Page:Outlines of Physical Chemistry - 1899.djvu/170

 ��OUTLINES OF PHYSICAL CHEMISTRY

��Heats of Formation (15°) — continued

��Metallic Oxide*

�Solid

�Dissolved

�Sulphides

�Solid

�Dis- solved

�(Tl* 0). .

�(Tl„ S). .

�m, 0, H,0).

� � � �Tl* 0„ aq).

� � � � �Hg* 0) ppt.

� � � � �(Hg, 0). .

� �(Hg,S) black; ppt. .

� � � � �(Hg, S)red. (Ag„ S) ppt. .

�(Ag* 0).

� �(Pt, 0) •

� � � � �(Pd, 0, aq)

� � � � �(Pd, 0„ aq).

� � � � ��Remarks. — 1. When a substance is endotherrnk (formed with absorption of heat) it is not to be assumed that the isolated atoms combine to produce a negative thermal effect.

The equation (I, H) = — 6*4 C. is really the expres- sion of a double decomposition : I 2 + H 2 = 2HI, and signifies that two free atoms of iodine and two free atoms of hydrogen, in forming two molecules of gaseous hydriodic acid, evolve less heat than in forming one molecule of iodine and one molecule of hydrogen.

In the same way (N 2 ,0) = — 20*6 C. denotes that in nitrous oxide the atoms are less firmly bound than they are in the elementary gases.

2. In order that thermo-chemical data may have a precise significance, it is necessary that the physico- chemical state of the reacting substances should be well defined. For instance, the heat of combustion is not the same for allotropic modifications of the same element : it is smaller for diamond than for amorphous carbon, also smaller for rhombic sulphur than for the monoclinic or amorphous variety. We see from this that different quantities of heat are dissipated in isolating the atoms of sulphur or of carbon from the different modifications of these elements.

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