Page 378 - Physical chemistry understanding our chemical world

BATTERIES 345

metal such as a piece of copper wire, e.g. in a bicycle headlamp. The battery pro-
duces electrons as a by-product of the redox reaction occurring at the cathode. These
electrons pass through the load (a bulb, motor, etc.) and do work, before re-entering
the battery where the anode consumes them. Electrochemical reduction occurs at the
positive pole (the anode) of the battery simultaneously with electrochemical oxidation
at the negative pole (the cathode).
There are several types of battery we can envisage. A majority of
the batteries we meet are classed as primary batteries, i.e. a chem- A battery is sometimes
ical reaction occurs in both compartments to produce current, but called a galvanic cell.
when all the chemicals have been consumed, the battery becomes
useless, so we throw it away. In other words, the electrochemical reactions inside
the battery are not reversible. The most common primary batteries are the Leclanch´ e
cell, as described below, and the silver-oxide battery, found inside most watches and
slim-line calculators.
By contrast, secondary batteries may be reused after regenerat-
ing their original redox chemicals. This is achieved by passing a In the shops, sec-
current through the battery in the opposite direction to that during ondary batteries are
normal battery usage. The most common examples of secondary usually called rechar-
batteries are the lead–acid cell (there is one inside most cars) and geable batteries.
nickel–cadmium batteries (commonly called ‘NiCad’ batteries).

What is the earliest known battery?

Battery types

We have evidence that batteries were not unknown in the ancient world. The Parthians
were a race living in the Mediterranean about 2000 years ago, from ca 300 BC until
AD 224, when they were wiped out by the Romans. They are mentioned in the Bible,
e.g. see The Acts of the Apostles, Chapter 2.
A device was found in 1936 near what is now modern Baghdad, the capital of
Iraq. It consisted of a copper cylinder housing a central iron rod. The identity of the
ionic electrolyte is now wholly unknown. The device was held together with asphalt
as glue.
If the copper was tarnished and the iron was rusty (i.e. each was covered with
a layer of oxide), then an approximate emf for this 2000-year-old battery would
probably be in the range 0.6–0.7 V. We do not know what the battery was used for.

The Daniell cell

One of the first batteries in recent times was the Daniell cell, The vertical dotted
.
2+. 2+ line in this schematic
Zn|Zn .Cu |Cu. This battery comprised two concentric terra- indicates a porous
cotta pots, the outer pot containing a zinc solution and the inner
membrane.
pot containing a copper solution. Metallic rods of copper and