The range of my results were from 0.20A to 1.00A, with an average discrepancy of 0.02A from the average reading, which although there was one large anomalous result is quite small, is quite a small variation, therefore The evidence is strong enough to say that the mass lost at the cathode equals the mass gained at the anode, and that q µ i, as the greatest error was only 0.01g, or 12.5%.
If This experiment was to be done more accurately, I would have to use more accurate apparatus, such as a newer ammeter, a balance with more digits, a more accurate way of controlling the current, maybe with a computer, and likewise with the temperature. I also could have kept the size and separation of the electrodes the same. I also could have made sure that the crocodile clips were completely out of the electrolyte. Also I could have taken a much wider range of readings, from 0.01A to 10A at smaller intervals, and I could have timed for different times, and I could have investigated the other variables, such as the temperature of the electrolyte, the concentration of the electrolyte, the separation of he electrodes, and the size of the electrodes. The Electrolysis Of Copper Sulphate Solution Using Copper Electrodes
Planning
Electrolysis is the decomposition of a substance by the passage of an electrical current. I a typical set up, two electrodes (conducting rods immersed in an electrolyte). Voltage is applied to the electrodes with a power pack. The electrolyte must be an ionic compound that is molten or in aqueous solution, in order for it to conduct electricity.
Electric current is caused by the movement of charged particles. In a normal circuit, theses charged particles are electrons, which are effectively pumped through the metal wire by the power pack. In the electrolyte these charged particles are mobile ions. At the electrodes electrons are given to the cat ions cathode (-), and are released at the anode (+), so the current flows. Therefore species are gaining electrons at the cathode, and so being oxidised, whilst electrons are taken away at the cathode (reduction).
At the cathode there is preferential discharge of ions according to the position of the element in the reactivity series. When aqueous copper salts are electrolysed, the cat ions present is he solution are hydrogen ions, which come from the water, and copper ions, so copper is formed at the cathode.
Cu"(aq) + 2e- > Cu(s)
At the anode the reaction occurring depends on the nature of the electrode. If the electrode is inert, then normally it is found that the ions are discharged in the order halide then hydroxide before sulphate. However, this order may change depending on concentration. An example of this is platinum electrodes. Those made of carbon behave similarly, but a carbon anode will react with oxygen as it is released forming oxides of carbon, like in an aluminum smelter. Copper electrodes are not inert, instead of incoming anions being discharged, the copper goes into solution:
Cu(s) > Cu" (aq) + 2e-
The reaction occurring at the anode during the electrolysis of a copper salt is the reverse of the cathode reaction. So for every two electrons passing through the external circuit, one copper ion should be formed at the anode and one copper ion discharged at the cathode. So overall copper is being transferred from anode to cathode, as is exploited in electroplating and in purifying copper. One would expect the mass loss of the anode to equal the mass gain at the cathode, as explained earlier, for every two electrons, at the cathode one copper ion is discharged, whilst at the anode, one copper ion is formed. Also the concentration should remain constant.
The amount of copper deposited on the cathode and lost from the anode depends on the number of electrons passing through the circuit, i.e. upon the charge passed through the cell. Now the charge passed, q (in Coulombs), is related to the current. I )in amps) and time, t (in seconds), by Faraday's law:
q=ixt
therefore I will predict that the mass change of the copper electrodes is directly proportional to the current and the time.
Abstract
This investigation looks at the factors which affect the rate mass of copper deposited at the cathode during the electrolysis of Cupric Sulphate solution using Copper electrodes. Both the time and the current flowing in the circuit was expected to affect the mass of copper deposited. However, the concentration of electrolyte was expected to have no effect. It was discovered in this investigation that both the time and the current was directly proportional to the mass of copper, and indeed the concentration had no effect. It was also found that due to the technical difficulties in measuring the gain of copper at the cathode, the loss of copper at anode was measures, since the two values should be the same.
Method
The beaker was filled with 100 ml of saturated solution of copper sulphate at 20oC. The electrodes were made by cutting 0.5 mm sheet copper into size (1 in x 6 in). The electrodes were sandpapered prior to use in order to remove the oxide layer on the sheet copper. This was attached to the side of beaker by bending the electrodes into shape so that it clings onto the side of the beaker. One of the electrodes was marked “+ve” with a compass. This was connected to the positive end (+5V) of the Power Supply Unit (PSU). The other electrode was marked “-ve” and was connected to the 0V terminal. The ammeter and the variable resistor was connected in series between the anode (+ve) and the +5V PSU terminal (Figure 1.1). After switching on the PSU, the apparatus was fiddled about by means of changing the voltage on the PSU and twiddling the knob on the variable resistor until a steady reading of 1.0A is obtained on the ammeter. The PSU was switched off, the electrodes were weighed with the 3-figure balance and the respective mass was recoreded.
The PSU was switched on for a minute. The electrodes were removed from the beaker, washed under the tap, dried with the lab coat, washed with acetone in order to remove all the water. The electrondes was then left on a central-heating radiator for 1 minute to dry, and they were weighted. The change in mass of cathode since the beginning of experiment was recorded. The change in mass of anode was also recorded for comparsion purposes. This was repeated three times in all to demonstrate that the results were reproducible.
The procedure described in paragraph two was repeated using currents from 0.1A to 1.5A in 0.1A increasments. It was also repeated using different ratios of saturated CuSO4 solution with distilled water, in order to give numerous different concentrations. The procedure was also repeated with different time by which the PSU was left on, starting from 1 minute to 10 minutes in 1 minute increasment.
