Chemistry - A Molecular Science

Chapter 11 Electron Transfer and Electrochemistry

11.6

COMMON BATTERIES Batteries are self-contained, galvanic electrochemical cells (Section 11.3). There are many types of batteries, but we consider only three.

steel case

graphitecathode (+)electrolyte

paste

zinc anode (-)

Figure 11.5 Dry cell battery

DRY CELL AND ALKALINE BATTERIES Figure 11.5 shows either a dry cell or an al

kaline battery, depending on the nature of

electrolyte paste. This is the most commonly

used battery in flashlights. The zinc base is

an active anode (-). A graphite rod, which is

immersed in an acidi

c electrolyte paste,

serves as an inactive cathode (+). The paste is a moist combination of MnO

, NH 2

Cl, 4

ZnCl

, and water that is thickened with starch. (^2) Anode reaction:
Zn(s)
→
Zn
2+(aq) + 2e
1-^
Cathode reaction: 2MnO
(s) + 2NH 2
1+ 4
(aq) + 2e
1-^
→
Mn
O 2
(s)+ 2NH 3
(aq) + H 3
O(l) 2
The cathode reaction is complicated and still not
fully understood. It is known to involve
the reduction of MnO
, but the reaction shown is but one possibility. The maximum cell 2
voltage is 1.5 V. One of the major disadvantages of the dry cell is that it has a very short shelf life because the zinc container reacts w
ith the acidic ammonium ions. To enhance the
shelf life, the paste is made alkaline (basic) by replacing the NH
Cl with KOH. The half- 4
reactions for an
alkaline battery
are
Anode reaction:
Zn(s) + 2OH
1-(aq)
→
Zn(OH)
(s) + 2e 2
1-^
cathodecan (+)
anode cap (-)
Zn in
electrolyte
Ag O
2
separator
(liquid junction)
insulatinggasket
Figure 11.6 Silver oxide or button battery
Cathode reaction:
2MnO
(s) + H 2
O(l) + 2e 2
1-^
→
Mn
O 2
(s) + 2OH 3
1-(aq)
MERCURY OXIDE AND SILVER
OXIDE (BUTTON) BATTERIES
Button batteries are the small batteries used to power calculators, cameras, watches, hearing aids,
etc

. The mercury(II) oxide (

E^

= 1.3 V)

and silver oxide (

(^) E= 1.6 V
) batteries
are essentially the same as shown in Figure 11.6. The only difference is the cathode reaction, which is either the reduction of s
ilver oxide or mercury(II) oxide. The porous
separator serves as the liquid junction.
Anode reaction:
Zn(s) + 2OH
1-(aq)
→
Zn(OH)
(s) + 2e 2
1-^
Cathode reaction:
Ag
O(s) + H 2
O(l) + 2e 2
1-^
→
2Ag(s) + 2OH
1-(aq)
or
HgO(s) + H
O(l) + 2e 2
1-^
→
Hg(l) + 2OH
1-(aq)