374 



REPORT 1886. 



§ 12. Consequence of the variation of the activity-coefficient iu 



homogeneous systems. 



[Since provisionally Arrlienius supposed the coefficient of activity to be 

 identical with molecular conductivity, he goes on to consider the effect of its 

 varying vrith pressure and temperature as conductivity does. Pressure-variation 

 he considers nil, or unknown. Temperature-variation he considers linear, 1 + bt. 

 So he writes (2a) — 



(1 - .r) a (1 + b^t) (n - .v) 8 (1 + b^t) = .r=/3 (1 + bj) y (1 + b^t), 



but then says that the b are about equal for all salts and for all bases, and fairly 

 equal for all monobasic acids ; so the temperature terms cancel each other in pairs- 

 nearly ; and he states the laws.] 



(38) Tfie equilibi-ium of a homogeneous system is independent of pressure. 



(39) The relative quantities of four bodies, like tivo acids ' and two salts, or two 



bases and tivo salts, or four salts, in a system in equilibrium, only va7"y 

 very slightly with temperature. 



The first is in accord with Bunsen and Berthelot ; the second with Ostwald,^ 

 who has verified it between 20° and 100° for HCl, HNO, and two salts of these 

 acids with a single base. He has further shown that these two acids have the 

 same activity between these temperatures by letting them act on calcic oxalate.^ 



M. Ostwald has also determined the relative coefficients of affinity of different 

 acids with regard to the same base. 



He let two acids act simultaneously on a given base — as, for example,. 

 KHO, MgO, &c. The acids employed were HCl, HNO3, and H.^SO^. The term 

 ' relative coefficient of affinity ' expresses the ratio between the fractions of the 

 base which the two acids seize. If one mixes KNO3 with HCl, one equivalent of 

 each, there will be formed .t equivalent of KCl and x of HNO3, and there will 

 remain (1 — .f) equivalents of KNO3 and of HCl. The coefficients of activity 

 being /3, y, a, b, respectively, x is given by the equation — 



{l-xfab = x^^y, 

 and so the relative coefficient of affinity of HCl : HNO3 i^ — 



« = i£s = ^/i;:)■ 



/3y> 



If instead of potassium one uses Bodium, y and 8 remain, but a and fi change, 

 Q, however, need not change unless a'j^' is different from a//3. 



(If acid salts are formed, a modification is necessary.) 



Molecular conductivities given above (§5) make one suspect that changing a base 

 from KHO to ZnO or MgO will diminish the value of Q for HNO, : H^SO^andfor 

 HCl : H,SO,. 



For the rest : because the activity-coefficientsofHNOjandHClare about equal^ 

 and the same for salts formed from these acids, their relative coefficient of aiEmity 

 ought to be nearly 1, whatever the base on which they are allowed to act. 



Here are Ostwald's numbers : — 



Table of Relative Affinities. 



• Except H2SO, and ^^VOi- 



' Ostwald, Journal fiir praktisohe Chemie (1877), T. 16, p. 



385. 