The results were graphed using GraFIG (Appendix 1) in the following manner: a graph of Current vs Change in Mass (dMass) was plotted for Experiment Nr. 1; a graph of Volume of saturated CuSO4 solution (which is directly proportional to the concentration) vs Change in Mass was plotted for Experiment Nr. 2; and a graph of Time vs Change in Mass was also plotted for Experiment Nr. 3.
Experiment Nr. 1
Time: One minute.
Concentration: Saturated CuSO4 solution at 20oC
Discussion
From the results obtained and the graphs, it is indicated that the mass of copper deposited at the cathode is directly porportional to the current. It is also indicated that the mass of copper is directly proportional to time. However, during the experiment it was realised that it was extremely difficult to measure the gain of copper on the cathode, since by cleaning the electrode with lab coat, and washing it under the tap, some of the copper deposited was knocked off. Nevertheless, this washing process was vital because the need the eliminate the copper sulphate crystals that may grow on the electrodes from time to time. The results would have been better if the experiment was left on for longer, say, 5 minutes, to compensate for the error. Also, it would have been more sensible if I had not left the electrodes in the Copper Sulphate solution over half-term. It was found that the mass of copper lost at the anode should be exactly the same as the mass of copper gained at the anode, since the concentration of Cu2+ ions in the electrolyte does not change. Every copper ion reduced at the cathode, one copper atom must be oxidized at the anode to keep the electrical charge balanced. This made it possible for the loss of copper at the anode to be used as a part of the results. The loss at anode can be more accurately measured since one cannot alter the mass of the anode significantly if it had been dried properly (since the copper does not drop off).
The concentration of electrolyte did not have a significant effect on the gain of copper at the cathode. However, when the concentration dropped to less than 0.1 M the conductivity of the solution was changed by such a huge amount that the PSU in use was no longer able to pump through the desired current. This alters the current, thus declaring the results invalid since current was expected to affect, and did affect the mass of copper deposited at cathode. The drop in the mass deposited in very low concentrations was due to the drop in current rather than the concentration itself. If a more powerful PSU was available (say, 100V max), and a more sensitive variable resistor was used, even at low concentrations, it should not have had an effect on the mass of copper deposited at the cathode.
As a matter of interest, during the experiment some voltmeters were found to be available, because the technician had got the wrong shunts. I shoved a voltmeter across the electrodes, and the voltage was found to be different whenever the current is different. This is due to the presence of the variable resistor in the circuit. The voltage from the PSU is always the same. By incresing the resistance of the variable resistor, I am essentially increasing the total resistance of the circuit. Using the equation
V = IR^2 [4]
when voltage (across PSU) is constant (at 5.0V), changing the resistance would change the current. However, the resistance across the electrodes is constant (equals the total resistance of the solution, the electrodes and the crocdile clips), by the same equation, when the current is changed, the voltage must change. Current must be the same everywhere in a series circuit. Therefore, the voltage across the electrodes cannot be seen as an experimental parameter. It is, in fact, the result of one’s fiddling with knob on the variable resistor. This can be tested easily by replacing the cell with a light bulb, where the resistance is constant and intensity varies with the current of the circuit, or the voltage across the bulb. By removing the variable resistor from the bulb circuit and using different voltage setting on the PSU, same effect on the buld can be achieved. Whether a extremely high voltage across the cell can lower the resistance of the cell by ionization of the the air around it remains to be seen.
METHOD
The apparatus was set up as in the diagram below:
Copper Sulphate solution (50cm3) was poured into a small beaker. The two copper electrodes were thoroughly cleaned using water and steel wool, to scratch off the layers of copper from previous experiments. The electrodes were weighed, their masses recorded and placed into the beaker containing Copper Sulphate solution. The electrodes were connected to a cell and ammeter. A steady current flowed (0.2 Amps) and the experiment was stopped at definite times (i.e. 5,10,15,20,25 minutes). At these times the current was switched off and both electrodes were removed from the solution. They were then washed by dipping in distilled water, and dried by dipping into propanone (a highly volatile liquid which readily evaporates) and placed near an electric heater.
Once clean and dry both electrodes were both carefully weighed and their subsequent masses recorded.
EVALUATION
Although this was a successful experiment, there were some factors of the experiment, which could have been improved to make it even more successful. One of these factors could have been the electrodes, which, even after a good clean were still quite dirty and obviously still had irremovable substances from previous experiments still attached to them. If this experiment were to be repeated for a second time, in need of greater accuracy, it would be imperative to have a new pair of electrodes, which have never been used before.
Another factor which may have affected the overall outcome of the investigation, may have been the fact that the practical work of the investigation was carried over from lesson to lesson, meaning that variables such as the concentration or the amount of the Copper Sulphate solution could have changed between lessons. To overcome this problem, a stock solution of Copper Sulphate should have been made so as the concentration remained the same at all times. The same electrodes and equipment should have been used throughout. Also, when weighing, the same electrical balance should have been used as there may have been slight differences between the two balances. This is what could explain the anomaly (“freak” result) in the graph.
I found this investigation very interesting and am looking forward to investigating more of the variables in this experiment, which may or may not affect the mass of copper deposited onto the cathode, such as changing the Current or Temperature variable.