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Generally then', let r= the thermal value of a chemical change: let the change be the formation of a definite amount of a compound? (viz. X,Y,Z), consisting of a parts by weight of the element X, 6 parts by weight of the element Y, and c parts by weight of the element Z; then r=[X, Y, Z]
..(1). Let the compound X,Y,Z, be produced as before, but in presence of a large excess of water which holds it in solution, then r=[X, Y, Z, Aq] ......
........(2). Let the substance X,Y,Z, already existing be dissolved in an unlimited amount of water, then r=[XYoZ, Aq]
............(3). Let the compound X Y be decomposed by the element z with formation of XZ and Y, we get the expression
r=[XY, Z]=[X, 2]-[X, Y) ..................(4), that is, the total thermal change consists of two parts, (a) the heat absorbed in separating XY into X + Y, and (6) the heat evolved in the union of X and 2 to form XZ.
Finally let the compound XY react on the compound ZV to produce XZ and YV, the value of r is found by the formula
r=[X, Z]+[Y, V]-[X, Y]-[2, ..............(5). Equations (1) to (3) have been already illustrated. As an cxample of the use of (4) we may take the action of zinc on hydrochloric acid whereby zinc chloride and hydrogen are produced ;
[Zn, 2HCl]=[Zn, C14) – 2[H, CI]; or that of iron on a solution of copper sulphate to produce ferrous sulphate and copper ;
[CuSOʻAq, Fe)=[Fe, SO‘Aq)--[Cu, SO‘Aq].
1 Thomsen, Thermochemische Untersuchungen, 1. 5 et seq.
? In many cases we may use the term “molecule' in place of definite amount', and 'atom' in place of 'parts by weight': but as we shall frequently deal with solids and liquids it is better at present not to speak of atoms and molecules.
As an illustration of (5) the decomposition of PbO by HS resulting in production of PbS and H,O, may be used;
[PbO, H'S]=[Pb, S]+[H?, 0]-[Pb, 0]-[H?, S]". 119. A distinction is generally drawn between so-called exothermic and endothermic changes; the former are accompanied by evolution, the latter by absorption of heat.
Let (P"Q") represent the energy in a compound formed of a parts of element P and b parts of element : let (Pa) and (Q“) represent the energy in a parts of P, and in b parts of l respectively; then, inasinuch as the energy in any system resulting from a definite chemical change is equal to the difference between the energy in the original system from which it was produced and that lost during the process, it follows that
(pa)=(Pa)+(2")-(Pa, 2), assuming that the heat evolved in the formation of P“Q% measures the total loss of energy. And
.: (Pa)+(Q)>(PaQ). This equation represents an exothermic change. But in some cases a chemical change occurs only when heat is added to the changing system from without; in such a case
(PQ)=(Pa)+(Q)+(Pa, Q'), and
.: (Pa)+(Q)<(PQ). This equation represents an endothermic change.
It has been stated that if an exothermic change is possible it will always occur. When we have advanced somewhat in our study of thermal chemistry we shall see how impossible it is to found a system of classification on the difference between exothermic and endothermic changes. In some cases, a chemical reaction which appears to be accompanied by
1 Thomsen appears to be the only chemist who systematically writes the indices above the symbols of elements in the formule of thermal chemistry. Thomsen also sometimes uses the colon in place of the comma to express chemical reaction between the substances whose formulæ are separated by this symbol.
absorption of heat is found, on more careful study, to form one member of a series of changes the thermal sum of which is represented by a positive quantity. Indeed
Indeed any chemical reaction is a most complex phenomenon when regarded from the thermal point of view; physical changes (expansion or contraction, passage from solid to liquid or gas, or vice versa, &c., &c.) form part of the total change, the thermal value of which is set down in a lump sum. But thermal chemistry aims at something more than this rough grouping together of positive and negative values. Thermal chemistry tries to disentangle the primary chemical, from the subordinate physical changes, and moreover to divide the chemical processes into those which consist of molecular decompositions, and those which consist of atomic combinations.
