260 261 CHAPTER XIV. RELATIONS BETWEEN CHEMICAL CHANGES AND CHANGES EVERY chemical change consists of two parts, a change in the form of combination of the matter of the system, and a change in the total quantity, or in the form, or in both the quantity and form, of the energy of the system. Energy is the power of doing work. Work is the "act of producing a change of configuration in a system in opposition to a force which resists that change." If one system does work on another system, one loses and the other gains energy; and the energy lost by one is equal to the energy gained by the other. If both systems are included in a larger, the total energy of this system is unchanged. If one part of a system does work on another part, the total energy of the system is unchanged, although one part has gained and another part has lost energy. The principle of the conservation of energy affirms that ;— "The total energy of any material system is a quantity which can neither be increased nor diminished by any action between the parts of the system, though it may be transformed into any of the forms of which energy is susceptible." (Clerk Maxwell.) The energies of actually existing material systems depend upon the states of these systems at any moment. The state of a system is conditioned by many variables; among the more important are chemical composition, pressure, temperature, and volume. If we wish to connect changes of energy with changes of chemical composition we must start with chemical systems of * The subject of energy is treated very shortly. The student should refer to a book on Physical principles, e.g. to Clerk Maxwell's Matter and Motion. definite and defined composition, in definite and defined states, and we must cause these to change to other definite and defined states; we must then determine the compositions of the resulting systems, and we must measure the changes of energy which have accompanied these changes of composition and of state. Of two equal quantities of energy one may be more 262 available for doing work than the other. Thus, in order to cause thermal energy to do work it is necessary to allow it to pass from a body at a higher to a body at a lower temperature. A certain body may be at a very low temperature and yet contain thermal energy; but it may be impossible to cause this energy to do work, because of the impossibility of framing an engine consisting of the cold body and another system at a lower temperature than the cold body. A quantity of heat as it exists in a hot body is more available for doing work than the same quantity of heat as it exists in a colder body. When energy passes from a more available, or higher, to a 263 less available, or lower, form it is said to be degraded. All forms of energy can be directly or indirectly transformed into heat. A given quantity of heat-energy cannot be wholly transformed into one of the higher forms of energy. Every transformation of energy involves the degradation of a portion of the energy. But every chemical change is accompanied by a transformation of energy from the form of chemical energy to other forms of which thermal energy is usually one; every chemical change therefore is accompanied by a degradation of It is not asserted that the whole of the energy energy. which changes form during a chemical change is necessarily degraded. The chemical system represented by the symbols 2H+0 264 contains more energy than the system represented by the symbol H2O. In the passage from one of these systems to the other energy is lost by the changing system; the energy so lost by the system is gained by neighbouring systems, by the vessel in which the change is accomplished, the surrounding air, &c. But although there is no destruction, there is degradation, of energy. If 2H represents 2 grams of hydrogen, O represents 16 grams of oxygen, and H2O represents 18 grams of liquid water, all measured at normal pressure and at about 15o-16o, then the change 2H + 0 = H2O is accompanied by the production of 68,360 gram-units of heat. If we assume that the whole of the energy which changes form during the chemical change 2H+0=H2O appears as heat, then 68,360 gram-units of heat represents the difference between the energies of the two systems 2H + O and H2O. Whether this quantity of heat does or does not measure the total difference of energy between the two systems, it is certain that the change from the one system to the other is always accompanied by the production of the same quantity of heat. And what is true of this chemical change is true of others also. Each definite chemical change from one system of defined composition, under defined conditions, to another system of defined composition, under defined conditions, is accompanied by the production or disappearance of a fixed quantity of heat. The following examples illustrate this point. In each case the original and final systems are under the normal pressure (760 mm.) and the temperature of each is about 16°. The symbols represent the combining, or reacting, weights taken in grams. Original New System Gram-units of heat 22,000 + 265 8,440 + In some of these changes heat disappears from the system, that is to say, energy is raised from the form of heat-energy to some other more available, or higher, form. Yet it has been asserted (par. 263) that every chemical change is accompanied by a degradation of energy. Take, for example, the change H+I=HI; in this change 6040 gram-units of heat disappear. But in order to bring about the formation of HI from H+I it is necessary to heat the system H+I to 300°-400°; that is to say it is necessary to add energy from without the system. This added energy is employed in bringing the system H + I into a condition such that chemical action becomes possible; chemical action results and this action is attended with a degradation of energy. Suppose a stone to rest at the bottom of an inclined plane AB (Fig. 19). Let the stone be moved from A to B ; to perform B E Fig. 19. -A this work a certain amount of energy must be used. The stone now possesses more energy, by virtue of its position, than it did when it was at A. Let the stone be moved a very little way over the crest of the plane AB, it will now roll down the inclined plane BC until it comes to rest at C. In the passage from B to С energy has been degraded; but the stone at C possesses more energy than it did at A because the level of C is higher than that of A. If the stone is rolled up the short incline CD, by the expenditure of energy, it will be in a position to descend from D to E; in this descent energy will be degraded. When the stone reaches E it will possess the same energy as when it was at rest at A. The system H + I corresponds to the stone at A; to bring the system into such a condition that chemical change can occur, energy must be expended. The system ready to undergo chemical change corresponds to the stone at B. Chemical change occurs; the system passes from B to C, from H + I to 266 HI, and this passage is attended with degradation of energy. But the system HI possesses more energy than the system H + I did before energy was expended upon it in causing it to pass into that condition in which chemical change could occur; therefore although the chemical change from H+I to HI has been attended with degradation of energy, yet the whole change from 1 gram of gaseous hydrogen and 127 grams of solid iodine, at say 15° or 16°, to 128 grams of gaseous hydrogen iodide at the same temperature, has been attended by a raising of energy from a lower to a higher form. The system HI is readily separated into the elements H+I. To effect this separation the temperature of the gas is raised; this corresponds to the rolling of the stone up the small incline CD. The separation of HI into H+I is attended with the degradation of a considerable quantity of energy, just as the descent of the stone from D to E is attended with the degradation of energy. In this example the energy required to bring the original system into that state in which chemical action could occur was furnished in the form of thermal energy from without the system. But the necessary energy for bringing a chemical system into that state in which chemical change can take place, and energy can be degraded, is frequently supplied by chemical actions occurring within a larger system of which the special system under consideration forms a part. Thus the change from 71 grams of gaseous chlorine and 16 grams of gaseous oxygen to 87 grams of gaseous chlorine monoxide (2CI + 0 = ClO) is accompanied with the disappearance of 17,900 gramunits of heat. Chlorine monoxide is produced by passing chlorine over mercuric oxide; the equation representing the chemical change is 2HgO + 4C1=C1,O+ Hg,OCI. This change consists of various parts; (1) formation of HgCl, and O, thus HgO+2C1 = HgCl,+O, this is accompanied by the production of 16,250 gram-units of heat; (2) production of the oxychloride Hg,OC1, thus HgCl,+HgO= Hg,OCI,, this is accompanied by the production of 8,900 gram-units of heat; (3) formation of Cl2O, thus 2C1+0=C1,0, this is accompanied by the disappearance of 17,900 gram-units of heat. Now 16,250 +8,900 =25,150; and 25,150-17,900 7,250. The complete change from 2HgO+4C1 to Hg,OCI, + Cl2O is accompanied by the production of 7,250 gram-units of heat. To bring the system 2C1+0 into that state in which |
