Transcript Al(OH) 4

Chemistry Project
Home Made Cell
Chemical cell is a device in which
chemical energy is converted into
electrical energy.
A simple chemical cell must contain
1. Two electrodes (Electrode: a metal or carbon rod that carries
electricity in or out of an electrolyte during electrolysis)
2. An electrolyte (a compound which, when molten or
dissolves in water (with mobile ions), conducts an electric
current and is decomposed during the process)
Theory of setup 1:
At anode:
*Al(s) + 4OH- (aq) ------> 3e- (aq) + [Al(OH)4]- (aq)
(x2)
At cathode:
*2H+ (aq) + 2e- (aq) ------> H2 (g)
(x3)
Overall:
[ Al(s) + 4OH- (aq) ] ,[Al(OH)4]- , H+ (aq), H2 (g) │Cu(s)
From the simple electrochemical cell, as in the electrochemical
series, aluminium has a higher tendency to lose electrons to form
Al3+ ions.
*At anode (oxidation):
Al(s) ------> Al 3+ (aq) + 3e- (aq) (Impossible!!!)
As aluminium (III) ions are unstable, and are difficult to
form, therefore Al will react with hydroxide ions in the
water to form [Al(OH)4]- which is more stable than
aluminium (III) ions in water.Although the 1st I.E. of
aluminium is low, from 1st I.E. onwards, the ionization
power of aluminium will be decreased.
From the graph, we have
If we want to get one more electrons from the Al + ion, it
needs to use more energy than the 1st I.E., so it is nearly
impossible for aluminium to lose the 3rd electron.
Therefore, at anode:
Al(s) + 4OH- (aq)--------> [Al(OH)4]- (aq) + 3e- (aq)
*At cathode (Reduction):
Cu2+(aq) + 2e- -------> Cu(s)
In the electrochemical series, Cu2+ (aq) is a stronger oxidizing
agent than H+ (aq) in the electrolyte of saturated salt solution
(NaCl), but the solution only contain Na+, Cl-, H+, OH-,
[Al(OH)4]- but not Cu2+ ion . Therefore, hydrogen ions will be
discharged at the cathode and formed on the surface of copper
electrode
2H+ (aq) + 2e- (aq) --------> H2 (g)
(Oxidizing Agent)
As time goes by, the reaction may be stopped because the
hydrogen gas bubbles formed hinder the direct contact
between the copper electrode and the electrolyte.
Electrolyte part :
Saturated salt solution, which contains plenty of mobile
ions, are used as electrolyte to complete the circuit.
If the concentration of the salt solution increase, the
concentration of mobile ions will also increased, as a
result, the current of the electrochemical cell will also
increase.
e.m.f ( Electromotive force )
E total (cell) = Eθcathode – Eθanode
.
E total cell =
Eθcathode from
[ 2H+ (aq) + 2e- (aq)-->H2(g) ]
- Eθanode from
[ Al(s) + 4OH-(aq)--> 3e-(aq) + Al(OH)4]- (aq) ]
The electromotive force of this cell is the maximum
potential difference (voltage) between aluminium and
hydrogen ions. The absolute potential cannot be measured
directly, but we can compare them with each other
Procedure of Setup 1:
Chemical : Tap water
200 ml of saturated salt solution, NaCl(aq)
1x Aluminium can
Apparatus: Copper wires
1 x Voltmeter
Sandpapers
1x
Scissors
Magnetic stirrer hotplate
Procedure :
1.
An aluminium can and the copper strip were cut to
suitable size.The inner surface of the can and the strip
were cleaned by sandpaper so that aluminium oxide
coated on the can,which hindered the reaction between
aluminium and hydroxide ions,
and copper oxide coated on the strip surface could be
removed.
2.
200 ml of saturated salt solution was added into the can.
3.
A circuit was connected as follow:
Result
Time ( min )
0
10
20
30
40
50
60
e.m.f. ( V )
0.30
0.30
0.40
0.40
0.40
0.35
0.35
The result table of the e.m.f. of setup 1
The e.m.f. of setup 1
e.m.f.(V)
0.50
0.40
e.m.f.(V)
0.30
0.20
0.10
0.00
0
10
20
30
40
Time(min)
50
60
70
Modifications:
Although a simple cell has been set up, there are rooms of
improvement. The e.m.f of the cell and the current generated by the
cell are quite low. The cell should be modified so that it can be used
in more electric appliances.
In order to complete the circuit, the electrodes should contact with
the electrolyte. The more the surface area of the electrodes contact
with the electrolyte, the higher the current generated by the cell.
This means the electrodes should be made as rough as possible.
Beside using sodium hydroxide solution as electrolyte, we are
going to add bleach in setup 2 and both bleach and sodium
hydroxide (caustic soda ) in setup 3.
Theory of setup 2:
Bleach, containing hypochlorite ions (OCl- (aq) ), have a
stronger reducing power compare with hydrogen ions ( H+ (aq) ).
OCl- (aq) + H2O( l ) + 2e-(aq) <===> Cl-(aq) + 2OH-(aq) ----(*)
As the difference between aluminium and hypochlorite ion in
electrochemical series is greater than that between aluminium
and hydroxide ion, the e.m.f. of the cell will be increased .
On the other hand, the hydroxide ions produced will react with
aluminium to form aluminate(IV) ions. This means the reaction
will be speed up and thus increase the current generated by the
cell.
