Li Po Chun United World College ChemistryHL IA
Tony Kwok
International Baccalaureate Diploma Programme
Chemistry Higher Level Internal assessment
Research question: How does the concentration of aqueous CuSO4·∙5H2O affect the rate of copper electroplating? School : Li Po Chun United World College
Candidate Number : 000638 0104
Candidate Name : Kwok Siu Pang
Teacher Advisor : Jon Chui
Word Count : 3063
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
Exploration
Research question: How does the concentration of aqueous CuSO4·5H2O affect the rate of
copper electroplating?
Background
Electrolysis is an important industrial process used to obtain reactive metals, such as sodium,
from their common ores. It works in the electrolytic cell where electricity is passed through an
electrolyte and electrical energy is converted into chemical energy. An electrolyte is a
substance which does not conduct electricity when solid, but does conduct electricity when
molten or in aqueous solution and is chemically decomposed in the process. As a result,
electrolyte stands an important role in the process of electrolysis.
The objective of the experiment is to investigate the correlation between the concentration of
the electrolyte and the rate of electrolysis.
Theory
In this experiment, copper plate is used as an electrode in anode. At the same time, carbon bar
will be used as an electrode in cathode and CuSO4·5H2O with different concentrations (0.2M,
0.4M, 0.6M, 0.8M and 1M) stands as an electrolyte. The reaction is expected as shown below:
Copper Plate
Carbon Bar
Own made figure.
Anode : Cu(s) → Cu2+(aq) + 2eCathode : Cu2+(aq) + 2e- → Cu(s)
During electrolysis of aqueous salt, there are usually four ions present. The cation will be
discharged at the negative electrode (cathode) and the anion at the positive electrode (anode).
At the cathode where Reduction occurs, Cu2+ is consumed to form Cu(s) by gaining two
electrons from the battery. At the anode where Oxidation occurs, Cu(s) is used and become
Cu2+ by losing two electrons. The copper bar (Cu) is used as a replacement of Cu2+ in order to
maintain the concentration of the electrolyte so that the result of the experiment can be more
significant. Besides, Cu2+ and SO42-, hydrogen ions and hydroxides ions from the water will
also be presented. In the electrochemical series, H2O has higher position than Cu. Normally,
H2O should be preferentially discharged as equation shown below,
2H2O(l) → 4H+(aq) + 4e- + O2(g)
However, as copper electrode is used, Cu(s) has much higher concentration than H2O in this
case. Positive electrode is oxidised itself. As a result, H2O is not involved in the reaction. In
general, OH- is attracted to cathode while H+ is more likely presented in anode. Theoretically,
cathode will become alkaline and anode will become acidic. It is also expected that the carbon
bar is the object to be plated. Copper bar is the plated metals. The mass of carbon bar will
increase as time pass because more and more Cu is formed as mentioned above. In the
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
meantime, the mass of copper bar will decrease because Cu become Cu2+ and dissolve into the
solution. Therefore, the mass of carbon bar increased represent the rate of electroplating.
Hypothesis
In this experiment, we assume that the increase of the concentration of electrolyte will result
in the higher rate of electrolysis. It can be explained that the higher concentration of
electrolyte has more solute in the solution. Particles become more crowded. Collision
frequency increases so effective collision frequency increases. As a result, the possibility of
Cu2+ (the solute) gaining electrons in cathode increases. The rate of electrolysis increases as a
consequence.
Variables
Independent variables
Dependent variables
Concentration of CuSO4 solution
Final mass of the carbon bar
Controlled variables
Significance
Solution
Time
The longer duration of
electrolysis, the more product to
be produced. This changes the
final mass of carbon bar and
therefore affect the experimental
data.
Every trial is conducted in 30
minutes.
Temperature
As temperature increases, the
average kinetics energy of the
particles increases. The particles
move faster so there will be more
effective collisions occurs. The
rate of electrolysis will be different
in varied temperature.
