Why does this produce electricity?
The
flow of current can be understood as the flow of ions from the more reactive metal to the less reactive metal. The ions moving from one electrode to the
other creates an electrical charge which is neutralized by the flow of
electrons across the wire.
Before considering the reaction of two
metals, consider what happens when we place a single metal electrode in an
electrolyte. Some of the metal atoms in the electrolyte go into solution as
ions while the remaining electrons create a negative charge on the metal. The
separation of ions and electrons leads to a separation of charge. However, this
build up of charge cannot continue indefinitely because as the negative charge
builds up in the metal it becomes increasingly difficult for positive metal
ions to go into solution. A similar build up in positive charge in the electrolyte
also prevents the build up of charge. This degree of charge build up depends on
the metal and represents the work required to separate electrons from the ions.
This is known as the electromagnetically principle
Similarly, if a copper strip is placed
in an aqueous Copper(II)Sulfate solution the copper will also lose ions. These
reactions are often written as Cu | Cu+2 this is the half-cell
reaction.
The tenancy for Zinc to lose ions is
greater than that of Copper. When the two cells are joined together (using a
copper wire to connect the electrodes and porous barrier that allows the ions
to pass known as a salt-bridge connect the electrolytes, the build up of
electrons on the zinc will flow to through the wire onto the copper.
The copper ions in the electrolyte gain
electrons and become copper atoms.
Thus the reaction can be written,
Zn | Zn2+ | | Cu2+
| Cu
Danile Cell
To continue the reaction, the charge
must be removed. This can be acheived by coupling a second reaction which uses
the electrons in the metal to convert the ions in the electrolyte into a metal.
For a more specific example, consider a zinc electrode in an electrolyte of
Copper(II)Sulphate solution.
The loss of electrons by the Zinc is
known as oxidation. Zn(s) → Zn2+ + 2e-.(1)
A wire connecting the Zinc electrode to
a Copper electrode, allows the electrons to flow to the Copper electrode.
Copper ions in the copper sulfate solution take up the electrons and become
atoms of copper on the copper electrode. The gaining of electrons by ions is
known as reduction
Cu+2 + 2e- → Cu(s).(2)
The net reactions is then,
Zn(s) + Cu2+ → Cu(s) + Zn2+
When the two electrodes are joined by a
wire the charge stored can flow and the electrons combine. The simplest kinds
of battery have two conductors made of different materials which are partially emerged in a solution which allow the electrons and ions to flow freely known
as an electrolyte.
At the copper electrode (cathode), the
acid dissolves the copper metal producing hydrogen gas, H+. The
reaction will continue until the supply of zinc is used up. The electrons, with
their negative charge, are attracted to the copper electrode which causes a
current to flow. One of the problems with this cell is that the current stops
flowing after a short time because the hydrogen burbles block the current.