The Foundations of Chemistry

the cathode solution to replace every Cu^2 ion reduced. Some Zn^2 ions from the anode
vessel and some SO 42 ions from the cathode vessel also migrate into the salt bridge.
Neither Clnor Kions are oxidized or reduced in preference to the zinc metal or Cu^2 
ions.
As the reaction proceeds, the cell voltage decreases. When the cell voltage reaches zero,
the reaction has reached equilibrium, and no further net reaction occurs. At this point,
however, the metal ion concentrations in the cell are notzero. This description applies to
any voltaic cell.
Voltaic cells can be represented as follows for the zinc–copper cell.

salt bridge

g

Zn
Zn^2 (1 M)

Cu^2 (1 M)
Cu

rp

species (and concentrations)

in contact with electrode surfaces

In this representation, a single line (
) represents an interface at which a potential develops,
that is, an electrode. It is conventional to write the anode half-cell on the left in this
notation.
The same reaction occurs when a piece of Zn is dropped into a solution of CuSO 4.
The Zn dissolves and the blue color of Cu^2 ions disappears. Copper forms on the Zn
and then settles to the bottom of the container. But no electricity flows in an external
circuit, because the two half-reactions are notphysically separated.

Problem-Solving Tip:How to Tell the Anode from the Cathode

The correspondence between the names anodeand cathodeand the charge on the elec-

trode is differentfor electrolytic cells than for voltaic (galvanic) cells. Students sometimes

get confused by trying to remember which is which. Check the definitions of these two

terms in Section 21-2. The surest way to name these electrodes is to determine what

process takes place at each one.

anodeI::Foxidation and cathodeI::Freduction

As a memory aid, both anode and oxidation begin with a vowel, whereas both cathode

and reduction begin with a consonant.

(Left) A strip of zinc was placed in a

blue solution of copper(II) sulfate,

CuSO 4. The copper has been

displaced from solution and has

fallen to the bottom of the beaker.

The resulting zinc sulfate solution is

colorless. This is the same overall

reaction as the one that occurs when

the two half-reactions are separated

in the zinc–copper cell (see Figure

21-6). (Right) No reaction occurs

when copper wire is placed in a

colorless zinc sulfate solution. The

reaction

Zn^2 Cu(s)88nZn(s)Cu^2 

is the reverse of the spontaneous

reaction in Figure 21-6; it has a

negative E^0 celland is nonspontaneous.