The experiment is conducted in
room temperature.
Voltage
The higher voltage of battery is
used, will increase the rate of
electrolysis. However, when a
high voltage is used (e.g. >5V),
there may some unknown metals
be oxidised or other materials
involved in the reaction. These
uncertainty could affect the data
hugely.
3V is used in this experiment. This
decision is made after balancing
the accuracy and efficiency.
The immersing surface area of
the two bars
(Surface area)
The higher surface area of
electrode will increase the contact
area among electrolyte and
electrode. More effective collusion
happens and lead to a higher rate
of electrolysis.
Same size of carbon bars and
copper plates are selected to use.
As only part of the electrodes will
immerse into the electrolyte, 2 cm
length is measured as a
immersing part by rule. The rest of
the part will be covered by
adhesive tape so that the
electrolyte can only touch the
immersing part.
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
Apparatus
1. Carbon bars x 7
2. A bottle of CuSO4 (s)
3. Copper plates x 7
4. Electrolytic Cell
5. 3V Battery
6. Stop watch
7. wires x 2
8. beakers x 7
9. electrical balance
Method
1. Weigh 9.9874g(0.2M), 19.9748g(0.4M), 29.9622g(0.6M), 39.9496g(0.8M) and 49.937g(1M) of
CuSO4·5H2O (249.685g). In the process of weighing, put the CuSO4·5H2O directly into the
beaker which will be use in electrolysis. It can be avoided the error that some CuSO4·5H2O
sticked to the beaker/container during transferring to another beaker.
Calculation:
molar mass of CuSO4·5H2O = [63.5+32.1+4(16.0)]+5x[2(1.0)+16.0]=249.7g
(MCuSO4·5H2O)x(200/1000)x(Concentration)= mCuSO4·5H2O required
e.g. 249.7g x 200/1000dm3 x 0.2M = 9.988g CuSO4·5H2O(s) required in 0.2M 200ml
solution
2. Fill 200ml water into the beaker and mix with relevant mass of CuSO4·5H2O to prepare
0.2M, 0.4M, 0.6M, 0.8M and 1M of solutions. Stir the solution until CuSO4·5H2O is
completely dissolved.
3. Preparing copper plates and carbon bars. Select the carbon bars with same size. Same
step to copper plates. Use a ruler to measure 2 cm length of the bars and plates which will
expose to the electrolyte. Use the adhesive tape to cover the bar except the exposure part.
4. Weigh and record the initial mass of copper bar and carbon bar.
5. Connect the battery to the electrolytic cell and fill in the beaker with CuSO4 solution.
6. Install the copper bar on the Anode and carbon bar on the cathode. Make sure the
exposure part of the carbon bar and copper are completely immersed into the electrolyte.
7. Turn on the power of the battery and start timing. Check if 3 voltage is selected.
8. Record the qualitative data during electrolysis.
9. After 30 minutes, turn off the power and weigh the final mass of copper bar and carbon bar
to see the changes.
10. Record the result
11. Repeat the reaction with CuSO4 solutions in different concentrations.
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
Analysis
Data Collection:
Qualitative data
All trials
Observation
Explanation
A yellowish brown layer was formed on the
surface of immersing part (without adhesive tape
cover) of carbon bar.
Copper was formed in the surface of carbon
bar.
Some
In anode, some black solids dropped from the
trials (not electrode.
specific)
The copper plate was impure. Some
unknown black materials were dropped
when Cu become Cu2+ and dissolve into
the solution. As the unknown materials are
insoluble, so they presented as black solid
form in the solution.
All trials
This may be due to some impurities sticking
into the surface of the copper. During the
electrolysis, the impurities dropped. The
copper plate is therefore changed its colour.
In addition, after the experiment, there may
some Cu2+ appear in the surface of copper
plate. Since Cu2+ is easier remove than Cu,
the surface of copper become smooth.
