Page:Text-book of Electrochemistry.djvu/253

 ��POTENTIAL DIFFERENCE.

��CHAP.

��Solution Pkessure, P, of the Metals in Atmospheres.

��Volts.

Magnesium 10**

Aluminium lO*^

Cadmium 10^

Iron 10»

��Volts.

Lead 10-«

Hydrogen 10^*

Copper 10-«

Mercury 10^**

Silver lO-'-^^

��The elements used in these determinations were (the sum

��of Metal I Zn and Zn volt : see p. 240) —

��MgSO, CdSO^ FeSO^ PbACj CuSO^ AgjSO^ Zn I ZnSO, | H^SO, | H^CPd) = 0760 .-. Hj | H,SO,

��Zn

�ZnSO,

�MgSO,

�Mg

�Zn

�ZnSO^

�CdSO,

�Cd

�Zn

�ZnSO,

�FeSO,

�Fe

�Zn

�ZnSO,

�PbACg

�Pb

�Zn

�ZnSO,

�CuSO,

�Cu

�Zn

�ZnSO,

�Ag,SO,

�Ag

��VoltB.

��/. Mg /.Cd .-.Fe /. Pb .'. Cu /. Ag

��Volta.

��Heat of lonisation. — ^As Ostwald has shown, we can calculate TV, in this case the heat of ionisation, from the formula already given —

o.. n-n^^ - 23,070f - W
 * dT T

if we kncTw P, the potential difference between metal and

solution. The value of j^ can easily be determined by

arranging in opposition to each other two surfaces, Metal

I Salt solution kept at different temperatures. Thus we

dP find for Copper | Copper acetate, -^^ = 000774 volt; for

Copper I Copper sulphate, 0000757 volt, or as the mean value 0000766 volt per degree. Therefore, if T = 290°—

23,070 X 0000766 X 290 = 23070 x 0*60 - TV,

from which —

��TV= 13,842 - 5124 = 8718 cal.

�� �