THERMO-CHEMISTRY, or THER MAL CHEMISTRY, that branch of physical chemistry which deals with the thermal changes which occur when chemical reactions take place or when a body or system (such as a solution) undergoes certain kinds of physical change. Its precise limits are not easy to define, since the subject merges into ordinary chemistry on the one hand and into thermodynamics on the other. Any chemical operation can be con sidered from two points of view, according as we are interested in the modification that it produces in the nature of the substances that are involved or in the quantity of energy which is absorbed, liberated or otherwise transformed at the same time. It is the province of thermo chemistry to investigate the transf3rmations of energy that occur in such cases. The com plete discussion of the energy-transformations that accompany a given chemical change should include the consideration of every type or form of energy which may be present; but the in vestigations which have hitherto been made have related chiefly to the quantities of heat which are liberated or absorbed, and it is to this circumstance that the science owes its pres ent name, The quantity of heat that is liberated or ab sorbed during a proposed chemical reaction can be determined by causing the given reaction to take place in the interior of a calorimeter. The particular form of calorimeter that is to be used will naturally depend to a considerable extent upon the nature of the reaction that is to be studied. If the problem consists in the deter mination of the quantity of heat that is liber ated when two given liquids are mixed, the calorimeter commonly consists of a platinum vessel, capable of containing, from 500 to 1,000 cubic centimeters, placed inside of another ves sel of silver; the space between the two vessels being filled with water. The liquids that are to be examined are brought to the same tempera ture as nearly as possible, and are then mixed in the platinum vessel. The rise of temperature of the calorimeter being noted, and the masses and specific heats of the various parts of the calorimeter (and its contents) being determined by separate experiments, we are then in posi tion to calculate the quantity of heat energy lib erated by the reaction. For detailed informa tion with regard to the various kinds of calori meters that are used, and for a discussion of the sources of error to which such instruments are liable, reference must be made to extended works upon heat and thermo-chemistry. (See the references at the end of this article).
In thermo-chemical work, the unit of mass is almost invariably the gram; and the gram is always understood, when no other unit is specifically mentioned. The unit of heat is
also understood to be the calorie, which, for thermo-chemical purposes, is defined as the quantity of heat required to raise the tempera ture of one gram of water by one Centigrade degree, when the temperature of the water is in the vicinity of 18° or 20° C. The notation that is employed in expressing the results of a thermo-chemical experiment upon the heat that is developed by a given chemical reaction is simple. The formula: of the substances that react are written within square brackets, and separated by a comma or a colon; it being understood that the number of grams that are present of any one substance is equal to the molecular weight of that substance. A sign of equality is written after the bracketed for mulae, and on the right hand side of this sign the number of calories of heat generated or absorbed by the reaction is written ; a positive sign being prefixed (or suffixed) when heat is evolved, and a negative sign when it is ab sorbed. The indices that are attached to the symbols of the various elements are written above those symbols, instead of below. For example, [W,0]=-*+ 68,360 signifies that when two grams of hydrogen and 16 grams of oxygen, both at about C. and under ordinary atmospheric pressure, combine to produce 18 grams of water (also at 18° C.), the quantity of heat that is evolved is sufficient to raise the temperature of 68,360 grams of water by one Centigrade degree; the temperature of the water being about 18° C. When a compound is broken up into its con stitutent parts, the bracketed formula are pre ceded by a negative sign. Thus the expres sion, — RI,C11=-22,000 signifies that when, by any means, 36.5 grams of hydrochloric acid are decomposed so as to set free 1 gram of hydrogen and 35.5 crams of chlorine, the change is accompanied by the ab• sorption of a quantity of heat that would be sufficient to raise the temperature of 22,000 grams of water by 1° C.
The "heat of formation" of a substance is the quantity of heat given out when the sub stance is formed from its constituents; it being taken as negative when the formation of the substance is accompanied by the absorption of heat. In general, any given substance may be prepared in various ways, from different mate rials or constituents; and in such cases the heat of formation will be different, according to the particular substances that are regarded as con stituents. For example, sulphuric acid might be prepared from sulphur, oxygen and water, according to the equation S + 30 + H20 or from sulphur dioxide, oxygen and water, ac cording to the formula