● In a voltaic cell electrons flow from the half-cell with a more negative potential to the half-cell with a more positive potential and generates an electromotive force (emf).

● The magnitude of the voltage depends on the difference in the tendencies of the two half-cells to undergo reduction.

19.1 Standard electrode potentials

● Since it is impossible to measure the electrode potential of only one half-cell, standard hydrogen electrode is used as a reference electrode.

● It is assigned a standard electrode potential of 0,00 V.

● Standard conditions:

● all solution must be 1.0 mol dm-3

● 100 kPa● all substances are pure● 298K● solid metals or Pt as an electrode

2 H+(aq) + 2e- H2(g)

● The standard electrode potential of a half-cell is determined by connecting that half-cell to the standard hydrogen electrode at standard conditions and measuring the e.m.f .

Zn (s) | Zn2+ (1 M) || H+ (1 M) | H2 (1 atm) | Pt (s)

● The sign of the standard electrode potential depends on the direction of electron flow:

● Standard electrode potentials are always given for the reduction reaction.

● The E° values for the oxidation will be of equal magnitude but have the opposite sign.

● In a spontaneous reaction the half-cell with the more negative standard electrode potential gives up electrons = is oxidised whereas the half-cell with the more positive standard electrode potential recieves electrons = is reduced.

= The electrons flow towards the half-cell with the highest Eº value.

Electrolysis of aqueous solutions

Electrolysis of a concentrated solution of NaCl (aq)

Electrolysis of CuSO4 (aq) using inert graphite electrodes

Electrolysis of CuSO4 (aq) using copper electrodes

How much Ca will be produced in an electrolytic cell of molten CaCl2 if a current of 0.452 A is passed through the

cell for 1.5 hours?