Experiment 12
Purpose : To investigate the effect of concentration on the electromotive force of a
cell.
Theory : Zn²+ (aq) + 2e ® Zn (s)
If the concentration of Zn²+(aq) ion decreases, the equilibrium position will
shift to left and EZn will become more positive. Thus, the electromotive
force of the electrochemical cell increases.
Apparatus : - 5 strips of filter paper
- 1 zinc plate and 1 copper plate
- Galvanometer
- Potentiometer / wheatstone bridge
- Jockey
- Five 30cm³ connecting wires with crocodile clips
- 2V accumulator or 2 dry cell (each 1.5V)
- Two measuring cylinders
- Six 100cm³ beakers
- One piece of sandpaper
- Onewash bottle filled with distilled water
Materials : KA 1 is 1.0 mol dmˉ³ aqueous copper (II) sulphate
KA 2 is 2.0 mol dmˉ³ aqueous zinc sulphate
KA 3 is 1.0 mol dmˉ³ aqueous zinc sulphate
KA 4 is 0.1 mol dmˉ³ aqueous zinc sulphate
KA 5 is 0.01 mol dmˉ³ aqueous zinc sulphate
KA 6 is 0.001 mol dmˉ³ aqueous zinc sulphate
A saturated solution of potassium chloride.
KA 1 and beaker B with 50.0cm³ of KA 2 . Immerse a strip of
filter paper into the saturated solution of potassium chloride;
then dip the ends of the filter paper into the 2 beakers A and B
to build a salt bridge. Clean the copper and zinc plate with
sandpaper, wash with distilled water, and dry. Clip the copper
plate with the crocodile clip and connect it to the end of the
potentiometer to which the positive terminal of the accumulator
is connected. The zinc metal is connected to a galvanometer to
which a jockey is attached. The negative terminal of the
accumulator is then connected to the other end of the
potentiometer as shown in the diagram below.
one minute has elapsed, touch the jockey along the length of
the potentiometer wire until the galvanometer shows a zero
reading indicating no current flow. Do not slide the jockey
along the length of the potentiometer wire to avoid a build up
of resistance. Note down the length of the potentiometer wire
as x cm and record your readings in the table on the next page.
Repeat the experiment above by replacing solution KA 2 with
solutions as shown in the table below. (New filter paper are to
be used for every new solution.)
Calculate Ecell = __________x_____________ × 2.0V
Length of potentiometer wire
Experiment | Beaker A | Beaker B | Length of Wire x/cm | Ecell /V | [ Zn²+ ] / mol dmˉ³ | lg [ Zn²+ ] |
1 | 50.0cm³ KA1 | 50.0cm³ KA2 | 40.3 | 0.806 | 2.000 | 0.301 |
2 | 50.0cm³ KA1 | 50.0cm³ KA3 | 44.5 | 0.890 | 1.000 | 0.0 |
3 | 50.0cm³ KA1 | 50.0cm³ KA4 | 50.5 | 1.010 | 0.100 | -1.000 |
4 | 50.0cm³ KA1 | 50.0cm³ KA5 | 64.0 | 1.280 | 0.010 | -2.000 |
5 | 50.0cm³ KA1 | 50.0cm³ KA6 | 71.1 | 1.422 | 0.001 | -3.000 |
Question :
Zn (s) / Zn²+ // Cu²+ (aq) / Cu (s)
write an equation for the overall cell reaction.
Beaker A: Zn (s) → Zn²+ (aq) + 2eˉ
Beaker B: Cu²+ (aq) + 2eˉ → Cu (s)________
Overall : Zn (s) + Cu²+ (aq) → Zn²+(aq) + Cu (s)
(e) Plot a graph of Ecell against lg [ Zn²+ ] in beaker B and explain how the e.m.f of
the cell varies with the concentration of Zn²+ in the beaker B.
(i) The solution in beaker A is replaced with 0.1 mol dmˉ³ aqueous copper (II)
sulphate?
The value of Ecell will decrease because the concentration of Cu²+ ions have
decreased.
(ii) The solution in beaker B is replaced with 0.1 mol dmˉ³ aqueous aluminium
chloride, and the plate of zinc metal is replaced with aluminium metal?
The electrode potential of the aqueous aluminium chloride and copper metal.
Therefore, the e.m.f becomes higher.
Conclusion : The electromotive force of a cell is inversely proportional to the
concentration of Zn²+ in beaker B.
Comment : 1. The resistance of copper wires are different because the wires used are
not the same.
2. The concentration of potentiometer which is not tight enough will cause
the reading recorded not accurately.
3. The contact between the jockey and the copper wire is not perfect and it
causes the reading not accurately.
4. Current provided in the dry cell will decreased during the experiment
and causes the reading not accurately.
1 comment:
thanks
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