Thermal measurements appear then to indicate differences between the action of zinc, magnesium, nickel, &c., and that of copper, mercury, and silver, on dilute nitric acid. The gaseous products of the action of the first class of metals are probably for the most part the results of secondary changes occurring between nascent hydrogen and the acid, whereas the same substances when arising from the action of metals of the second class may rather be regarded as the results of a direct deoxidising action exerted by the metal on the acid'.

Traube, as we found in chapter II. (pars. 43, 44), from investigations conducted on lines very different from those of thermal chemistry, was led to regard the action of copper on nitric acid as essentially a deoxidising action.

125. From what we have learned regarding atomic and molecular systems, and from a consideration of the preceding paragraphs of this section, it follows almost necessarily that change from one allotropic modification of an element to another must be accompanied by absorption or evolution of heat. A few thermal measurements are given here to shew that this conclusion is fully justified by facts.

A. [P2, O3]=369,100 units + when P2 is 62 grams of ordinary

2

phosphorus (Pa);

[P2, O3]=326,800 units+ when P, is 62 grams of amorphous

=

phosphorus (PB);

.. the change of Pa to PB 21,150 units of heat +.

In the oxidation of 31 grams Pa to H3PO4 in aqueous solution by hypochlorous acid, 209,500 thermal units are evolved;

in the oxidation of 31 grams P8 to H,PO, in aqueous solution by hypochlorous acid, 181,200 thermal units are evolved;

.. the change of Pa to PB = 28,300 units of heat +.

Hence mean value of this change = 24,725 gram-units +.

B. [20z=30]= 59,200 units of heat+; that is to say

the separation of 2 gram-molecules of ozone (O3) into 3 gram-molecules of oxygen (O2) is attended by the evolution of 59,200 units of heat.

1 For thermal details concerning the action of metals on sulphuric and nitric acids see Naumann, loc. cit. 477-482.

M. C.

18

The comparative thermal instability of the molecule O helps us to understand why ozone is so much more active as an oxidising agent than ordinary oxygen'.

C. If Sa represent 32 grams of octahedral sulphur, Ss the same mass of prismatic, Sy the same mass of soluble amorphous, and S♪ the same mass of insoluble amorphous sulphur; then

the change of Ss to S, is accompanied by the evolution of

126. Too little has as yet been done to allow of the application of thermal measurements to the classification of the elements in any but a very general way.

The relations existing between the members of a group of elements are sometimes summarised in the thermal values of comparable reactions undergone by these elements. Thus, (see table p. 240) taking Mendelejeff's group II. we have,

The heats of formation in aqueous solution of the haloid salts of these metals are arranged in the following table (data from Naumann's book):

1 According to van der Meulen (Ber. 16. 1853) the thermal value of the

change in question, 203=302, is about 68,000 units.

2 For more details see Naumann, loc. cit. 486.

Hence we conclude that in each case the value for Ba > Sr > Ca > Mg, and for Mg > Zn > Cd > Hg. In other words, the thermal value of the change [M, X, Aq] increases as the atomic weight of M increases, when M is a member of an even series belonging to group II. but decreases as the atomic weight of M increases, when M is a member of an odd series of the same group. The difference between the values of [M, X, Aq] for each pair of elements is nearly constant. Thus

The close relationship of magnesium to calcium, and also its relations to barium and strontium, and the comparatively feebly marked relations existing between magnesium, zinc, cadmium, and mercury, are brought into forcible relief by these numbers1.

127. The comparative study of classes of compounds, no less than that of classes of elements, has already been considerably advanced by the application of thermal methods. Thus the relations between the oxides and oxyacids of nitrogen, phosphorus, and arsenic are suggested by the following data

1 Attention has already been drawn to the fact that there exists a well-marked connection of a periodic character between the atomic weights of the elements and their heats of combination with chlorine, bromine, and iodine. par. 109.)

(See ante,

The superior thermal stability of the oxides of arsenic as compared with the analogous compounds of nitrogen, and the comparatively very great stability of the oxides of phosphorus, are rendered evident by these numbers. A comparison of the heats of formation of nitric, phosphoric and arsenic acids (although the formula of the first is not strictly comparable with that of the second and third), establishes the same point. Thus

[N, O, H, Aq]=49Ioo+:

[P, O, H3, Aq]= 305,300+:

[As, O, H3, Aq]=215,200,

If the heats of formation of the three oxyacids of phosphorus are compared, it is seen that the change from hypophosphorous, or phosphorous, to phosphoric acid, is thermally very probable,

[P, O, H3, Aq]=305,300:

[P, O3, H3, Aq]=227,600 :

[P, O2, H3, Aq]=139,800.

A comparison of the thermal changes accompanying the formation and decomposition of the trichlorides of phosphorus, arsenic, antimony, and bismuth serves to illustrate the relations which exist between analogous chemical changes, and gains or losses of energy by the changing systems.

[P, CI3]=75,300 : [As, Cl3]=71,500: [Sb, Cl3]=91,400 : [Bi, Cl3]=90,600. [PC13, Aq]=65,100; giving H,PO3+3HCl.

In the decomposition of SbCl, by water, the greatest development of heat (8,900 units) corresponds to the formation of the oxychloride SbOCI,; the further change of this substance to Sb,O, and HCI involves absorption of a little heat.

.. [BiCl, Aq]=7,800-(-14,200))
- 14,200)} if BigO3. 3H2O+6HCI

were produced.

=3 = - 6,400

2

These numbers associate the stability of BIOC with great loss of energy in the formation of this compound.

Another way of stating the thermal reactions of analogous antimony and bismuth hydroxides illustrates the fact, that while antimony hydroxides are acid substances the corresponding bismuth compounds are marked by basic characters.

Thus [2SbO3H3, HCIAq]= 2,400; (forming SbOgCl2) :

but

[BiO3H3, HCIAq]= 14,200; (forming BIOCI).

The complete decomposition of a haloid salt by water may produce either hydroxide, hydrochloric acid, and water; or oxide, hydrochloric acid, and water. Taking the latter case, Thomsen has calculated the difference between the heats of formation, in presence of water, of oxides and chlorides, and has shewn that for all the nonmetals, except tellurium, antimony (trichloride), and bismuth, this difference is positive. We are not concerned here with tellurium; for antimony and bismuth the differences are

[Sb2, O3, H2O]-[Sb, C13]= 7,680 units -.
[Bi2, 03, H2O] - [Bi, Cl3]=21,700,,

Hence we should conclude that SbCl, and BiCl, would differ from other analogous chlorides in being only partially decomposed by water, and that the decomposition would be carried further in the case of antimony than in that of bismuth. This expectation is confirmed by the actually occurring reactions'; in the case of SbC1,, ths of the total decomposition (i. e. decomposition into oxide, hydrochloric acid, and water) is accomplished by the formation of Sb,O,Cl,; in the case of BiCl, the formation of BiOCI.H,O represents 3rds of the total decomposition.

A comparative study of some of the thermal relations of the hydracids and oxyacids of the halogens helps towards a classification of the latter group of acids.

1 For more details see Thomsen, Thermochemische Untersuchungen, 2. 25-39; 298-304; and 364-374.

2 See Thomsen, loc. cit. 1. 150-155, and 240-253; also Jahn, loc. cit. 136-138.