THERMO-CHEMISTBY. 212 THERMO-CHEMISTBY. Tj, if the oquilibi'ium at some given temperature T, and the average of the energy-changes (heats given oft' or taken up by the reaction ) at the two temperatures are known. In this manner it is possible to calcuhite, for instance, the degree of dissociation of ammonium chloride (see Decom- position; Dissociation) at different tempera- tures, if the degree of dissociation at some one temperature, and the heat of dissociation, are known. In connection with the influence of changes of temperature on chemical equilibrium, it is necessary to mention Gibbs's phase rule, which is of great importance in classifying the phenomena of equilibrium in material systems. By the 'phases' of a chemical system Gibbs means its several homogeneous parts that can be separated from one another mechanically. For instance, let water be shaken up in a bottle with ordinary ether, and three layers will form — a solution of ether in water, a solution of water in ether, and, over the liquid, a mixture of the vapors of water and ether; the three homogeneous layers can be separated by mechanical means, and hence they constitute the three 'phases' of the system. Since all gases and vapors form homogeneous mi.xtures, it is evident that no system can contain more than one gaseous phase. According to Gibbs's phase rule, n phases in equilibrium with one an- other cannot possibly be made up by less than n — 2 independent chemical substances, or, what is the same, n — 2 independent chemical sub- stances cannot possibl}' form more than n phases in equilibrium with one another. When n — 2 independent substances do form n phases, then the slightest change of temperature destroys the equilibrium, a transformation takes place, and one of the phases disappears. For instance, let » — 2 = 1, and therefore n := 3, as is the case at the melting-point of ice, when ice, liquid water, and water vapor may form three phases made up of a single chemical substance — water. As long as the temperature is constant, the three phases remain in equilibrium : let the tempera- ture rise, and all the ice will have melted away; let the temperature fall, and all the water will have frozen. The melting-point of water is sometimes referred to as its triple point, because of the three phases which may exist in equilib- rium at that point. Above and below that point, in the regions of the simultaneous existence of only two phases (liquid water and its vapor, or ice and its vapor), a change in temperature causes a corresponding change in the equilibrium, the vapor-tension, and hence the concentration of the gaseous phase varying with the temperature; but none of the phases necessarily disappears, i.e. the equilibrium of the several phases is not necessarily dcslrni/ed. In the case of ether and water forming tliree phases (see above), the number of independent substances being only one less than the number of phases, a change in temperature would likewise cause a change in equilibrium ; but ordinarily none of the pha.se3 would necessarily disappear, i.e. again, the equi- librium would not necessarily be destroyed. If, however, the temperature should fall to the point at which water would begin to freeze out of the aqueous solution of ether, then a fourth phase (ice) would be added to the three phases of the system; the number of phases (4) would then exceed by two the number of independent sub- stances (2, ether and water) composing them, and the equilibrium that would then ensue would be destroyed by the slightest variation of tem- perature from that 'quadruple point' (i.e. the point of four phases) of the sy.stem. Tempera- tures like the triple point of water and the quad- ruple point of water and ether are referred to generally as the multiple points of chemical sys- tems. The above examples, involving physical transformations alone (melting, freezing, evapo- ration, solution), have been considered here for simplicity's sake. But the phase rule is equally applicable to systems in which verj' complex chemical phenomena may be taking place, and its value lies largely in the fact that it likens very complex plienomena of chemical equilibrium to the simple phenomenon of the physical equi- librium between water and ice. It further, evi- dently, permits of classifying the phenomena of chemical equilibrium with reference to the num- ber of substances taking part in them, and thus possesses considerable didactic importance. Final- ly, it can serve as a guide to the discovery of new substances (for instance, new hydrates of inorganic salts) that may appear as phases in chemical systems in equilibrium and thus render service to certain branches of purely experimen- tal research. One more important application of thermody- namics to chemical phenomena has been made and I'equires mention in the present sketch. Thermodynamics, in studying a transformation of some material system, endeavors to ascertain the maximum mechanical work that might be produced by the transformation, the 'maximum work' meaning the work that might be obtained by the use of some ideal mechanical device, frictionless and permitting, so to speak, of no leakage of energy. The importance of knowing this maximum of work is very great. Any natu- ral change taking place of itself — whether it be the falling of a stone, the expansion of a com- pressed gas, the combustion of coal, or any other change, mechanical or chemical — may be used to produce mechanical work; and no material sys- tem is capable of changing unless it possesses the eapacity for producing work — or, as Helm- holtz terms it, 'free energy-.' In other words, it may be said that it is because a system can pro- duce mechanical work that it is capable of changing spontaneously. The burning of coal (i.e. the chemical transformation of the system, carbon and oxygen), once started, can go on of itself because it can be used to produce mechani- cal work, or, what is the same, because the sys- tem carbon and oxygen possesses a certain amount of free energy. ^Vhen, therefore, the free energj' of a system has been used, without loss, to produce mechanical work, and that work has been measured, we have a measure of the cause of the given transformation. The cause of chemical transformations is generally termed 'chemical afKnity.' Obviously, then, the maxi- mum work that can be produced by a chemical transformation is a measure of the chemical, affinities involved in it, and this is why the de- termination of maximum work has great impor- tance for chemical theory. But it may also be valuable for purely practical purposes. Take, for instance, again the combustion of coal. It is well known that steam engines are very waste- ful of energy. In connection with the problem
