Page 72 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry

Phase and Chemical Equilibria 57

(b) Galvanic Cells, which extract energy as non-PV work in spontaneous
reactions. This provides the link with thermodynamics. Examples are
dry battery cells, lithium-ion cells, fuel cells, etc.

6.2.8. Half-Reactions. Redox Reactions
A half-reaction is a process in which there is a transfer of electrons from
one substance to another. If there is a loss of electrons, the half-reaction is
referred to as oxidation; if there is a gain of electrons, the half-reaction is
referred to as reduction. Any electrochemical reactioncanbe expressedas
the sum of two half-reactions.
As an example, consider the redox (oxidation–reduction) reaction

Cu 2+ (aq) + Zn(s) → Cu(s) + Zn 2+ (aq) (6.26)

which is assumed to be composed of the two half-reactions

−
Cu 2+ (aq) + 2e → Cu(s) [reduction of Cu 2+ ] (6.27a)
Zn(s) → Zn 2+ (aq) + 2e − [oxidation of Zn] (6.27b)

If these are used in a cell (for example, the Daniell Cell) which may be
characterized as

Zn(s)|ZnSO 4 (a) CuSO 4 (a)|Cu(s) (6.28)

the oxidation half-reaction will deposit electrons on the cathode pole via
the external circuit.
The half-cell potentials of the reduction and oxidation reactions referred
to above are respectively +0.34 and +0.76 volt, giving an overall sum value
o
of E =1.18 volt.
If this reaction is carried out reversibly by letting the produced current
act against a maximum resistance, the electrical work produced by the
cell will be

o
w other =∆G T,P = −n E , (6.29)
where n is the number of moles of electrons, is the Faraday constant
o
equal to 96,485 coulomb/mol −1 of electrons and E is the total potential
of the half-cells. The electrical work, for the Cu–Zn cell is (observing that