The colour of electrode became light brown from
dark brown. The surface of copper became
smooth from tough. The copper in the surface is
easy to be removed. There is also fractal pattern
in the surface of the copper plate.
TABLE 1 SHOWING THE OBSERVATION DURING THE EXPERIMENT.
Quantitative Data
Trai Concentrati initial
l
on:
mass of
(±0.001g)
copper bar
(g±0.001g)
final mass
of copper
bar
(g±0.001g)
The
change in
mass of
copper bar
(g±0.002g)
initial
mass of
carbon bar
(g±0.001g)
final mass
of carbon
bar
(g±0.002g)
The
change in
mass of
carbon bar
(g±0.002g)
The rate
of
electroly
sis (g/
hours) :
1 0.2M
CuSO4
(9.987g)
13.179
13.174
0.005
1.070
1.099
0.029
0.058
2 0.2M
CuSO4
(9.982g)
12.670
12.635
0.035
1.080
1.122
0.042
0.084
3 0.2M
CuSO4
(9.985g)
11.957
11.931
0.026
1.071
1.111
0.040
0.080
4 0.2M
CuSO4
(9.979g)
12.098
12.067
0.031
1.070
1.109
0.039
0.078
5 0.2M
CuSO4
(9.991g)
13.024
12.988
0.036
1.089
1.132
0.043
0.086
6 0.4M
CuSO4
(19.959g)
11.919
11.883
0.036
1.070
1.130
0.060
0.12
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
7 0.6M
CuSO4
(29.983g)
8.822
8.774
0.048
1.082
1.160
0.078
0.156
8 0.8M
CuSO4
(39.967g)
12.986
12.930
0.056
1.091
1.180
0.089
0.178
9 1.0M
CuSO4
(49.924g)
11.932
11.588
0.344
1.079
1.179
0.100
0.2
TABLE 2 SHOWING THE RATE OF ELECTROLYSIS IN DIFFERENT CONCENTRATION OF
ELECTROLYTE AND OTHER RELEVANT DATA.
Calculation:
Change in mass of copper = Initial mass of copper — Final mass of copper Change in mass of carbon = Final mass of carbon — Initial mass of carbon !"#$%!!!""!!"!!"#$%& − !!"#"$%!!"##!!"!!"#$%&!!
!
x 60 mins 30!!"#$
Rate of electrolysis = Trial :
0.2M
CuSO4
(g
±0.001g)
1
(9.987g)
Concentr
ation
0.2M
(g/h)
0.058
2
(9.982g)
0.084
3
(9.985g)
0.080
4
(9.979g)
0.078
5
(9.991g)
0.086
Average
0.082
Standard
deviation
0.0037
Percenta
ge of
Standard
deviation
4.453%
TABLE 3 SHOWING THE AVERAGE RATE OF ELECTROLYSIS OF 0.2M CONCENTRATION OF
AQUEOUS CUSO4.
First trial (0.2M CuSO4 (9.987g)) does not take into account in this case. It is estimated that
a significant error was made in first trial. This is probably because the poor conduct of
electricity of old wire was used.
Calculation:
Average rate of electrolysis =
e.g. Average rate of electrolysis of 0.2M trials=
=0.082g/hrs
Percentage of standard deviaEon of 0.2M trials= = 4.453% After balancing the time consumption and the accuracy of the experiment, 5 trials was
only conducted in 0.2M CuSO4 concentration in order to find the significance of random
error.Assuming that the trials in other concentrations have the same percentage of
standard deviation, a table of value of random errors will be collected as following,
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Li Po Chun United World College ChemistryHL IA
0.2M CuSO4
(Average)
Random errors
(3 sig.f.)
