This explanation makes reference the Zn/Cu2+ cell shown above.
The cell cannot operate unless the circuit is complete. The
oxidation half-cell originally contains a neutral solution of Zn2+ and SO42- ions, but as Zn atoms
in the bar lose electrons, the solution would develop a net positive charge from the Zn2+
ions entering. Similarly, in the reduction half-cell, the neutral solution of Cu2+
and SO42- ions
would develop a net negative charge
as Cu2+
ions leave the solution to form Cu atoms. A charge imbalance would arise and stop cell operation if
the half-cells were not
neutral. To avoid this
situation and enable the cell to operate, the two half-cells are joined by a salt
bridge, which acts as a
"liquid wire," allowing ions to flow through both compartments and complete the circuit.
The salt bridge shown in the diagram is an inverted U tube containing a solution of the nonreacting ions Na+
and SO42 - in
a gel. The solution cannot pour out, but ions can diffuse through it into and out of
the half-cells.
To maintain neutrality in the reduction half-cell
(right; cathode compartment)
as Cu2+
ions change to Cu atoms, Na+ ions move from the
salt bridge into the
solution (and some SO42- ions
move from the solution into the salt bridge). Similarly, to maintain neutrality
in the oxidation half-cell (left; anode compartment) as Zn atoms change to
Zn2+
ions, SO42- ions
move from the salt bridge into that solution (and some Zn2+ ions move from the
solution into the salt bridge). Thus, as the diagram shows, the circuit is completed as electrons
move left
to right
through the
wire, while anions move right to left and cations move left to right
through the
salt bridge.
