THERMO-CHEMISTRY. 210 THERMO-CHEMISTRY. due in part also to the purely physical changes of state that often accompany chemical reac- tions : and for this reason the scope of thermo- chemistry must extend over physical as well as chemical changes of matter. For the principles of thermo-chemical notation, see Chemistry, section on Transfuntiations. What has thus far been accomplished in the domain of thermo-chemistr}- niaj' be summed up as follows: First, a large number of transforma- tions have been subjected to calorimetric meas- urement, and so the total amounts of heat given oft' or absorbed during a great many changes have been determined experimentally; secondly, the principles of thermodynamics have been suc- cessfully applied to the consideration of chemical changes, and thus thermochemistry has been highly developed theoretically. Experimental Tiiermo-Chemistry. The ac- tual execution of thermo-chemical measurements is a matter of some ditliculty, owing to the considerable errors that may be caused by more or less heat being lost by radiation while the measurement is being carried out. The heat given off or absorbed is determined by keeping the vessel in which the reaction takes place im- mersed in a known quantity of water, and ob- serving the temperature of the latter before and after the reaction. But whatever the details of such a 'calorimetric' arrangement may be, what- ever the precautions taken to isolate the calori- meter from its surroundings, the loss of heat during an experiment would render the observa- tion worthless in every case in which the chemi- cal change studied would take place slowly. In consequence of this, thermo-chemical knowledge would necessarily be confined to rapid reactions alone, if it were not for the fact that early in the history of thermochemistry a principle be- came known that permitted of ascertaining the heat of slow reactions, too, by indirect methods. The principle in question is known as the lair of constant heat-sums. While clearly established by Hess in 1844, i.e. before the law of the conserva- tion of energy became known, it is nothing but a special form of the law of conservation. It is as fol- lows: The amount of heat given off or taken up ichen a given chemical system is changed into another is the same whatever the way in which the change may take place. Let, for example, the given chemical system consist of 17 grams of gaseous ammonia in one vessel, its eqviivalent 36.5 grams of gaseous hydrochloric acid in an- other vessel, and a large quantity of water. This system may he changed into a dilute aque- ous solution of ammonium chloride in two differ- ent ways: (1) ammonia and hydrochloric acid may be caused to combine in the gaseous state, yielding solid ammonium chloride and develop- ing 42,100 calories of heat; then the ammonium chloride may l)e dissolved in the water — a change accompanied by the ahmirption of 3900 calories; or (2) the gaseous ammonia may be dissolved in a large amount of water — a process developing 8400 calories ; the gaseous hydrochloric acid may he dissolved in a separate large quantity of water — a process developing 17.300 calories; and, finally, the dilute aqueous ammonia may be mixed with the dilute aqueous hydrochloric acid ■ — a process developing 12.300 calories. Which- ever the way adopted, the result is the same — viz. a dilute aqueous solution of ammonium chloride. The heat developed when the first way is adopted is 42,100-3900 = 38,200 calories; the heat developed when the second wav is adopted is 8400 + 17,300 + 12.300 = 38,000 calo- rics. The ligures 38,200 and 38,000, differing by only 2 parts in 382 (little more than '/:> per cent.), i.e. by less than the unavoidable cxjieri- mcntal error, must be considered as equal — which is in accordance with the law of constant heat sums. If, for some reason, it were impossible directly to measure, sa.v, the heat produced by the combination of gaseous ammonia and gaseous hydrochloric acid, that heat might be calculated, according to the law of constant heat-sums, by adding 3900 calories (the heat absorbed when one equivalent of ammonium chloride is dissolved in much water) and 38,000 calories (the total heat produced during the transformation, by the second way, of gaseous ammonia and hydrochloric acid into dilute annnonium chloride). The sum, 41,900 calories, would be aj near the truth as the 42,100 calories found by direct experiment. To take another, even simpler example, suppose it were asked. How nmch heat would be evolved or absorbed in the transformation of 12 grams of amorphous carbon into diamond? The trans- formation, although accomplislied on a minute scale by Jloissan, in his electric furnace, is of course inaccessible to direct calorimetric meas- uiement. But the law of constant heat-sums ])ermits of answering the question by measuring the heat of combustion of amorphous carbon and tliat of diamond. The transformation of amor- phous carbon into carbon dioxide, whether ac- complished by direct combustion or by first changing the carbon to diamond and then burn- ing the latter, nuist be accompanied by the evo- lution of the same amount of heat, viz. 97.650 calories; and as the heat of combu.stion of diamond is 94.310 calories, the transformation of amorphous carbon (12 grams) into diamond nnist, according to the law of constant heat- sums, be accompanied bv the evolution of 97,650 — 94,310 = 3340 calories. In a similar manner Hcss's law permits of ascertaining the heat that would be developed during the formation of compounds (e.g. the majority of organic compounds) whose forma- tion from the elements could not be directly .stud- ied calorimetrically. I>et it be required, for in- stance, to ascertain the lieat that would be de- veloped or absorbed if ordinary alcohol (C,H„0) were made from its elements — carbon (in the form of diamond), hydrogen, and oxygen. To do this, we may determine calorimetrically the heat (call it a) developed by the comhustion of one gram-molecule of alcohol and the heats of combustion (6 and c) of quantities of isolated carbon (diamond) and hydrogen equal to those contained in one gram-molecule of alcohol. The three combustions uiay be represented by the following equations: C,H.,OH + 60 = 2C0. -f 3H„0 + n calories; 2C + 40 = 200, + 6 calories: 3H; +30 = 3H,0 -f c calories. Adding the second and third equations, we get: 2C + 3H, -f 70 = 2C0, + 3H.0 + 5 -f c, and subtracting the first equation from this, we get: 2C + 3H, + O — CH.OH = 6 -f e — a, or 2C + 3H. + O = aH,OH-f 6 + c — a. This last equation, expressed in words, means