It is not possible to enter on any full discussion of the terms exothermic and endothermic as applied to chemical phenomena until the subject of affinity has been treated; at present I wish to insist on the inadvisability of making the conception implied in these terms the basis of a system of classification of chemical reactions, and at the same time to draw attention to some processes which are suggested by the terms in question.
Naumann' shewed that no action occurs when dry sulphuretted hydrogen is passed into a solution of iodine in dry carbon disulphide, but that as soon as water is added, hydriodic acid and sulphur are produced. The reaction
2H2S + 21, = 4HI + S2
(gaseous) (solid) (gaseous) (solid) would be thermally represented as
[2H2S, 21°)= 4[H, I]-2[H?, S]
= - 24800 - 9200
- 34,000. When water is present, the reaction
2H2S + 212 4HI + S2 (in solution) (in solution) (in solution) (solid)
i Ber. 2. 177; and Annalen 151. 145.
would be thermally represented as
=52,800 – 18400
= 34,400'+. The reaction of dry sulphuretted hydrogen on dry iodine would be markedly endothermic; but when this change is made one of a series the thermal value of which, taken as a whole,
a is positive, then the complete cycle of change proceeds rapidly.
But the more concentrated an aqueous solution of hydriodic acid becomes the less heat is there evolved on each addition of the acid, until the specific gravity of the liquid is 1'56”, after which no more heat is evolved; the liquid is saturated. If therefore the hydriodic acid produced in the foregoing reaction is allowed to accumulate in the liquid, no more water being added, a point will be reached at which the sum of the thermal changes is equal to zero; at this point the chemical change stops, but proceeds again on the addition of a little water. It is possible to obtain an aqueous solution of hydriodic acid of specific gravity 167; if sulphur is shaken with this liquid a little sulphuretted hydrogen and iodine are produced, i.e. the change
S2 + 4HI 2H,S + 12
proceeds until the hydriodic acid becomes reduced to specific gravity 1'56, when equilibrium is again established.
Portions of this cycle of change are exothermic, other portions are endothermic. Variation of the mass of one of the members of the changing system determines whether the thermal value of the complete change shall be positive or negative, and also determines the direction in which the change shall proceed. This reaction may be taken as typical of most if not all chemical processes. Such processes consist of portions having positive thermal values and portions having negative values; small variations in the conditions may
1 No notice is taken in these thermal expressions of the change, if any, which accompanies the decomposition of 21, and the production of S.,. See post, par. 132.
? This liquid contains about 25 per cent. of HII.
determine whether the process as a whole shall belong to the class of exothermic or to that of endothermic changes.
120. Direct measurements of the thermal changes which accompany chemical changes can only be made in a few simple cases; it is generally necessary to have recourse to indirect methods. The truth of the following deduction from the theory of energy is assumed in all these methods of calculation.
The total loss of energy by a chemical system in passing from a definite initial to a definite final state is independent of the intermediate states.
The total loss of energy is of course measured by the heat evolved and the work done by the system in its passage from one state to the other. But for our purpose the energy given out in forms other than that of heat may be overlooked, and we may put the statement in this form ; the total thermal change during a chemical process is dependent only on the initial and final states of the chemical system.
In applying this statement, it is necessary to arrange series of reactions each beginning with the same materials in the same conditions and ending with the same products under the same conditions ; all the processes which form one of the cycles of change must be capable of calorimetrical measurement, and all the processes in the other cycle, except that one the thermal value of which is to be determined, must also be capable of measurement by the calorimeter : if this be done, it follows from the principle just stated that the difference between the total thermal values of the two cycles of changes represents the thermal value of that special portion of one of the cycles which it is wished to determine. Each cycle may however consist of various parts, so that it is sometimes a little difficult to unravel all the changes, and to find that portion of one cycle the thermal value of which has to be determined by calculation.
I shall now give some examples to shew how the thermal values of various chemical changes are deduced from the results of experiments. M. C.