NOAH N Electrochemistry Report FINAL2
Stage 1 Chemistry
Electrolysis Practical Investigation
Name:
Noah Nishihara
Home Group:
E05
Report
of experiment on:
The Mass Difference of Electrodes inthe Electrolysis
of Copper
Date of
Experiment: 26 May 2017
Work Finalised: 2 June 2017
Table
of Contents
Aim
To
investigate the effect of electrolyte concentration on the process of copper
refining, this experiment will determine the anode mass loss during ten minutes
of copper electrolysis.It will also attempt to confirm that the theoretical
amount of mass lost from the anode equals the actual amount of mass lost from
the anode copper electrode.
Outline
of Procedure
Apparatus
·
100 mL and 250 mL size beakers
·
100 mL measuring cylinders
·
Copper electrodes of assorted sizes
·
Alligator clips to attach electrodes onto the side of
the beaker
·
Two insulated wire pieces
·
DC capable power supply (regulated)
·
Aqueous copper(II) sulphate, 0.5 M
·
Paper towels
·
Steel wool
·
Stopwatch
·
Ruler
·
Incubator
·
Top loading balance (max 450 g)
·
Blu-tack
·
Distilled water (in containers)
Diagram:Anode
copper mass loss before and after the electrolysis; comparison
of the practical results and theoretical results and a change of concentration
of electrolyte
Procedure Description:
This experiment was carried out twice with the
same concentration of 0.5 M copper sulphate and then repeated twice with 0.2 M
copper sulphate for a total of four measurements.
1.It was ensuredthat the
copper electrodes were clean by rubbing them with steel wool. This resulted in
shiny electrodes without the possibility of contamination or moisture which
could potentially affect data collection and measurements.
2.The mass of the two labelled
(+and - ) copper electrodes were measured separately with a top loading balancewith
the lid covering it andthe results were recorded.
3.90 mL of 0.5 mol / L
copper (II) sulphate aqueous solution was addedinto a measuring cylinder,
4.This was poured into a 100
mL beaker to assemble the circuit shown in the figure above.
5.The electrodes were
clamped to the rim of the beaker using crocodile clips ensuringthat the
electrodes did not touch. The distance between electrodes was kept as constant
as possible.
6.The clips were not touching
the electrolyte. The positive anode copper was connected with the positive end
of the power supply. The negative cathode was connected to the negative end of
the power supply.
7.The area of the two copper
plates immersed in the copper (II) sulphate aqueous solution waskept as similar
as possible.
8.The power supply was set
to 2.0 V to prevent water from reacting and producing hydrogen ions H+
and hydroxide ions (OH–).
9.The current intensitywas
set to 3.1 A (or equivalent maximum value)
10.Current was allowed to
flow for 10 minutes (600 seconds) by pressing the on button and starting the
stopwatch simultaneously.
11.The currentreading was
continuously monitored and recorded along with the time on the stopwatch.
12.Movement of the
electrodes was avoided.
13.The copper electrodes
were removed, rinsedcarefully withdistilled water and left to evaporate on
blu-tack (for the cathode mass gain measurement). The anode was wiped and the
copper oxide was not included in the mass measurements.
14.The mass of each copper
plate was weighed.
15.The process was
repeated another three times in totalwith
0.2 mol/L of copper (II) sulphate and different electrodes which were weighed
again.
Principles
The measuring principle
utilised here is integer counting. The data is collected directly as values
from measuring devices. The top loading balance used had a resolution of 1 to
0.001 g and could weigh up to 450 grams. Calibration of the device was not
carried out as the digital readout was deemed correct before use.
Basic Formulas
Used
Concentration Calculations: C1V1 = C2V2
To find
Charge:
Formulas used in analysis
Number of
moles:
Note:-1.602×10-19〔C〕×
6.022×1023〔/mol〕≒ -9.65×104〔C/mol〕Since
the quantity of electricity Q of one electron is constant at -1.602 × 10
-19 C, in electrolysis, the amount of changing substances is proportional
to the quantity Q of electricity passing through.
Hypothesis:If the
concentration of copper (II) sulphate is raised, then the efficiency of the
reaction will also rise in the electrolytic refinement of copper.
