with anhydrous acid not yet having been made, let us take for a point of departure the ordinary hydrated acid and add to each of the subsequent results the quantities of heat evolved by the saturation of a part of the acid () by ammonia,

 Acid Quantity of heat evolved Sum By ammonia By water 595.8 . . . . . 595.8 518.9 77.8 595.7 480.5 116.7 597.2 446.2 155.6 601.8 Mean . . . . . . . . . . 597.6

19. Since the anhydrous acid evolves in all 510.1, and in order to have the quantity which it evolves in becoming it is necessary to subtract from this number 38.9, the sum of the heat evolved by the supposed anhydrous acid in combining with liquid ammonia would be 1,069.1.