the close of Chap. XII.; we have learnt that a number can be found for each acid, and each base, which expresses the amount of chemical change which this acid, or base, is capable of producing under defined conditions.

It is probable that, as investigation proceeds, specific affinity-constants will be determined for the members of other classes of compounds besides acids and bases.

We have now learnt something about chemical composition 258 and chemical classification. We have also found that many, and probably all, chemical reactions brought about by the compounds classed together as acids are quantitatively conditioned by the affinity-constants of these acids.

We ought now to inquire into the connexions between the compositions of acids and the values of their affinity-constants. But we are not yet ready for this inquiry; we must learn more regarding chemical composition. (s. Chap. XVII.)

No attempt has been made in this chapter to analyse the 259 meaning of the term affinity; we have not asked why this body chemically interacts with that; we have not inquired as to the nature of chemical affinity. We have been content to call affinity that property of elements and compounds by virtue of which they interact to produce new combinations. We have found it possible to assign quantitative values to this property in the cases of acids and bases.

Before proceeding to consider in some detail the generally accepted theory regarding the mechanism of chemical change, we shall briefly glance at the relations between chemical changes and the changes of energy which invariably accompany them.

260

261

CHAPTER XIV.

RELATIONS BETWEEN CHEMICAL CHANGES AND CHANGES
OF ENERGY *.

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 energy. It is not asserted that the whole of the energy which changes form during a chemical change is necessarily degraded.

The chemical system represented by the symbols 2H+O 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-16°, 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

265

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.

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

85,820 +

2HClÅq+O

10,270 +

HClAq + O

9,380 +

103,240 +

124,560 +

21,320 +

9,320

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 Idid 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 C 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