Download 3.-Electrochemical-Cells-V2-

After completing this topic you should be able to :
• State electricity can be produced in a cell by connecting two different metals in
solutions of their metal ions.
•
Electrons flow in the external circuit from the species higher in the electrochemical
series to the one lower in the electrochemical series.
•
State the purpose of the ‘ion bridge’ (salt bridge) is to allow the movement of ions
to complete the circuit.
•
Use the direction of electron flow in an electrochemical cell to describe where
oxidation and reduction takes place.
•
Use the Electrochemical Series to produce a cell when at least one of the halfcells does not involve metal atoms.
•
State that some batteries are rechargeable, e.g. the lead-acid battery.
•
Describe the reactions, which take place in a hydrogen fuel cell.
When
is placed into
solution, the
displaces the
The
.
transfer
to the
.
(aq) +
(s)
The
The
form
are OXIDISED.
in the solution each gain
from the
to
.
(aq) +
(s)
The
An
can be set up to show that
current can be detected.
are
.
and a
Here is an
made using
V
and
.
Each metal is placed in a
solution containing its own
ions.
The
completes
.
.
(aq)
(aq)
The
completes the circuit for
between the solutions.
The
flows from the
. It allows
produces a voltage of about 1.0 V.
to the
.
to move
DICTIONARY - ELECTROCHEMICAL CELL
An ELECTROCHEMICAL CELL makes an electric current from
chemicals.
This name is often shortened to a CELL.
For
into
to flow from the
, the
have to change
.
(s)
The direction of the
occurs at the
(aq) +
(
) shows
.
The
enter the solution of
The mass of the
decreases.
(aq)
.
At the
,
solution are attracted to the
When the
are
(aq) +
.
touch the
, they each gain
changed into
The
,
(aq) in the
flowing in from the
, and are
.
.
(s)
The mass of the
increases
as
are deposited on the
electrode.
(aq)
The electrochemical cell does not work without a
is an
.
The
. Another name for a
ensures the solutions are always electrically neutral by allowing
to move between the solutions.
-ve ions
+ve ions
The number of
(aq) in
solution increase as the
(s)
give off
and
changes into
(aq).
The solution needs –ve ions to
remain electrically neutral.
The number of
(aq)
decrease as they change into
(s).
The solution needs +ve ions to
remain electrically neutral.
The
is made from potassium nitrate as it is soluble, and the
potassium and nitrate ions do not form precipitates with any other ion.
The
can be used to work out:
•
which pair of metals give the largest voltage;
•
if a displacement reaction will happen;
•
the direction of the current (electron flow) in an electrochemical cell.
ELECTROCHEMICAL SERIES RULE 3
In an electrochemical cell the electrons (the current)
will flow from the metal higher in the Electrochemical
Series to the metal lower down.
Here are the
and
equations as listed on the
.
The
is
. The
equation is written in reverse.
from
transfer
to
.
(s)
(aq) +
(s)
(aq) +
The
is
. The
equation is as written.
The
flow from the
to the
.
A
the
The
reaction occurs when the reactants are located on the corners of the “ ” on
.
,
An
(aq), and
(aq), can transfer electrons to
can be made using
solution,
(aq).
,
(l).
solution,
Here is
,
(aq) /
,
(aq)
V
carbon Carbon electrodes
make the electrical
rod
connection with the
solutions.
carbon
rod
Carbon as graphite
conducts electricity,
and does not react
with the solutions.
(aq)
flow from the
solution
(aq)
solution to the
.
Here are the ion-electron equations from the
substances in the cell.
for all the
The
cannot react with the
, it is a spectator ion.
The
is
.
The equation is written in
reverse.
(aq)
from
+
transfer
to
.
+
(aq) +
The
is reduced. The equation is as written.
(aq)
(l)
(aq) +
(aq)
Here are the
reaction.
(aq)
(aq)
(aq)
is
+
and
reactions combined to give the
(aq) +
(l)
+
(aq)
+
(l)
(aq) +
+
(aq)
(aq) +
(aq) +
(aq)
is
.
.
Complete the ELECTROCHEMICAL SERIES RULE 3 examples on page 6 of
the Metal Chemistry and Electricity & Chemistry Examples Booklet.
are batteries which, when they go ‘flat’, can be charged
and re-used. This means during recharging the chemicals used in the redox
reactions are reformed.
The lead-acid battery is the oldest type of rechargeable battery.
The battery is made from
plates and
. A car
battery is an example of this type of battery.
Some of the lead plates are covered in
,
.
The following reactions occur at the electrodes when the battery
is producing electricity, i.e. during discharge.
(s)
(s)
+
(s) +
+
(aq) +
(s) +
(aq) +
(aq)
(s) +
(aq) +
(s) +
(aq)
(l)
(s) +
(l)
In a battery:
• The
reaction occurs on the negative electrode producing
• The
reaction occurs on the positive electrode taking in
.
.
During charging the chemical reactions are reversed. The following reactions occur
at the electrodes when the battery is being charged.
(s) +
(s) +
(s) +
(s)
(l)
(l)
(s)
(aq)
+
(s)
+
+
(aq) +
(aq) +
(s)
+
(aq) +
(aq)
The first small rechargeable battery produced for
appliances, used a reaction between cadmium and
a compound of nickel (nickel oxyhydroxide
NiOOH) to produce a current.
More advanced rechargeable batteries such as lithium-ion
batteries have been developed. The main advantage of
these batteries is they are much lighter.
A
produces electricity by reacting
reaction.
with
(from air) in a
The
consists of 2 electrodes, which are separated by a special membrane,
called a PROTON EXCHANGE MEMBRANE (PEM). This acts as an electrolyte in
the cell.
At the other electrode
the
ions meet
molecules
from the air and
from the
external circuit.
These react to form
.
At one electrode,
reacts
with a catalyst producing
( ) and
.
are
.
(g)
(aq) +
PEM
The
(g) +
+
( ),
, move through
the (PEM) to the other
electrode.
(aq)
(l)
Here are the
reaction.
and
2
(g) +
reactions combined to give the
(l)
(aq) +
(g)
(g) +
(g)
(aq) +
The
.and
cancel.
(l)
(aq) +
+
The
reaction is
multiplied by 2.
This ensures both
reactions have
.
(aq) +
(g)
(g)
(l)
only produce
and therefore reduce
emissions of carbon dioxide.
can be made by
electrolysis of
.
Electric vehicles, which are powered by
already being used.
are
(aq)