The Foundations of Chemistry

Cu^2  2 e88nCu 0.337 Vm88reduction potential

Zn88nZn^2  2 e 0.763 Vm88oxidation potential

Cu^2 Zn88nCuZn^2  E^0 cell1.100 V

The positive E^0 cellvalue tells us that the forward reaction is spontaneous at standard

conditions. So we conclude that copper(II) ions oxidize metallic zinc to Zn^2 ions as they

are reduced to metallic copper. (Section 21-9 shows that the potential of the standard

zinc–copper voltaic cell is 1.100 volts. This is the spontaneous reaction that occurs.)

The reverse reaction has a negative E^0 and is nonspontaneous.

nonspontaneous

reaction: CuZn^2 88nCu^2 Zn E^0 cell1.100 volts

To make it occur, we would have to supply electrical energy with a potential difference

greater than 1.100 volts. That is, this nonspontaneous reaction would have to be carried

out in an electrolytic cell.

EXAMPLE 21-3 Predicting the Direction of Reactions

At standard conditions, will chromium(III) ions, Cr^3 , oxidize metallic copper to copper(II)

ions, Cu^2 , or will Cu^2 oxidize metallic chromium to Cr^3 ions? Write the cell reaction, and

calculate E^0 cellfor the spontaneous reaction.

Plan

We refer to the table of standard reduction potentials and choose the two appropriate half-

reactions.

Solution

The copper half-reaction has the more positive reduction potential, so we write it first. Then

we write the chromium half-reaction as an oxidation, balance the electron transfer, and add

the two half-reactions and their potentials.

E^0

3(Cu^2  2 e88nCu) (reduction) 0.337 Vo

2(Cr88nCr^3  3 e) (oxidation) 0.74 V0o

o2Cr3Cu^2 88n2Cr^3 3Cu E^0 cell1.08 V 0 o

Because E^0 cellis positive, we know that the reaction is product-favored (spontaneous).

Cu^2 ions spontaneously oxidize metallic Cr to Cr^3 ions and are reduced to metallic Cu.

You should now work Exercise 54a.

870 CHAPTER 21: Electrochemistry

Problem-Solving Tip:The Sign of E^0 Indicates Spontaneity

For a reaction that is spontaneous at standard conditions, E^0 must be positive. A negative

value of E^0 cellindicates that the reverse of the reaction written would be spontaneous at

standard conditions.