.. [C2H3, O3] at 150°=310,600-14,011=296,589 (say 296,600) gram-units. The thermal values of reactions of various kinds may be determined by the use of the principle laid down in par. 120. The following are examples. L. The heat of liquefaction of the hydrate H2SO. H2O is found to be - 3680 from these data, [H2SO4H2O, Aq]=7120: [H2SO1H2O, Aq]= 10,800. M. The mean thermal value of the fixation of each molecule of water by a salt when undergoing hydration may L.-L.; be found by using the formula Hence, calculating from the salt Na2SO. 10H,O, the mean thermal value of the reaction [Na2SO‘, H’O] is — 1834 units; and calculating from the salt Na2SO. 5'4 H2O, the mean value for the same reaction is — 1785 units. If the heat of liquefaction of water is subtracted from the difference L.-L,, we get the thermal value of the combination with solid water of the salt in question. Thus, taking sodium butyrate, for and for CH,NaO2; L=4240 units: but heat of liquefaction of 3 gram-molecules water-1430.3=-4290: : .. L.-L=800 units; i.e. the mean thermal value for the combination of each molecule of water, in the solid form, with C,H,NaO, is represented by 1697 gram-units. N. The heat of formation in solution of a double salt, is the difference between the heat of solution of the double salt, and the sum of the heats of solution of its constituents; O. One other example of the calculation of a heat of formation will be given, as it serves to shew how very indirect are the methods sometimes adopted. Required the thermal value of the reaction [CI, O]. Data, (1) for finding the value of [H, Cl, O, Aq]: (a) the reaction 2NaOH + Cl2= NaCl + NaClO+H,O, if expanded thermally becomes [2NaOH, Cl2]=[H, Cl, Aq]+[H, Cl, O, Aq]–[H2, O] but +[NaOHAq, HCIAq]+[NaOHAq, HCIOAq]=24,600 : [H, Cl, Aq]=39,300 [H2, O]=68,400 [HClAq, NaOHAq]=13,700 [HCIOAq, NaOHAq]= 10,000; .. 24,600=[H, Cl, O, Aq]+39,300+13,700+ 10,000 - 68,400 .. [H, Cl, O, Aq]=30,000. [H, Cl, O, Aq] - 5,400 : (b) the decomposition of aqueous HCIO by aqueous HI, viz. 2HI+HCIO=HCl + H2O+Ig, if expanded thermally becomes [HCIOAq, 2HIAq]=[H, Cl, Aq]+[H2, O]–[H, Cl, O, Aq] but [H, Cl, Aq]=39,300 [H3, O]=68,400 2 [H, I, Aq]= 26,350: -2 [H, I, Aq]=51,400: .. 51,400 81,350-[H, Cl, O, Aq]: .. [H, Cl, O, Aq]=29,950. Hence mean value of [H, Cl, O, Aq]=(29,975) say 30,000. (2) Further data for finding [Cl2, O]: the reaction ClO + H2O=2HCIO, if expanded thermally becomes [CIO, H2O] = 2 [H, Cl, O, Aq] - [H2, O] - [Cl2, O]= 9400. But we have already found 2 [H, Cl, O, Aq]=60,000: and [H2, 0]=68,400 : .. [CI2, O]=- 17,800. 121. From these examples we may provisionally conclude that a chemical change which is accompanied by considerable loss of energy to the changing system will generally occur unless prevented by actions outside of the system. The following processes may be taken as illustrative of this somewhat vague generalisation. The acids of the acetic series readily yield chloro-, or bromo-derivatives by the direct action on them of chlorine or bromine, iodine however does not react under similar conditions to form iodo-acids. The thermal values of the reactions of the three halogens on acetic acid are as follows, X=Cl=30,000 units +. C2H2O2+X2=C2H2XO2+ HXX=Br=4,800 +. (X=I = 18,000", The reverse action in the case of iodine, viz. CH,IO,+HI=C2H2O2+I2 is represented thermally thus, [C2H3IO2, HI]=18,000. This action occurs provided a concentrated aqueous solution of hydriodic acid is employed. hence it follows that the decomposition of 2HI into H,+I2 in dilute solution would absorb 38,000 12,400 = 25,600 units of heat. These thermal numbers shew that the process which is accompanied by a large loss of energy occurs, whereas that which would involve gain of energy to the system does not occur. But why does a concentrated aqueous solution of hydriodic acid act as an energetic reducing agent? We have already learned (par. 119) that little or no heat is evolved during the absorption and solution of gaseous hydriodic acid by a solution of that gas containing about 20-25 per cent. of HI; hence a concentrated solution of this compound contains a considerable quantity of HI, as distinguished from HI.xH2O. But the numbers given above shew that HI contains much more energy than HI.H,O; hence a concentrated aqueous solution of hydriodic acid is much more energetic than a dilute solution of the same compound'. The following tables' contain thermal data for discussing the action of sulphuretted hydrogen as a reagent for precipitating certain metals from acid solutions, and other metals only from neutral or alkaline solutions. TABLE I. Base Reaction Cdo PbO CuO HgO TO Cu2O Ag2O [Base 2TICIA} (1) 27,300 29,200 31,700 45,300 38,500 38,500 58,500 [Base Aq, 2HClAq] (2) 20,300 15,400 15,300 19,000 27,500 14,700 42,600 (1) (2)= + 7,000 13,800 16,400 26,300 11,000 23,800 15,900 TABLE II. Base Reaction [Base, H2S] (1) 32,100 Cao PbO CuO Hgo TI,O Cu2O Ag2O [Base, 2HC1] (2) 55,000 62,200 49,300 77,200 18,900 6,000 13,900 To illustrate the application of these data, take the case of cadmium. [CdO2HClAq, H SAq]=27,300 : i.e. the thermal value of the change which occurs when aqueous HS reacts on a dilute solution of CdO in HCl is represented by 27,300 units +. [CdOAq, 2HClAq]=20,300 : i.e. the thermal value of the change which occurs when CdO in aqueous solution is neutralised by a dilute solution of HC1 is represented by 20,300 units +. The former number exceeds the latter by 7,000, .. the action of H,S, in solution, on CdO, in dilute HCl solution, is accompanied by evolution of 7,000 units of heat; this action readily occurs. But [CdO, H2S] = 32,100; and [CdO, 2HCl]=55,000: i.e. the formation of CdS, by the action of gaseous HS on CdO, is accompanied by the evolution of 22,900 units of heat less than attends the action of gaseous HCl on CdO, .. CdS is decomposed by gaseous HCl with formation of CdCl,. Moreover the numbers [2HCl, Aq] = 34,600; whereas [H2S, Aq]=4,800 shew, that, comparing equivalent quantities of hydrochloric acid and sulphuretted hydrogen, the former when in the state of gas possesses an excess of energy, measured by about 34,000 thermal units, above what it possesses when in dilute solution, whereas the excess of energy of gaseous H.S above that possessed by H,S. HO is measured by about 5000 thermal units. But the more concentrated an aqueous solution of hydrochloric acid, the less is the quantity of heat evolved by adding hydrochloric acid gas to that solution; in other words, a concentrated aqueous solution of this acid is nearly as energetic a reagent, provided it is used in sufficient quantity, as gaseous hydrochloric acid. Hence we should conclude, and our conclusion is verified by experiment, that cadmium sulphide will be decomposed by concentrated aqueous hydrochloric acid. |