The energy released in a spontaneous
redox reaction can be used to perform electrical work. This occurs in a galvanic cell (aka a voltaic cell) which is a device in
which electron transfer is forced to take place through an external pathway,
rather than directly between reactants.
The two solid metals (Zn & Cu) are
called electrodes. The electrode at which oxidation occurs is
called the anode (Zn). The electrode at which reduction occurs is
called the cathode (Cu).
A voltaic cell consists of two half-reactions (reduction and
oxidation). Zn is oxidized at the anode to
produce electrons, which flow through the external
circuit to the cathode, where Cu2+ is reduced. Oxidation of Zn introduced extra Zn2+
ions into the anode compartment – unless this positive charge is neutralized,
no further oxidation can take place (also reduction of Cu2+ leaves
excess negative charge in the cathode compartment).
Electrical neutrality is maintained by
migration of ions through a salt bridge
(a glass U-tube filled with cotton batting soaked electrolyte solution - usually sodium nitrate).
The Na+ ions migrate to the cathode and the NO3-
ions migrate to the anode. The salt bridge also acts to close the circuit.
The
resulting electrochemical cell can
produce a voltage of 1.10 V
For the cell in which the reaction Zn°(s) + Cu2+(aq) →
Zn2+(aq) + Cu°(s) takes place, it can be written in a
short form, following the format below.
The │represents a phase boundary like that
between the electrode and the electrolyte.
The ║represents a physical boundary like the salt bridge.
anode(-) │electrolyte
║ electrolyte │cathode(+)
Zn(s)│Zn2+(aq)║
Cu2+(aq)│Cu(s)
TryIt!
For the Cu(s)│Cu2+(aq)║ Ag+(aq)│Ag(s) cell:
- Draw the cell, including beakers, specific electrodes, specific electrolytes, salt bridge , wires & voltmeter.
- Label the anode & cathode.
- Place the half cell reactions under the appropriate half cell.
- Show the direction of electron flow.
- Show the direction of ion flow.
- Write
out the net cell reaction.
Check out this video for the answer.
Homework:
Practice, p. 397 # 1
Section Questions, p. 400 # 4-7abcdef, 8
Answers: