NATURE 



[_yan. 3, 1884 



If the compounds X Y and Z V react to produce X Z and 

 Y V then 



r=[X,Z]+[Y,V] -[X,Y]-[Z,V]. 

 These equations illustrate the methods by which the 

 thermal vakie of a chemical change can be indirectly 

 calculated. The total loss of energy by a chemical sys- 

 tem in passing from a definite initial to a definite final 

 state is independent of the intermediate states ; assum- 

 ing, as we may do for most purposes, that the total loss 

 of energy is measured by the quantity of heat evolved, it 

 follows that the total thermal change accompanying a 

 chemical change depends only on the initial and final 

 states of the system. Hence, if we have series of reac- 

 tions beginning with the same materials in the same con- 

 dition, and ending with the same products in the same 

 condition, and if all the thermal changes in one series 

 may be measured, and all except one in the other series 

 may be measured, it follows that we can calculate the 

 thermal value of the unknown member of the second 

 series of changes. Thus, it is required to determine the 

 thermal value of the synthesis of 46 grams of formic 

 acid (CHjjO.,). Twelve grams of carbon, 2 of hydrogen, 

 and 48 of o.xygen combine to produce 44 grams of carbon 

 dioxide and 18 grams of water: but the same quantities 

 of the same materials might theoretically be combined to 

 produce 46 grams of formic acid, and then from this, 44 

 grams of carbon dioxide + 18 grams of water would be 

 produced. The following are the thermal values of the 

 various parts of these two series of changes : — 

 [C, 0=] = 96,960 gram-units + ; [H^, O] = 68,360 + ; 

 [CH'O-, O] = 65,900 + ; 

 but 



[C, O^] + [H2, O] = [C, H=, 0-] -f [CH^O=, O] = 1 65,320 -f 

 . ■ . [C, H^ 02] = [C, 0=] + [H^ O] - [CH^'O^ 0] = 99,420 +. 

 Such calculations sometimes become very complex • 

 corrections must frequently be introduced for quantities 

 of heat evolved or absorbed during purely physical 

 changes which form integral parts of the cycle of chemical 

 change under investigation. 



The thermal study and comparison of classes of chemi- 

 cal changes leads to the conclusion that a chemical change 

 which is accompanied by considerable loss of energy to 

 the changing system will generally occur, unless prevented 

 by the action of forces external to the system. This 

 generalisation, vague though it be, helps to explain many 

 classes of chemical reactions, e.g. the action of con- 

 centrated and dilute solutions of hydriodic acid on 

 sulphur, and on many hydroxyl-containing carbon com- 

 pounds ; and the action of sulphuretted hydrogen in 

 precipitating certain metallic sulphides in the presence of 

 acid, and others only form alkaline liquids. 



Thomsen has devoted much time and care to the 

 thermal investigation of the mutual actions of acids and 

 bases : the greater part of his first volume is devoted to 

 this inquiry. The " heat of neutralisation of an acid by a 

 base " is defined as the number of gram-units of heat 

 evolved on mixing equivalent quantities in grams of 

 the acid and base in dilute aqueous solution, the products 

 of the action being also soluble in water. Thomsen 

 employs a solution of 2 NaOH (grams) in about 200 H^O 

 (grams) as the standard base : he measures the thermal 

 values of the following reactions : — 



r2NaOH Aq, 2H.\ Aq] in the case of a monobasic acid. 

 [2NaOH .■\q, H.,X Aq] „ „ dibasic 



[2NaOH Aq, ^HjXAq] „ „ tribasic 



[2NaOH Aq, iH.iXAq] „ „ tetrabasic „ 



The commoner acids may be broadly divided into four 

 groups, according to the values of the " heats of neutrali- 

 sation." This value is for Group I. about 20,000 gram- 

 units; II., about 25,000 ; III., about 27,000; and IV., about 

 30,000 gram-units. The study of heats of neutralisation 

 has led Thomsen to the conception of the avidity of an 

 acid, i.e. the striving of one acid to displace another from 

 its combination with a base. Thus, when equivalent 

 quantities of NaOH, HNO3, and H^SO^ are mixed in 

 dilute aqueous solutions, two-thirds of the NaOH com- 

 bines with the HNO3, and one third with the HjSOj ; the 

 avidity of HNO3 for NaOH is said to be twice as great 

 as that of HjSOj for the same base. HNO3 in aqueous 

 solution is therefore a stronger acid than HjSO^. 



Measurements of the heats of neutralisation of mono- 

 basic, dibasic, «-basic acids has led Thomsen to classify 

 some of these acids in ways difterent from those gene- 

 rally adopted in the text-books. His results as regards 

 dibasic and tiibasic acids may be thus summarised : — 

 Dibasic Acids 

 Group I. Typical formula R H., e.g. SiFc . H, 

 „ II. „ „ RfOH)., £-.?. S0o(6H), 



„ III- „ „ R(0H)H.^.^. S02(0H)H. 



Tribasic Acids 

 Group II. Typical formula ^{OU^^.c.g. C4H504(0H)3. 

 „ HI. „ „ HR(OH)H.f.^^.HP03(OH)H. 



These e-xamples will serve to show the suggestiveness 

 of the results of thermal chemistry. Thomsen's three 

 volumes teem with suggestions : his results throw light 

 on such questions as are connoted by the expressions 

 allotropy, molecular compounds, classification of elements 

 and compounds, isomerism, and affinity. 



It is in examining the subject of chemical affinity from 

 the point of view of thermal chemistry that one becomes 

 aware of the complexity of the problems included under 

 this expression. 



From the following numbers, 

 [H, CI] = 22,000 + ; [H, Br] = 8440 -f ; [H, I] = 6050 - ; 

 it might be concluded that the affinity of chlorine for 

 hydrogen is much greater than that of bromine, and that 

 the affinity of iodine for hydrogen is much less than that 

 of bromine. But these thermal equations are not com- 

 parable; at ordinary temperatures chlorine is a gas, 

 bromine a liquid, and iodine a solid ; hence, on this 

 ground alone, no precise conclusions can be drawn from 

 the above data regarding the relative affinities for hydro- 

 gen of the three halogen elements. Again, looking at the 

 numbers, 



[C, O] = 28,600 -f; [C, O-'] = 97,000 -f , 

 it might be said that when oxygen combines with carbon 

 in quantities of 16 grams at a time, the union of the 

 second parcel of 16 grams is attended with evolution of 

 much more heat than accompanies the addition of the 

 first parcel of 16 grams. But measurement of the heat of 

 oxidation of carbon monoxide, [CO, t>] = 68,400+, at 

 once negatives this conclusion, and rather points to the 

 nu mber 68,400 Xi 136,800 as representing the thermal 

 value of the transaction, C -t- Oj = COj, where C repre- 

 sents 12 grams of gaseous carbon. 



