PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 402

while the activity of hydrogen ions is equal to one,aH+= 1, and the fugacity of hydrogen is
equal to the standard pressure,fH 2 = 101325 Pa.

Note:The standard hydrogen electrode consists of a platinum strip covered with platinum

black, saturated with hydrogen under a pressure of 101325 Pa, and dipped into a solution

of hydrochloric acid of the molality 1.2 mol kg−^1 (the corresponding activity of hydrogen

ions is unity).

11.8.7 Nernst equation for a half-cell

An electrochemical reaction occurs at the electrode in a half-cell, e.g.

Bz

+

+z+e−= B. (11.122)

For this reaction we can write

Φ = Φ◦−

RT

z+F

ln

aB

aBz+

. (11.123)

The quantity Φ is called theelectrode potential, Φ◦is thestandard electrode potential.
The electrode potential is defined as the equilibrium potential (electromotive force) of a
galvanic cell in which one electrode is the standard hydrogen electrode and the other is the
studied electrode. When oxidation proceeds at the studied electrode, we speak about the
oxidation potentialΦox, when reduction occurs, we speak about thereduction potential
Φred. It applies that
Φox=−Φred, Φ◦ox=−Φ◦red. (11.124)

Note: Based on an international agreement, we can find only the values of standard

reductionpotentials in tables.

11.8.8 Electromotive force and electrode potentials

Electrode potentials allow for the calculation of the potential a cell using the relation

E= Φox+ Φred, (11.125)