Tony Kwok
0.4M CuSO4
(19.959g)
0.00365
0.6M CuSO4
(29.983g)
0.00534
0.8M CuSO4
(39.967g)
0.00695
1.0M CuSO4
(49.924g)
0.00793
0.00891
Calculation:
EsEmated random errors of other concentraEons = 4.453% x Rate of electrolysis e.g. 4.453% x 0.12 = 0.00365 g /hrs (3 sig.) in 0.4M trial
Data processing
To illustrate the trend of the changes in rate of electrolysis in different concentrations, a graph
is plotted as shown below, GRAPH 1 SHOWING THE CORRELATION BETWEEN THE CONCENTRATION OF ELECTROLYTE AND
THE RATE OF ELECTROLYSIS BY USING THE POLYNOMIAL TREND LINE.
Rate of electrolysis (g/hours)
0.22
0.186
y = 0.0733ln(x) + 0.195
R² = 0.9871
0.152
0.118
0.084
0.05
0
0.2
0.4
0.6
0.8
1
1.2
Concentration (M)
GRAPH 2 SHOWING THE CORRELATION BETWEEN THE CONCENTRATION OF ELECTROLYTE AND
THE RATE OF ELECTROLYSIS BY USING THE LOGARITHMIC TREND LINE WITH ERROR BARS.
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
Interpretation
In graph 2 above, there is positive linear trend shown with increasing the
concentration of electrolyte. The correlation of the line of best fit to the plotted points is
high suggesting that the data is quite accurate (since the line of best fit represents the
“theoretical” values), this is represented by the very high R2 values that indicate close
correlation. R2 value is over 0.9 showing that the concentration and the rate of
electrolysis has a very strong positive correlation. This indicates that the assumption
made above is correct.
First trial (0.2M CuSO4 (9.987g)) does not take into account in this case. It is estimated
that a significant error was made in first trial. This is probably because the poor
conduct of electricity of old wire was used. The is a anomaly and therefore is
eliminated. However, the elimination of this trial lead to the insufficient data to
represent the random errors. This will be discussed in “limitation.”
The random errors can be shown by standard deviation. Since 4 trials (supposed to be
five, one trial is eliminated) was only conducted in 0.2M CuSO4 concentration, the
percentage of standard deviation is calculated and used in other concentrations.
Therefore, random errors are bigger when the concentration is higher. But the random
error is only 4.453% which is insignificant. This means that the data collected is precise
and accurate. This can be matched with the high R2 values.
Graph 1 is plotted incorrectly even though there is a higher R2 value. As the graph
below has shown, when the concentration is higher than 2M, the rate of electrolysis
will drop. According to this graph, the hypothesis is also disapproved. However, this
does not make sense to the collision theory which indicate the increase in concentration
should result in more effective collision occurring and lead to a higher rate of reaction.
Graph 3 showing the equation of y = -0.0821x^2+0.2456x+0.036
Graph 1 is therefore replotted to be graph 2.
Graph 4 showing the equation of y = 0.0733ln(x) + 0.077
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
Evaluation
Conclusion
The report provides observation and explanation in the qualitative data. However, some
observations are hardly explained, for instance, the fractal pattern appears in the surface of
the copper plate. To illustrate the trend of the changes in rate of electrolysis in different
concentrations, several graphs are plotted. The general trend of the graph shows a strong
positive correlation, which is justified by the 0.9871 𝑅2 value between the concentration and
the rate of electrolysis. This justifies the hypothesis is correct. Logarithmic trend line is a
more reasonable trend line than polynomial trend line. The hypothesis will be disapproved if
polynomial trend line is used. But logarithmic trend line still increase and become flatter
when the concentration is higher.
Scientific context
In the logarithmic trend line, the increase in rate of electrolysis is decrease when the
concentration is getting higher. This can be explained by collision theory. When concentration
increased, the density of particles increased. Since the surface of copper plate is constant,
there is a limited area for the particles to react. Assuming that each particle needs one unit2 of
surface area for the reaction occur, the table below is showing how the the increase in rate of
electrolysis is limited.
surface
area (unit2)
5
5
5
5
5
5
5
Number of
particles
1
2
3
4
5
6
7
Reaction
rate
1
2
3
4
5
5
5
The table above shows that after 5 particles present, the reaction rate is limited to 5.