Procedure of Setup 2:
Chemical: 1x Aluminium can
100 ml Saturated salt solution, NaCl(aq)
100ml Bleach
Mercury Chloride, HgCl2 (aq)
Apparatus: Copper wires
1 x Voltmeter
1 x Ammeter
Sandpapers
1 x Scissors
1 x Forceps
Cotton wool
Magnetic stirrer hotplate
Procedure:
1.
An aluminium can was cut to suitable size, the inner
surface was cleaned by sandpaper. The can was then
washed using mercury chloride by the aid of forceps and
cotton wool.
2.
The copper strip was cleaned by sandpaper. The strip was
bent for many times and a few cuts were made using the
scissors to increase the surface area.
3.
100 ml of saturated salt solution and 100 ml of bleach was
added into the can.
4.
The circuit was connected as follow:
Result of setup 2:
Time (min)
0
10
20
30
40
50
60
e.m.f.(V)
1.1
1.2
1.0
1.0
0.9
0.9
0.8
Current (A)
0.2
0.2
0.2
0.2
0.2
0.2
0.2
The result table of e.m.f. and current of setup 2 (using
saturated salt solution and bleach and sodium hydroxide)
Theory of Setup 3:
Sodium chloride solution, bleach and sodium hydroxide are
used as electrolyte. The increase in the concentration of
hydroxide ions also increases the rate of reaction with
aluminium, and the current generated by cell will be increased.
Moreover, refer to the equation (*), the increase in the
concentration of hydroxide ions causes the equilibrium to shift
to left, and more hypochlorite ions will be available for the
reduction.
The overall reaction is as follow:
2Al (aq) + 3OCl- (aq) + 2OH- (aq) + 3H2O(l)
<===> 2[Al(OH)4]- (aq) + 3Cl- (aq)
Procedure of Setup 3:
1. Repeat procedure 1-4 in setup 2, then 5 tablets (about 2g) of
solid sodium hydroxide (95%) were added, so that the electrolyte
of cell in setup 3 contains saturated salt solution, bleach and
sodium hydroxide.
Result of setup 3
Time (min)
0
10
20
30
40
50
60
e.m.f. (V)
1.4
1.2
0.3
0.2
0.1
0.1
0.0
Current (A)
0.20
0.25
0.20
0.20
0.10
0.05
0.00
The result table of e.m.f. and current of setup 3 (using saturated
salt solution ,bleach, sodium hydroxide as electrolyte)
The comparsion of the current between setup 2 and 3
Current(A)
0.30
0.25
Setup2
0.20
Setup3
0.15
0.10
0.05
0.00
0
10
20
30
40
Time(min)
50
60
70
The comparison of emf between setup 1 to 3
1.6
1.4
1.2
1.0
Setup1
e.m.f.(V)
0.8
Setup2
0.6
Setup3
0.4
0.2
0.0
0
10
20
30
40
Time(min.)
50
60
70
Precaution:
1. The size of the aluminium can be used in setup 1-3 and
the surface area cleaned by sandpaper should be similar for
fair comparison.
2. The volume of electrolyte in setup 1-3 should be the
same for fair comparison.
3. The number of cuts and the level of bending of copper
strip used in setup 2 and 3 should be similar for fair
comparison of the function of sodium hydroxide.
4. In order to complete the circuit, the electrodes should
contact with the electrolyte. The more the surface area of
the electrodes contact with the electrolyte, the higher the
current generated by the cell. This means the electrodes
should be made as rough as possible.
5. Beside using sodium hydroxide solution as electrolyte,
we are going to add bleach in setup 2 and both bleach
and sodium hydroxide (caustic soda ) in setup 3.
Limitation:
1.
The home- made cell is in liquid form and it is difficult
to handle it.
2.
The chemicals inside are corrosive, so it is dangerous if
use it carelessly.
3.
The cell is so large that it is not portable.
4.
Aluminium reacts with oxygen in air very easily, so the
cell cannot be leave in air for a long time.The voltage
of the cell cannot be estimated.The aluminium can will
become thinner and thinner as Al(s) is oxidized to
[Al(OH)4]- . So it cannot be used for a long time.
Conclusion:
From the result, we can see that,
Setup 1, with saturated sodium chloride solution only as
electrolyte, has generated stable but low e.m.f. that it is
not suitable to be used in most of the electrical appliances.
Setup 2, with saturated sodium chloride solution and
bleach as electrolyte, has generated a quite high and stable
e.m.f. It does not release so much heat and the reaction is
not very violent.
Setup 3, using saturated sodium chloride solution, bleach
and sodium hydroxide as electrolyte, has given the
highest e.m.f. among the three setups. But the e.m.f. has
dropped sharply after all the NaOH(aq) had reacted.
It was because the NaOH(aq) added could speed up the
reaction, thus increased the e.m.f. generated. On the
other hand, the disadvantage is that the life of the cell
would be shortened, and the leakage of the can would be
sped up.
Therefore, we can conclude that the cell using sodium
chloride solution and bleach as electrolyte is the best.
Group Members
Lau Ka Yan Connie (14)
Lee Po Man Andrew (15)
Li Heung Wing Henry (18)
Or Ming Kuen Walter (22)
Wu King Fung Ken (32)