Electrolysis of Copper in Copper Sulphate Solution

Introduction

Aim

This is an investigation into how temperature affects the amount of
copper lost from the Anode and gained on the Cathode during
electrolysis.

Electrolysis is…

The chemical change by passing electric charge through certain
conducting liquids (electrolytes). The current is conducted by
migration of ions - negative ones from the Anode (positive electrode),
and positive ones to the Cathode (negative electrode). Reactions take
place at the electrodes by transfer of electrons in the direction of
the current.

In this case, copper electrodes in a copper sulphate solution are
used.

In the electrolysis of copper, copper atoms in the Anode become copper
ions by losing electrons which go into the circuit.

Cu - 2e à Cu

The positive ions are attracted to the negative Cathode but only
copper ions are discharged.

Cu - 2e à Cu

So this electrode gets coated with copper.

Diagram

[IMAGE]

Also attached to this circuit will be a variable resistor. An ammeter,
placed in the circuit, will register the flow of current: from this
indication, the electric circuit can be deduced to be complete.Also, a
stopwatch will be used.

The 'cell' is a power pack.

Variables

The variables in this experiment are:

· The electrodes

· Resistance

· The electrolyte

· Temperature

· Position of the electrodes (Distance between)

· Time

Only the size of the electrodes and times are being investigated,
therefore in order for this to be a fair test, the other factors must
be kept constant.

The same CuSO will be used throughout, so the concentration is the
same, and the same spacing between electrodes will be used. The size
of the electrodes should be the same, but they will be reused, so the
size will change from one experiment to another.

Time intervals of 5 minutes for each experiment will be used, until it
equals 35 minutes in total.

Prediction

I predict that the longer the electrolysis lasts, the more copper will
be deposited onto the Cathode as the ions will be passed from the
anode to the cathode.

Analysis

The concentration of the solution isn't expected to have a significant
effect, since the concentration of ions do not make it any easier for
the electrons to flow, nor does it affect any factors in the ionic
equation of reaction, so it should not have an effect. However, at
very low concentration, the resistance of water increases due to the
lack of dissolved ions, and this will have an effect on the current
and therefore having an effect on the mass of copper deposited
indirectly.

The time allowed for the electrolysis to run will obviously have an
effect on the mass of copper deposited, since the deposition of copper
is a continuous process, and providing there is a fixed current it
happens at a fixed rate. Therefore, the longer the time, the more
copper ions would be allowed to deposit and the bigger the combined
mass. The mass should be directly proportional to time since the rate
is fixed.

Apparatus

· 1 large beaker

· 2 copper electrodes

· 1 power pack

· 1 ammeter

· 1 variable resistor

· 4 long wires

· 1 stopwatch

· 300 ml copper sulphate solution

· 1 piece sandpaper

· 1 bottle propanone

Plan

An experiment will be conducted to find out how much copper is
dissociated from the anode to the Cathode in a certain time.

The resistor will be monitored to make sure that the experiment is
kept at the same resistance constantly, to prevent the occurrence of
anomalous results.

300ml of COSO will be used every time the experiment is undertaken.

Once the experiment has been set up, it will be initiated for 5
minutes, and then stopped. The electrodes will then be cleaned and the
weight of each electrode will be recorded in a results table.

The experiment will be repeated until there have been 35 minutes worth
of results gathered.

In addition, the same person will perform each experiment to keep it a
fair test

Results

Two experiments were undertaken to ensure that the results were more
reliable. Both are as follows:

Time (minutes)

Anode (+ve)

Weight (g)

Cathode (-ve)

Weight (g)

0

72.89

92.54

5

72.88

92.58

10

72.88

92.70

15

72.84

92.51

20

72.78

92.60

25

72.74

92.63

30

72.68

92.68

35

72.60

92.73

Exp 1 -

92.70

92.51

An explanation will be given to this unlikely decrease in mass after
the results of the first experiment have been analysed.


Time (minutes)

Anode (+ve)

Weight (g)

Cathode (-ve)

Weight (g)

0

72.59

92.56

5

72.56

92.58

10

72.53

92.61

15

72.50

92.64

20

72.49

92.66

25

72.49

92.68

30

72.45

92.71

35

72.43

92.70

Exp 2 -

92.71

92.70

Again, an explanation of this very small decrease in mass will be
given once the results of this experiment have been analysed.