Variables
Independent Variable: Concentration of copper (II) sulphate electrolyte
How the Independent Variable is changed: Alternate the electrolyte
conditions from 0.5M to 0.2M(Copper (II) sulphate concentration)
Dependent Variable: The amount of the anode copper that is lost and from
that, the efficiency can be derived (more mass loss may mean higher efficiency)
How the Dependent Variable is Measured: Mass measuring top loading
balance,
Other factors
held Constant in the Experiment:
- Type of
electrodes –Copper anode and cathode – anode quality
- Time
taken to complete reaction – 10 minutes(stopwatch
for 10 minutes of time)
- Volume
of solutions used in the electrolysis reaction
- Surface
area of electrode dipping into the electrolyte solutions – measured to 5
cm
- Maintain
voltage with regulated power supply – 0.15V-0.30V (industrial) here it is 2.0 V
- Current
of the electrolysis circuit – 10-20 mA/cm2(industrial) 0.02 A
if lengthy reaction – cathode current density - rate at which the anode is
dissolved and the rate at which the cathode is plated
- Attempt
to maintain constant temperature – or take into consideration its
fluctuations – recommended industrially to be 60 degrees Celsius (22 degrees
Celsius)
- Distance
between electrodes – for the 1st and 3rd reactions
the distance was less
Results
Table 1 Main Results
|
Order
of Attempt |
Type of
Electrode |
Concentration
(Mol/L) |
Mass Before
Electrolysis (grams) |
Mass After
Electrolysis (grams) |
Difference
in Mass (grams) |
|
1 |
Anode |
0.5 |
0.978 |
0.897 |
0.081 |
|
1 |
Cathode |
0.5 |
0.88 |
0.944 |
0.064 |
|
|
|
||||
|
2 |
Anode |
0.5 |
1.617 |
1.574 |
0.043 |
|
2 |
Cathode |
0.5 |
0.94 |
0.981 |
0.041 |
|
|
|
||||
|
3 |
Anode |
0.1 |
8.809 |
8.777 |
0.032 |
|
3 |
Cathode |
0.1 |
8.936 |
8.972 |
0.036 |
|
|
|
||||
|
4 |
Anode |
0.1 |
1.13 |
1.107 |
0.023 |
|
4 |
Cathode |
0.1 |
0.862 |
0.886 |
0.024 |
Table 2 Current Changes of the Reactions
|
1st |
Recording |
2nd |
Recording |
3rd |
Recording |
4th |
Recording |
|
Time
(s) |
Current
(A) |
Time (s) |
Current
(A) |
Time (s) |
Current
(A) |
Time (s) |
Current
(A) |
|
0 |
0.325 |
0 |
0.247 |
0 |
0.157 |
0 |
0.117 |
|
50 |
0.326 |
60 |
0.248 |
40 |
0.158 |
3 |
0.118 |
|
80 |
0.327 |
195 |
0.249 |
95 |
0.159 |
5 |
0.119 |
|
100 |
0.328 |
480 |
0.25 |
138 |
0.16 |
25 |
0.118 |
|
120 |
0.328 |
600 |
0.25 |
195 |
0.161 |
220 |
0.119 |
|
230 |
0.329 |
250 |
0.162 |
270 |
0.119 |
||
|
318 |
0.33 |
320 |
0.163 |
390 |
0.119 |
||
|
445 |
0.331 |
430 |
0.164 |
490 |
0.12 |
||
|
588 |
0.332 |
520 |
0.165 |
600 |
0.12 |
||
|
600 |
0.165 |
Using
the 5th Equation, the percent error formula, the results of the
experiment (anode mass losses) were compared with the theoretical calculations
of the mass lost at the anode. The second attempt was shown to have the
greatest error.
The results are shown in the table
below:
Table 3 Percent Error of Reactions
|
Error
with Lowest Current (%) |
Error
with Highest Current (%) |
|
|
1 |
0.3115 |
2.439 |
|
2 |
11.885 |
12.955 |
|
3 |
3.226 |
1.84 |
|
4 |
0.433 |
2.954 |
Calculations
Following
on from the Percent Error Formula calculations which indicate the ratio of the
observed and true values, the 6th equation is used to find the
percent uncertainty. For the first attempt at the lowest current, it is 4.867%.
This is given by:
The
experimental result is compared with the theoretical result in % error, while %
uncertainty is found to give a better idea of how well a value can be
determined. This uncertainty value also reveals how precise the experiment was.
The values for uncertainty are shown below:
Table 4 Uncertainty in Measurements
|
Uncertainty with Lowest Current (%) |
Uncertainty with Highest Current (%) |
|
|
1 |
4.867 |
38.109 |
|
2 |
276.395 |
301.279 |
|
3 |
100.813 |
0.575 |
|
4 |
18.826 |
128.435 |
The
values somewhat correlate with the values for percent error.
The
statistical standard deviation was found for the mass loss using both theoretical
values of low and high current and the experimental value.
Table 5 Standard deviation of Mass loss
|
Mean (g) |
Standard deviation |
|
|
1 |
0.0646 |
±0.0008718 |
|
2 |
0.0471 |
±0.003533 |
|
3 |
0.0319 |
±0.0008083 |
|
4 |
0.0233 |
±0.0003843 |
This
calculation falsely leads to the belief that the experiment was not too spread
far out. This was not the case, with an alternative interpretation possible of
the second attempt being the most error-ridden and the first and third attempts
at around the same variance. Only the fourth attempt has satisfactory accuracy.
It should be noted however, that the electronic top-loading balance used only
measured to the third decimal place.
Concentration:
Moles of copper atoms:
Since
copper (II) ionsare divalent cations, two electrons are necessary for one
copper (II) ion to become a copper atom.
