Page 138 - Basic physical chemistry for the atmospheric sciences

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+
L ( aq) - Li(s) couple, or Oi(g)for the OJ(g) - Oi(g) couple] will react
.
to a significant extent (i e . , spontaneously) with the oxidized f o rm o f
i
2
g
any couple [e. . , Fe + ( aq) f o r the Fe + ( aq) - Fe(s) couple, or
)
Ag + ( aq f or the Ag + (aq) - Ag(s) couple] that lies below it in the table
o f electrode potentials, but it will not react to a significant extent with
the oxidized f o rm o f a couple that lies above it in the table.
Exercise 6 . 7 . By (a) using the rule just stated above, and (b) by
calculating the standard cell potential , determine whether, under stan­
3
dard conditions , Fe + ( aq) will oxidize c1 - ( aq) to Clz(g) to a signifi­
cant extent.
Solution. (a) c1 - ( aq) is the reduced form of Cl (g). Therefore , it will
z
react to a significant extent with the oxidized form of any couple that
lies below it in Table . 2 . FeH (aq) is the oxidized (rust) form of Fe(s),
6
but it lies above Cl ( aq) in Table 6.2. Therefore , Cl ( aq) will not react
-
-
to a significant extent with Fe H ( aq). 8 (b) The half-reactions and their
electric potentials are , from Table . 2
6
2
2CI ( aq)� Clz(g) + e - �x = - ( 1 .36 V) = - 1 .36 V
-
2
3
2Fe + (aq) + 2e - � 2Fe + (aq) �ed = 0 .77 1 V
Net: 2FeH (aq) + 2c1 - (aq)� �ell = - 0 . 5 89 V
2
Cl2(g) + 2Fe + (aq)
The negative value of �ell confirms that the reaction will not proceed
to a significant extent in the forward direction.
To balance the electrons in the oxidation and reduction half-reaction
in Exercise 6 . 7, we had to multiply by 2 the reduction half-reaction
given in Table 6 . 2 . However, the magnitude of the electrode potential
was unaffected . This is because the magnitude of E° is determined
only by the concentrations of the species in the half-reaction (1 mole
per liter under standard conditions , not by the amounts of the species
)
(in moles).

6.6 Standard cell potentials and free-energy change

We saw in Section 2 . 2 that if a chemical transformation is spontane­
ous , and pressure and temperature are constant , the Gibbs free energy
(G) of the system will decrease. We have seen in the above section
that a redox reaction will proceed spontaneously under standard con­
ditions if its standard cell potential (�en) is positive. Therefore , there
should be a quantitative relationship between AG° under standard