Concise Physical Chemistry

c14 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

HALF-CELLS 221

concentration, causing Zn^2 +(aq) to go from the solution onto the rod, taking up two

electrons from the metal:

Zn(s)→Zn^2 +(aq)+2e−

or

Zn^2 +(aq)+2e−→Zn(s)

Either way, an electrical potential difference is produced between the electrode and the

solution such that the sum of the chemical potential and the electrical potential, called

theelectrochemical potential, on one side balances the electrochemical potential on

the other side, and equilibrium is reached.

If the same thing is done using a copper rod in contact with a CuSO 4 solution,

an electrochemical balance is quickly reached in the same way, but the electrical

potential on the Cu electrode is not the same as it was on the Zn electrode because

the chemical potential of solid Cu is not the same as it is for solid Zn(s). Zn(s) is said

to be moreactivethan Cu(s) because it has a greater tendency to dissolve in a dilute

solution of its ions than Cu(s). If a Zn; Zn^2 +electrodeat potentialφis connected to a

Cu; Cu^2 +electrode across a digital voltmeter and the circuit is completed by means

of a neutral salt bridge, the difference in potential, called thevoltageof the cell, can

be measured. It turns out to be about 1.1 volts, depending on the concentrations of the

ZnSO 4 and CuSO 4 in the electrode compartments. The entire arrangement is called

a Daniell cell.^2

If a resistance or small motor replaces the digital voltmeter, heat or work can be

obtained. This is the principle of all electrochemical cells. Historically, two or more

cells connected were called abatteryof cells by analogy to gun batteries. What we

call the 1.5-volt AA “battery” is really a single cell.

14.2 HALF-CELLS

There are a great variety of cells available for research and practical use. The Cu; Cu^2 +

and Zn; Zn^2 +electrodes can be replaced by other metals, leading to many metal–metal

ion combinations. These M; Mz+systems, wherezis the number of electrons in-

volved in the electrode reaction, are calledhalf-cellsbecause any two of them can

be combined to form a cell. The semicolon is used to denote the possibility of

electron exchange and is often, but not necessarily, a physical interface such as the

solid;solution interface in Zn; Zn^2 +. The electrochemical standard is thehydrogen

half-cell(Fig. 14.1). Hydrogen is allowed to pass through the cell under 1 atm

(^2) Invented as early as 1837. The original Daniell cell was a copper cup containing the CuSO 4 in which
a porous cup containing the Zn;Zn^2 +solution was suspended. The porous cup soaked with salt solution
took the place of the salt bridge. The Daniell cell had many practical uses and played an important part in
early electrochemistry.