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 gradation 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 corre-
sponds to the rolling of the stone up the small incline CD.
The separation of HI into H + I is attended with the de-
gradation 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 (2Cl + 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 2HgO + 4C1 = C1,0 + Hg, OCI,. This change consists of various parts; (1) formation of HgCl, and 0, 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 2Cl + 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 + 401 to Hg,OCI, + C1,0 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,+ O and Hgo + HgCl, = Hg, OC1,, 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 be necessary 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 Pb0, 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 (H,S) 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 = Nao Aq+ 21,0 + HS energy must be expended : when Na So 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

1

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 + O 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, +1,0 + Aq=2HIO, Aq; this change is accompanied by
tắe disappearance of 1790 gram-units of heat. [The formulae
represent reacting weights taken in grams; Seð, 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 genera-
lisation that every chemical change is accompanied by degra-

dation of energy.
268 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 xii. Suppose that the masses of ferric chloride and potassium sulphocyanide shewn by the formulae FeCl, and KCN S. (=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+Fe, Cl Aq

+ x' KCNSAq = Fe, (CNS), Aq+6KCIAq + x Fe, Cl Aq + ác KCNSAT (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

of the energies of the system would also be produced by raising the temperature of the system ; hence raising the temperature of the system would also alter the distribution of the elements forming the members of the system, i.e. would cause chemical change to occur.

Now consider a chemical change, one of the products of which is a solid under the conditions of the experiment. Suppose aqueous solutions of barium chloride and sodium sulphate to be mixed in the ratio BaCl, : Na Son Chemical change occurs ; energy is degraded ; there is change from liquid to solid, and more energy is degraded. As one of the products of the chemical change (barium sulphate) is removed from the sphere of action, by precipitation in the solid form, none of the energy degraded in the direct change can be used to bring the products of this change into a state in which they can chemically react to reproduce the original substances; hence the whole, or at any rate nearly the whole, of the energy degraded to the form of heat passes out of the system. The system is, so to speak, rapidly rolling down hill. Chemical change proceeds until the whole of the energy which can be degraded to heat has been degraded. The system is now in its final state of equilibrium. And this final state has been reached without adding an excess of either of the interacting substances.

270

We have now gained some fairly clear conceptions regarding chemical change.

Elements and compounds interact to produce other elements and compounds. Numbers are given to the elements expressing the masses of them which combine or interact with unit mass of one element chosen as a standard, and which also interact or combine with each other. These numbers we have called the combining weights of the elements. Numbers are also given to compounds which express the smallest masses of them which chemically interact with each other. These numbers we have called the reacting weights of the compounds. But it is necessary in chemistry to have regard not only to composition but also to properties. Elements are classified in accordance with their properties into metallic or positive, and non-metallic or negative, elements. They are also classified in groups in accordance with the properties and compositions of their oxides, hydrides, haloid and oxyhaloid compounds, &c. This classification of elements