Moles of electrons:
10 minutes
are 600 seconds
The values are summarised in the table below:
Table 6 Moles of electrons comparison
|
Moles Of Electrons – From Experiment Data Of Electrode Mass
Difference (Mol) |
Moles Of Electrons – Theoretical Values (Lowest Current)
(Mol) |
Moles Of Electrons – Theoretical Values (Highest Current)
(Mol) |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Calculations
of: Theoretical mass lost/gained of Copper
Moles of
Copper
Mass of
Copper:
The amount of charge gives the mass of copper lost or
gained in the calculations using equation 4. These values were used for the
percent error calculations and revealed the second attempt to be the most
inaccurate, while the first mass measurement of the anode which gave 0.081
grams was at a 26% deviation.
This is summarised in the table below:
Table 7 Mass change-accepted values
|
Mass at Low Current (g) |
Mass at High Current (g) |
|
|
1 |
0.0642 |
0.0656 |
|
2 |
0.0488 |
0.0494 |
|
3 |
0.031 |
0.0326 |
|
4 |
0.0231 |
0.0237 |
Discussion
The Reactionduring
electrolysis:
A transfer of
copper metal from the anode to the cathode is the objective.
Each
copper atom loses two electrons (oxidation) to become copper ionsat the anode.
Thecopper cations move to the cathode, where two electrons are gained (reduction)
to become copper atoms that are deposited on the cathode.The sulphate negatively
charged ion moves to the anode but does not reactas it is stable and the
concentration of ions is constant as the reaction of copper occurs at both ends.
The reactivity and concentration of ions determines this.
The
theoretical values attained using Equation 2 were lower for the first attempt,
and greater for the second and fourth attempts compared to experimental values.
Only the third attempt fitted the theoretical values. The first resultwas due
to errors in the measuring of mass. The cleansing process may have affected the
values for others. The current reading could have been lower than the actual
amount, causing the data to be lower. Contrastingly, the low error of the
fourth attempt indicated a close fit to the theoretical mass values, further
proving the misleading nature of the values of moles of electrons calculated
with Eq.2.
The
main findings of this experiment were mass loss values found to decrease with
each experiment. The reactions resulted in different mass loss values of the
anode. There was a correlation between the concentration and the
mass loss, due to differing current. Higher concentrations had greater mass
loss due to a raised current value.Lower concentrationshad less mass loss due
to a lowered current value.
In
the graphs of current change by time, it was observed that for the first
reaction, the current increased by 0.003 A in the first two minutes, but then
took over five minutes to increase by another 0.003A. Similarly, the second
attempt saw more rapid increase of current in the first few minutes as opposed
to later, when the value fluctuated until the time limit was reached. The third
round had the greatest increase of current, with an increase of 0.005 A in four
minutes and 0.003 A in another four minutes. The fourth reaction had the
current fluctuating from 0.118 A to 0.119 A from 3:40 to 4:30. The increase
overall was 0.003A. The amount of current change was reduced by longer
distances between electrodes. Current change always rose steeply and then became
stable.
Reaction
conditions including voltage can be lowered to be more effective in returning
metal ions to the original metal form. The experiment outlined here was
performed under time constraints. Current was not set to allow the voltage to
fluctuate due to lower performance. However, a controlled lower voltage over a
longer period may be an improvement over the current experiment design.
The
top-loading balance required a lid to be placed on top. It was uncalibrated due
to lack of knowledge.
Slight
differences in the surface area exposed to the electrolyte were unavoidable due
to the different shapes of the electrodes. An attempt was made to use
like-sized electrodes together and ensure that the length submerged was no more
than 5 cm.
The
drying of the cathode was done carefully with evaporation attempted and no
contact to it by any material. The meniscus was found at eye level for the
measuring and distribution of copper sulphate solution. Measuring cylinders of
100 mL capacity have less obvious formations of a meniscus, however, it was
ensured that the bottom of the meniscus was precisely on the calibration mark.
Although
the experimental values for the balance were to three decimal places, the
theoretical mass values were to four decimal places. A balance with greater
accuracy and less maximum measurable mass would have increased the precision of
the data analysis.
Using
the percent error formula, the second attempt was found to have a lower
accuracy than the other attempts due to balance fluctuations. This may have
been caused by a failure in the operation of the electrolysis. The other
measurements of anode mass loss were within five percent of the theoretical
values.
Systematic
errors affected the accuracy of the practical in the form of the use of
non-homogenous solutions. The result of this may be different current values
and subsequently changes to the electrode mass loss, and would lower the
accuracy of the collected data. To eliminate this error, the higher
concentration could be raised to 1.0M and compared with a lower solution of
0.5M.
The
accuracy of experimental values in this report could be increased if measuring
instruments are calibrated or solutions are standardised. Different apparatus
could be used.By repeating the experiment, the reliability can be increased.
The validity of this experiment was high, as the overall data proves what was
hypothesised, though with some measurement errors.
To
apply the results contained within the report, further studies will be
necessary. Distance between electrodes should be minimised for maximum increase
in current. Higher concentrations of copper(II) sulphate work well because of
the current increase.
Conclusion
High
values of current with 0.5 M copper(II) sulphate saw greater efficiency than
low current values with copper(II) sulphate of lower concentrations. The
calculations of the theoretical mass loss of the anode reflected the actual
mass loss for the 0.2 M copper(II) sulphate experiments only.
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