Electromotive Forces from Thermochemical JData. 225 



Observed 

 Average Value. 

 "Zn, CI,, Aq] - [Pb, CI2, Aq] = 36870 c. = '796 volt \ Volt. 

 "Zn, Ci;, Aq] - [Pb, CI2] = 30070 c. = -649 volt j '591 

 'Zn, Bro, Aq ]-[Pb,Br2,Aq]= 36550 c. = -789 volt \ .„ 

 ;Zn,B4Aq]-[Pb,Br2] =26510 c. = '572 volt j ^'^ 



The zinc-lead iodide cell being calculated from the heat of 

 formation of solid Pb I2 accords almost exactly with the 

 experimental value, the difference being only "002 volt. 



Similarly with a zinc-lead nitrate cell, while theE.M.F. cal- 

 culated from the heat of dissolved lead nitrate is "737 volt, after 

 allowing for the large negative heat of solution, —7600, one 

 gets for [Zn,0,N205,Aq]-[Pb,0,N205], 102,510-(68,070 

 + 7600) =26,840, giving a calculated E.M.F. of -580 volt, 

 which agrees exactly with the experimental value '580 to 

 •591 volt. 



A reference to the effect of concentration of the solutions 

 on the electromotive force of a zinc-lead nitrate ceU confirms 

 my views. Wright and Thompson find that increasing the 

 solution strength from "25 to 2M"(N03)3, 100 H2O causes an 

 increase in theE.M.F. from "580 to "591, while they calculate 

 that it should decrease from '759 to "716. Hence, instead of a 

 fall of "043 volt there is a rise of "Oil volt, the cause of the 

 increase being in all probability due to the fact that the heat 

 of dilution of zinc nitrate is positive in strong but negative 

 in weak solutions; so that with moderately strong solutions the 

 whole of the combining energy of zinc nitrate with water 

 may be converted into electric energy, while in weak solutions 

 a portion of that part of the energy of chemical change is 

 expended in the absorption of heat due to the greater dilution 

 of the salt; 



It may be well to give here some collateral evidence in 

 support of this view. In the first place, if lead in chloride 

 solutions, either not saturated with or entirely free from lead 

 chloride, had a tendency to form Pb CI2, represented by the 

 heat of formation of the dissolved salt, viz. 75,970 c, this 

 metal should be without action on a solution of stannous 

 chloride, the heat of formation of which in aqueous solution 

 is 81,140 calories ; but, as a matter of fact, a piece of pure 

 clean lead placed in an acid solution of stannous chloride 

 becomes immediately covered with a dull grey film, and after 

 a short time small brilliant crystals of deposited tin appear aU 

 over its surface. For the same reason a cell consisting of 

 lead and tin plates, both immersed in either dilute hydrochloric 

 acid, stannous chloride, or lead chloride solutions, furnishes a 

 current the direction of which indicates that lead and not tin 



Phil. Mag. S. 5. Vol. 27. No. 166. March 1889. Q 



