SAT Subject Test Chemistry,10 edition

Solution: Ag+ would be reduced to Ag (s) and Tl (s) would be oxidized to Tl+, since Ag+ has the
higher E°red. Therefore, the reaction equation would be:

Ag+ +   Tl  (s) →   Tl+ +   Ag  (s)

which   is  the sum of  the two spontaneous half-reactions.

Note that reduction and oxidation are opposite processes. Therefore, in order to obtain the
oxidation potential of a given half-reaction, the reduction half-reaction and the sign of the
reduction potential are both reversed. For instance, from the example above, the oxidation half-
reaction and oxidation potential of Tl (s) are:

Tl  (s) →   Tl+ +   e− E°ox =   +0.34   V

Standard reduction potentials are also used to calculate the standard electromotive force (EMF or E
°cell) of a reaction, the difference in potential between two half-cells. The EMF of a reaction is
determined by adding the standard reduction potential of the reduced species and the standard
oxidation potential of the oxidized species. When adding standard potentials, it is very important to
note that we do not multiply by the number of moles oxidized or reduced.

EMF =   Ered    +   Eox

The standard EMF of a galvanic cell is positive, while the standard EMF of an electrolytic cell is
negative. A spontaneous redox equation, therefore, will have a positive EMF, but a negative free
energy change, and vice versa for a nonspontaneous reaction. We shall specify further the relation
between EMF and ∆G below, but for now you should keep this reversal of sign in mind.

Example: Given that the standard reduction potentials for Sm3+ and [RhCl 6 ]3− are −2.41 V and
+0.44 V, respectively, calculate the EMF of the following reaction:

Sm3+    +   Rh  +   6Cl− →  [RhCl 6 ]3− +   Sm

Solution: First, determine the oxidation and reduction half-reactions. As written, the Rh is