http://www.ck12.org Chapter 23. Electrochemistry
The single vertical lines represent the phase boundaries between the metal electrodes and the solutions. The double
vertical line represents the salt bridge. The anode is conventionally written on the left and the cathode on the right.
The molarities of the half-cell solutions are also indicated.
Types of Voltaic Cells
Several variations on the basic voltaic cell presented above are in common use today. A few of those will be
described, including the dry cell, the lead storage battery, and the fuel cell.
Dry Cells
Many common batteries, such as those used in a flashlight or a remote control, are voltaic dry cells. There are several
kinds of dry cells in common usage that differ by the substances that are undergoing redox reactions. Shown below
(Figure23.2) is a zinc-carbon dry cell.
FIGURE 23.2
A dry cell is commonly known as a bat-
tery, like those used in a flashlight. They
are relatively inexpensive, but they do not
last a long time and are generally not
rechargeable.These batteries are called dry cells because the electrolyte is a paste instead of an aqueous solution. In a zinc-carbon
dry cell, the anode is the zinc container, while the cathode is a carbon rod through the center of the cell. The paste is
made of manganese(IV) oxide (MnO 2 ), ammonium chloride (NH 4 Cl), and zinc chloride (ZnCl 2 ), plus just enough
water to allow current to flow. The half-reactions for this dry cell are:
Anode (oxidation): Zn(s)→Zn^2 +(aq)+2e−
Cathode (reduction): 2MnO 2 (s)+2NH+ 4 (aq)+2e−→Mn 2 O 3 (s)+2NH 3 (aq)+H 2 O(l)The paste prevents the contents of the dry cell from freely mixing, so a salt bridge is not needed. The carbon rod
serves only as a conductor and does not participate in the actual reaction. The voltage produced by a fresh dry cell
battery is 1.5 V, but this value decreases somewhat over time as the battery is used up.
An alkaline battery is a variation on the zinc-carbon dry cell. The alkaline battery has no carbon rod and uses a paste
of zinc metal and potassium hydroxide instead of a solid metal anode. The cathode half-reaction is the same, but the
anode half-reaction is different.