Time (Minutes)

Original Mass Of Anode (grams)

Final Mass Of Anode (grams)

Change In Mass At Anode (grams)

5

72.89

72.88

-0.01

10

72.88

72.88

-0.00

15

72.88

72.84

-0.04

20

72.84

72.78

-0.06

25

72.78

72.74

-0.04

30

72.74

72.68

-0.06

35

72.68

72.60

-0.08

Comparison of increase and decrease in mass - Exp 1

Time (Minutes)

Original Mass Of Cathode (g)

Final Mass Of Cathode (g)

Change In Mass At Cathode (g)

5

92.54

92.58

+0.04

10

92.58

92.70

+0.02

15

92.70

92.51

-0.19

20

92.51

92.60

+0.09

25

92.60

92.63

+0.03

30

92.63

92.68

+0.05

35

92.68

92.73

+0.05

Time (Minutes)

Decrease in mass at Anode (g)

Increase in mass at cathode (g)

5

-0.01

+0.04

10

-0.00

+0.02

15

-0.04

-0.19

20

-0.06

+0.09

25

-0.04

+0.03

30

-0.06

+0.05

35

-0.08

+0.05

As seen above, the figure -0.19 is totally unexpected. The only
explanation is that a lot of copper was washed off when the cathode
was cleaned.

Comparison of increase and decrease in mass - Exp 2

Time (Minutes)

Original Mass Of Anode (grams)

Final Mass Of Anode (grams)

Change In Mass At Anode (grams)

5

72.59

72.56

-0.03

10

72.56

72.53

-0.03

15

72.53

72.50

-0.03

20

72.50

72.49

-0.01

25

72.49

72.49

-0.00

30

72.49

72.45

-0.04

35

72.45

72.43

-0.02

Time (Minutes)

Original Mass Of Cathode (g)

Final Mass Of Cathode (g)

Change In Mass At Cathode (g)

5

92.56

92.58

+0.02

10

92.58

92.61

+0.03

15

92.61

92.64

+0.03

20

92.64

92.66

+0.02

25

92.66

92.68

+0.02

30

92.68

92.71

+0.03

35

92.71

92.70

-0.01

Time (Minutes)

Decrease in mass at Anode (g)

Increase in mass at cathode (g)

5

-0.03

+0.02

10

-0.03

+0.03

15

-0.03

+0.03

20

-0.01

+0.02

25

-0.00

+0.02

30

-0.04

+0.03

35

-0.02

-0.01

The figure -0.01 is unexpected, but is only a very small decrease in
mass, so it is very likely that this small amount of copper was washed
off during the cleaning of the electrodes.

Experiment 1

[IMAGE]




Experiment 2

[IMAGE]




Average results

Time (Minutes)

Decrease in mass at Anode (g)

Increase in mass at cathode (g)

5

-0.02

+0.03

10

-0.015

+0.025

15

-0.035

-0.08

20

-0.035

+0.055

25

-0.02

+0.025

30

-0.05

+0.04

35

-0.05

+0.02

[IMAGE]




After examining each graph carefully, it is clear that experiment 2
was more of a success. The ideal results would have made a graph
something like this:

[IMAGE]




The results to make this graph would have been:

Time (Minutes)

Decrease in mass at Anode (g)

Increase in mass at cathode (g)

5

-0.03

0.03

10

-0.04

0.04

15

-0.02

0.02

20

-0.03

0.03

25

-0.02

0.02

30

-0.04

0.04

35

-0.01

0.01

Evaluation

Electrolysis is the name given to the chemical process which occurs,
for example when an electric current is passed between two electrodes
dipped into a liquid solution.

There are many factors which could have made this experiment more of a
success. For example, the variables could have been more carefully
controlled.

The electrodes; even after a good clean were still quite dirty and
obviously had irremovable substances from previous experiments still
attached to them. Also, if the electrodes had been closer together,
there may have been more interaction/transference between them.

Resistance; A more accurate resistor could have been used to ensure
that the resistance was kept at a constant rate.

The Electrolyte; The same CuSO and amount of it was used each time, so
it is unlikely that this had any measurable effect.

Temperature; A water bath could have been used to ensure a constant
temperature. During the experiment, room temperature was used; this
could have been rapidly reduced simply by somebody opening a window.

Time; A more accurate stopwatch could have been used to ensure
non-anomalous results.

I predicted that the amount of copper lost on the Anode would be the
amount of copper accumulated on the Cathode, but the results do not
show this. In fact they are quite uneven.

As ions dissociate from the Anode, an equal amount of ions should
attach to the Cathode. In the experiment conducted, this did not
happen.

The experiment was not a success as the results do not show much even
exchange of mass.

It would have been preferable to repeat the experiment a minimum of
ten times and to take the mean of these results. They would then have
been plotted to render anomalous results invalid.

During experiment 1, the Cathode decreased 19g in mass. As it has been
previously explained, it is possible that 19g of copper was washed off
when the electrode was cleaned in preparation for the next experiment.

To ensure more reliable, accurate results the variables in this
experiment could have been kept constant under more carefully
controlled conditions. For example, the temperature could have been
kept at something other than room temperature as it is not reliable.
To do this, a water bath could be used.