CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 401
Note:The same relations apply between the standard cell potential (electromotive force)
and the standard reaction thermodynamic functions.In a reversible cell, relation (11.108) applies between the electrical work and equilibrium
electromotive force. If the electrodes are connected in a short circuit, the electrical work is
zero. If there is an electrical device between the electrodes, the following analogy of relation
(11.108) applies for the electrical work
Wel=−zF Etv, (11.117)whereEtv< Eis the so-calledterminal voltage, the measured voltage between the electrodes.
If the cell works reversibly, we write for the heat supplied to the cell
Q=T∆rS=zFT(
∂E
∂T)p, [T, p,reversible process]. (11.118)If, however, the anode and the cathode are connected in a short circuit, i.e. there is no electrical
device between them and consequently no electrical work is done, the heat is equal to the
enthalpy change
Q= ∆rH=zF
T(
∂E
∂T)p−E
, [T, p, Wel= 0]. (11.119)In real (irreversible) cells in which the anode and the cathode are connected via an electrical
device,Qis found between the values determined from equations (11.118) and (11.119). We
can calculate them using the first law of thermodynamics
Q= ∆rU−Wel, (11.120)where ∆rUis the reaction internal energy andWelis the electrical work calculated from relation
(11.117).
11.8.6 Standard hydrogen electrode.
The standard hydrogen electrode is such at which the following reaction occurs
H++ e−=1
2
H 2 (g), (11.121)