EXPERIMENTAL THERMO-CHEMISTRY. The ac tual execution of thermo-chemical measu•emerts is a matter of some difficulty, 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 thermo-chemistry a principle be came known that permitted of ascertaining the heat of slow reactions, too, by indirect methods. The principle in question is known at the lace 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 when 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 equivalent 36.5 grams of gaseous hydrochloric acid in an other vessel, and a large quantity of water. This system may be ehanged 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 ammonimn chloride and develop ing 42,100 calories of heat ; then the ammonium chloride may he dis-solved in the water—a change accompanied by the absorption 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 way is adopted is 8400 + 17,300 + 12.300 = 38,000 calo ries. The figures 38,200 and 38,000, differing by only 2 parts in 382 (little more than 12 per cent.), i.e. by less than the unavoidable experi mental error, must be considered as equal—which is in accordance with the law of constant heat If, for some reason, it were impossible directly to measure, say, 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 3S,000 calories (the total heat produced during the transformation, by the second way, of gaseous ammonia and hydrochloric acid into dilute ammonium chloride). The sum.
41,900 calories, would be near the truth as the 42,100 calories by direct experiment. To take another, even simpler example, suppose it were asked, How much heat would be evolved or absorbed in the transformation of 12 grams of amorphous carbon into diamond? The trans formation, although accomplished on a minute scale by Aloissan, in his electric furnace, is of course inaccessible to direct calorimetric meas urement. lint the law of constant permits of answering the question by measuring the heat of combust-ion of amorphous carbon and that of diamond. The transformation of amor phous carbon into carbon dioxide, whether ac complished by direct combustion o• by first changing the carbon to diamond and then burn ing the latter, must be accompanied by the evo lution of the same amount of heat, viz. 97.650 calories; and as the heat of combustion of diamond is 94,310 c,alories, the transformation of amorphous carbon (12 grams) into diamond must, according to the law of constant heat sums. be accompanied by the evolution of 97,650 — 94,310 = 3340 calories, in a similar manner Hess's law• permits of ascertaining the heat that would lie 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. Let it be required, for in stance, to ascertain the heat that would be de veloped or absorbed if ordinary alcohol were made from its elements—carbon (in the fo•m of diamond), hydrogen, and oxygen. To do this, we may determine calorimetrically the heat (call it a) developed by the combustion of one gram-molecule of alcohol and the heats of combustion (b and e) of quantities of isolated carbon (diamond) and hydrogen equal to those contained in one gram-molecule of alcohol. The three combustions may be represented by the following equations: C,H0H + (10 = + 311,0 + a calories; 2C + 40 = + b calories; 30 = 311,0 + r calories.
Adding the second and third equations. we get: + + 70 = + 3H,0 + c, and subtracting, the first equation from this, we get : 2C' + 31-1, + — = b + c — a, or 2C + 3H„ + 0 = + b + c— Tins last equation, expressed in words, means that if carbon, hydrogen, and oxygen combined to form ordinary alcohol, an amount of heat would be formed equal to the heat of combustion of the isolated elements of alcohol minus the heat of combustion of alcohol itself; b, the heat of combustion of two atoms (i.e. twice 12 grains) of carbons, is found to he 94,300 X 2 = 183.600 calories; c, the heat of combustion of three molecules (i.e. 3 X 2 grains) of hydrogen, is found to be 67.500 X 3 = 202.500 calories; finally, a-, the heat of combustion of one molecule (46 grams) of alcohol, is found to be 340.000 calories. Hence, b c— a, the heat of tion of alcohol, is 138.600 + 202,500 — 340,000 = 51,100 calories. See COMBUSTION.