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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




Fig. 19.


In the passage

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. 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



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


gaseous oxygen to 87 grams of gaseous chlorine monoxide (2C1 + 0 = CI,O) 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 2 HgO + 4C1 = CI, O + Hg, OCI,. This change consists of various parts; (1) formation of HgČI, and O, thus HgO + 2Cl = HgCl, + O, this is accompanied by the production of 16,250 gram-units of heat; (2) production of the oxychloride Hg, Oci, thus HgCl, HgO = Hg, OCI,, this is accompanied by the production of 8,900 gram-units of heat; (3) formation of CI, O, 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 + 4Cl to Hg,OCI, +1 O is accompanied by the production of 7,250 gram-units of heat.

To bring the system 2Cl + O into that state in which


chemical union can take place, work must be done upon the system, energy must be expended; this energy is supplied by the other chemical changes HgO + 2Cl = HgCl, + and Hgo + HgCl, = Hg, OCI,, in each of which energy is degraded.

Every chemical change is attended with the degradation of energy; but to bring a given system into that state in which chemical change can take place it may


to expend energy upon the system; this energy is sometimes supplied by processes altogether outside the system, sometimes by making the required chemical change one part of a cycle of changes in some of which energy is set free under such conditions as to be directly available for bringing the special part of the whole system into that state in which the wished for chemical change can occur.

We have had examples of oxidation accomplished by arranging a series of chemical changes so that oxygen shall be produced in contact with the substance to be oxidised. Thus lead monoxide (Pb0) was oxidised to lead dioxide (PbO,) by suspending the monoxide in concentrated warm potash solution and passing chlorine into the liquid : potassium hypochlorite (KCIO) was thus produced, but was at once decomposed to potassium chloride (KCI), and oxygen which combined with the lead monoxide to produce lead dioxide. If lead monoxide is suspended in water or potash solution and oxygen is passed into the liquid no lead dioxide is formed. Now to bring the system PbO + O into that state in which the system PbO, can be produced, energy must be expended : but the interaction of potash solution with chlorine, to produce potassium chloride, water, and oxygen, is attended with the setting free of a large quantity of energy; a portion of this energy is employed in bringing the part of the whole changing system represented by the symbols PbO + O into that condition in which chemical action is possible (s. Chap. XI. par. 158.).

When zinc and dilute sulphuric acid interact a solution of zinc sulphate, and hydrogen gas, are produced ; if a solution of sodium sulphite (Na S03) is added to this changing system, hydrogen sulphide (HS) is evolved. But if hydrogen gas is passed into a solution of sodium sulphite, hydrogen sulphide is not produced (s. Chap. xi. par. 175.) To effect the change Na SO,Aq+6H = Na OAq + 2H2O+HS energy must. be expended : when Na 80 Aq is added to dilute sulphuric acid in contact with zinc, both the material and the energy needed for effecting the chemical change are provided ; and

moreover the energy is provided exactly in the form in which

it is required for accomplishing the chemical work to be done, 267 When energy is degraded in a chemical change, a portion, or perhaps in some cases the whole, of the energy

is degraded to the form of heat. But the quantity of heat produced seldom if ever affords a direct measure of the chemical energy degraded. Chemical changes are always accompanied by more or less marked physical changes; the production of the energy which appears as heat is generally due partly to the chemical, and partly to the physical, portion of the complete change. Thus when gaseous hydrogen and oxygen combine to form liquid water a large quantity of energy is degraded, and much heat is produced 2H +0= H,O = 68,360 gram-units of heat +]; some of this heat is due to the chemical change from the system 2H+ 0 to the system H,O, some of it is due to the physical change from gaseous water to liquid water.

Some chemical changes, with their accompanying physical changes, occur with the disappearance of heat. Thus selenion dioxide dissolves in water to form a solution of selenious acid, Seo, +H,0 + Aq=H, SEO, Aq; this change is accompanied by the disappearance of 920 gram-units of heat. Similarly iodine pentoxide dissolves in water to form a solution of iodic acid, 1,0, +H O+ Aq=2HIO Aq; this change is accompanied by the disappearance of 1790 gram-units of heat. [The formulae represent reacting weights taken in grams; Seo, e.g. means here 111 grams of selenion dioxide.] In such cases we may suppose that the whole of the energy degraded in the chemical part of the complete change, and some of the energy degraded in portions of the physical change, are degraded to heat, but that this quantity of heat is wholly used in effecting other portions of the physical change. Or we may suppose that only a portion of the chemical energy is degraded to heat, and that this heat is used, along with other forms of energy produced by the degradation of the chemical energy, in effecting the physical portion of the complete change. Either supposition is in keeping with the fact that the total change occurs with the disappearance of heat, and with the generalisation that every chemical change is accompanied by degra




dation of energy.


It is important to note once again that the statement that every chemical change is accompanied by degradation of energy does not assert or imply that the whole of the energy







which changes form during a chemical change is degraded : some of it is degraded, but some of it may be raised to a more available form.

In chapters xii. and xIII. we saw that many chemical 269 changes may be justly regarded as proceeding in two directions simultaneously, and that equilibrium results when the velocities of the direct and reverse changes become equal. We saw also that such chemical equilibrium may generally be overthrown by changing the temperature, or sometimes the pressure, of the system, or the relative masses of the interacting substances. The considerations concerning the relations of energy-changes and chemical changes shortly developed in the present chapter may be applied to the conception of chemical equilibrium gained in chaps. XII. and xIII. Suppose that the masses of ferric chloride and potassium sulphocyanide shewn by the formulae Fe, Cl, and K.CNS(=6KCNS) are mixed in dilute aqueous solution; the system is in a condition in which chemical change can occur; chemical change occurs, and a system is produced the composition of which may be represented by the equation Fe, Cl Aq+6KCNSAq + « FeCl Aq

+ x' KCNSAq= Fe, (CNS).Aq +6KCIAq + x Fe Cl. Aq + KCNSA (comp. Chap. XII. par. 238). Some energy is degraded in this change ; a portion of this energy appears as heat, a portion is probably employed in effecting some of the physical changes (contraction or expansion of volume &c.) which accompany the chemical change; a portion of the energy degraded in one part of the change is also probably employed in bringing the products of the change into a state in which they can interact to reproduce the original substances. After a very short time the system settles down into a state in which there is equilibrium of energy and of chemical distribution of the interacting substances. A little more potassium sulphocyanide is now added; chemical change again occurs; energy is degraded ; and after a short time equilibrium is established. Potassium sulphocyanide is added little by little until the whole of the ferric chloride originally present has been changed; the addition of more sulphocyanide cannot now cause chemical change; no more energy can be degraded by chemical processes; the system has reached its state of final equilibrium. Each addition of potassium sulphocyanide disturbed the equilibrium of the energies of the system, and this disturbance was attended by chemical change ; but a disturbance of the equilibrium


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