A spontaneous redox reaction involves the
transfer of electrons
In principle, this transfer or movement of
electrons can be used to do a type of work
- specifically, electrical work
A type of device that makes use of redox
reactions to produce electron flow, and to
allow electrical work, is known as a
The development of the voltaic cell
begins as follows...
Let’s look at the spontaneous redox
reaction that involves the metals Zinc (Zn)
and Copper (Cu).
Write the oxidation and reduction half-
reduction for aqueous copper sulfate and
Oxidation: Zn(s) → Zn2+(aq) + 2e-
Reduction: Cu2+(aq) + 2e- → Cu(s)
There would appear to be a movement, or flow,
of electrons from the Zinc metal to the Copper
-At the surface of the Zinc metal, the
oxidation of Zinc atoms will occur at
the same time the reduction of Cu2+
-Electrons from the oxidation of the
zinc will reduce the copper ions to
elemental copper on the surface of
-Over time, the zinc metal will
dissolve (as solid zinc is oxidized to
zinc ion) and zinc ion will build up in
-Also over time, the solution will no
longer contain copper ions, and the
copper will be present in elemental
What if we separated the
Cu(s)/Cu2+ from the Zn(s)/Zn2+?
The reaction will not occur because there is no way for
the electrons released by the oxidation of Zinc metal
to get over to the Copper ions and reduce them to
What if we provide a path of conductance for the electrons
released by the oxidation of the zinc to get over to the
Now that the electrons have a path to the Cu/Cu2+ side it
would appear that the reaction can proceed
But we have a problem….
We need a way to
charge build-up in
So, what do
the solutions due
we need to
to the change in
We originally have a neutral aqueous salt solution, e.g. ZnSO4 (i.e.
the concentration of cation equals the concentration of anion)
As the redox reaction proceeds:
We build up Zn2+ ions in the solution where the Zinc is being oxidized.
We remove Cu2+ ions from solution where the Copper ion is being
The anion concentration (sulfate ion in this case) does not change.
We are building up a net positive charge in the zinc solution, and a
net negative charge in the copper solution.
These charges will oppose the flow of electrons. The positive charge
in the zinc solution will make it harder for the negative electrons to
leave. Likewise, the negative charge in the copper solution will repel
the electrons that are trying to come over from the zinc side.
What if we had a tube filled with aqueous solution that
connected the two redox reactions?
This would allow
anions to move
from the copper
reduction side to
and keep the
In turn, this would now allow the electrons to flow
However, as the reaction proceeds, we have yet another problem...
After a while the net concentration of ions in the zinc oxidation side will be
greater than in the copper reduction side. This concentration gradient will
oppose movement of anions.
What it means is that we have to realize that charge neutralization
can occur by either anions moving to the left or cations moving to
So, in the connecting
tube of solution we
have net movement of
both types of ions
• Anions are going into the oxidation
The connecting tube of solution is
• Cations going into the reduction called a Salt Bridge
Summary of the movement of ions, electrons and the redox
half-reactions in a voltaic cell:
The two solid metals in the different half-reactions are called
The metal in the half-reaction where oxidation is occurring is called
The metal in the half-reaction where reduction is occurring is called
The cathode is often labeled with a "+"; "this electrode attracts
The anode is often labeled with a "-"; "this electrode repels
What causes the electrons to flow
from the anode to the cathode?
The electrons flow from the
anode to the cathode because
of a difference in the potential
energy of the electrons at the
anode compared to the
The potential energy of the
electrons is higher at the
anode than at the cathode
Like water molecules in a
waterfall, the electrons move in
a direction from high potential
energy to low potential energy
(i.e. "downhill" in an energy
Redox reactions that are driven by an external current are called
electrolysis reactions and take place in electrolytic cells.
The electrolytic cell contains two electrodes and either a molten salt solution
(as with NaCl) or some other type of solution (e.g. aqueous)
Electrolysis: chemical reaction caused by passing an
electric current through a liquid, making the molecules of
the liquid split up
External current is provided by a source of direct current (sometimes
another voltaic cell, or battery)
This acts as an electron pump to push electrons onto one electrode
(and drive a reduction reaction) and to withdraw electrons from the other
electrode (and drive an oxidation reaction)
The electrode where oxidation occurs is still
called the anode. However, for an electrolytic
cell it is labeled as "+."
This indicates that the electrode where electrons
are being withdrawn by the external current.
The electrode where reduction occurs is still
called the cathode. However, for an
electrolytic cell it is labeled as "-."
This indicates that this is the electrode where
electrons are being pumped into by the external
Electrolysis of molten NaCl produces chlorine
gas and sodium metal
The process of covering an object made of some
metal with a layer of another metal by means of
Involves the reduction of half of the electrolytic
Current is supplied from an outside source to force
On the cathode side, a source of ions must be