During electrolysis of aqueous salt, there are usually four ions present. The cation will be
discharged at the negative electrode (cathode) and the anion at the positive electrode (anode).
At the cathode where Reduction occurs, Cu2+ is consumed to form Cu(s) by gaining two
electrons from the battery. At the anode where Oxidation occurs, Cu(s) is used and become
Cu2+ by losing two electrons. The copper bar (Cu) is used as a replacement of Cu2+ in order to
maintain the concentration of the electrolyte so that the result of the experiment can be more
significant. Besides, Cu2+ and SO42-, hydrogen ions and hydroxides ions from the water will
also be presented. Since carbon bar is the object to be plated and copper bar is the plated
metals, the mass of carbon bar increased as time passed because more and more Cu is formed.
In the meantime, the mass of copper bar decreased because Cu become Cu2+ and dissolve into
the solution. Therefore, the mass of carbon bar increased represent the rate of electroplating.
Limitation:
In this experiment, 5 trials was only conducted in 0.2M CuSO4 concentration in order to
find the significance of random error even there is one trial was eliminated due to
systematic error. The random error of the 0.2M trials may not represent quality of the
data in this investigation is limited by the fact that there is only one data point for the
rest of the concentrations which means that there is the potential for random errors to
influence the data. The quality of the data could be enhanced by replicating each data
point 5 times so that an average value can be calculated. An average value will help to
eliminate the effect of random errors and give a value that is more accurate. In spite of
this shortcoming high degree of correlation between the data and the lines of best fit
suggest that the processed data is quite accurate. Increasing the number of increments
in the concentration would also make a trend in the data clearer.
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
The following table highlights the possible sources of error and corresponding
improvements:
Limitation
Types
Significance
preventing action
Improvement
Purity of copper
Systematic error
The impurities
cover the copper
plate. This reduces
the surface area of
the copper
contacting solution
and therefore lead
to a decrease in the
rate of electrolysis.
Use sandpaper to
rub the surface of
the copper plate so
that the plate has a
cleaner surface.
Since the process
of the experiment
does not consume
the whole copper
plate, the increase
in the surface of the
plate is enough.
Using 100% pure
copper plate will
increase the
accuracy of the
data collected.
The way of drying
Systematic error
Pro: more accurate
data collected
Con: Higher budget
needed
After conducted
Use tissue to wide
electrolysis, the
softly.
copper plate is wet.
Using tissue to
wipe the copper out
of the plate. This
may reduce the
mass of the copper
and directly lead to
a decrease in the
rate of electrolysis.
Wait until it dry.
The mass of solute Random error
are slightly different
to the mass
theortically.
The mass of solute
affects the
concentration of the
solution and lead to
error. As all trials
have the same
errors, the sum of
the errors can
make significant
impact.
Measure the
CuSO4·5H2O mass
close to +/- 0.01 to
the theoretical
mass.
Measure the
CuSO4·5H2O mass
same to the
theoretical mass.
Pro: more accurate
data collected
Con: Hardly
achieved artificially.
poor conduct of
electricity of old
wire
The first trial failed
due to this error.
The conductivity of
wire directly affect
the the number of
electrons
transferring and
affect the rate of
electrolysis.
/
Use the new wire
Systematic error
Pro: more accurate
data collected
Con: less efficient
Pro: more accurate
data collected
Con: Higher budget
needed
Safety and Ethics
Finishing the experiment, I dispose of the waste improperly which I did not rinse my CuSO4-containing
beakers into the inorganic waste bottle. Instead, I have placed the unrinsed beakers into the
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Li Po Chun United World College ChemistryHL IA
Tony Kwok
detergent basin and created a much larger volume of frothing toxic waste that is very difficult to
dispose of safely.
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