with thin films of chromium. A chrome bumper requires approximately 3 seconds of elec-
troplating to produce a smooth, shiny surface only 0.0002 mm thick. When the metal
atoms are deposited too rapidly, they are not able to form extended lattices. Rapid plating
of metal results in rough, grainy, black surfaces. Slower plating produces smooth surfaces.
“Tin cans” are steel cans plated electrolytically with tin; these are sometimes replaced by
cans plated in ^13 second with an extremely thin chromium film.VOLTAIC OR GALVANIC CELLS
Voltaic,or galvanic, cellsare electrochemical cells in which spontaneousoxidation–
reduction reactions produce electrical energy. The two halves of the redox reaction are
separated, requiring electron transfer to occur through an external circuit. In this way,
useful electrical energy is obtained. Everyone is familiar with some voltaic cells. The
batteries commonly used in flashlights, portable radios, photographic equipment, and
many toys and appliances are voltaic cells. Automobile batteries consist of voltaic cells
connected in series so that their voltages add. We will first consider some simple labora-
tory cells used to measure the potential difference, or voltage, of a reaction under study.
We will then look at some common voltaic cells.THE CONSTRUCTION OF SIMPLE VOLTAIC CELLS
A half-cellcontains the oxidized and reduced forms of an element, or other more complex
species, in contact with one another. A common kind of half-cell consists of a piece of
metal (the electrode) immersed in a solution of its ions. Consider two such half-cells in
separate beakers (Figure 21-6). The electrodes are connected by a wire. A voltmeter can
be inserted into the circuit to measure the potential difference between the two electrodes,
or an ammeter can be inserted to measure the current flow. The electric current is the
result of the spontaneous redox reaction that occurs. We measure the potential of the cell.
The circuit between the two solutions is completed by a salt bridge.This can be any
medium through which ions can slowly pass. A salt bridge can be made by bending a piece
of glass tubing into the shape of a “U,” filling it with a hot saturated salt/5% agar solu-
tion, and allowing it to cool. The cooled mixture “sets” to the consistency of firm gelatin.
As a result, the solution does not run out when the tube is inverted (see Figure 21-6), but
the ions in the gel are still able to move. A salt bridge serves three functions.1.It allows electrical contact between the two solutions.
2.It prevents mixing of the electrode solutions.
3.It maintains the electrical neutrality in each half-cell as ions flow into and out of
the salt bridge.A cell in which all reactants and products are in their thermodynamic standard states
(1 Mfor dissolved species and 1 atm partial pressure for gases) is called a standard
cell.21-8
858 CHAPTER 21: Electrochemistry
These are named for Allesandro Volta
(1745–1827) and Luigi Galvani (1737–
1798), two Italian physicists of the
eighteenth century.
Neither of these meters generates
electrical energy.
Agar is a gelatinous material obtained
from algae.
When we show a concentration in a
standard cellas 1 M,it is assumed to be
exactly 1 M.