180 OUTLINES OF PHYSICAL CHEMISTRY
intimate mixture of the molecules must always be obeyed, whatever be the energy used up in the disintegration.
(b.) Changes which go on in dilute solutions frequently give rise to a negative thermal effect. As instance of this we may take the equation already quoted in the chapter on neutralisation phenomena (page 168) :
KNa 2 S0 4 aq, £H 2 S0 4 aq] = - 0-92 Cal.
I would here urge a comparison of the two following equations, which have been experimentally established by Thomsen:
[£Na 2 S0 4 aq, HN0 3 aq] = - 1-752 Cal. [NaN0 3 aq, |H 2 S0 4 aq] = + 0-288 Cal.
Starting with two initially different systems, these reactions lead to the same final equilibrium ; and this equilibrium becomes established whatever be the thermal sign of the transposition which takes place. The thermo- negative reaction is, therefore, quite as normal as the thermo-positive.
In spite of these grave objections, Berthelot has not abandoned the principle of maximum work. On the contrary, he maintains his first statement, but limits its application to phenomena of a purely chemical nature. But it very often happens that the quantities of heat of purely chemical origin are heavily masked by thermal effects due to physical phenomena which take place at the same time as the chemical changes. Before we can class a reaction as essentially endothermic and consequently exceptional, we must, therefore, eliminate from the gross result of the calorimetric experiment, the secondary effects which correspond to external work, to change of state (fusion, volatilisation), and to the intervention of the solvent (heat of solution). With these eliminated we should have a corrected heat of reaction, referred to the solid state, and generally in agreement with the principle of maximum work.
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