Method and Obtaining Evidence
Diagram
Obtaining evidence
Current (amps) Anode before (g) Anode after
(g) Mass loss
(g) Cathode before (g) Cathode after (g) Mass gain (g)
0.2 1.39 1.36 -0.031.33 1.38 0.05
0.2 1.35 1.31 -0.041.37 1.40 0.03
0.2 average 1.37 1.34 -0.041.35 1.39 0.04
0.40 1.29 1.20 -0.091.40 1.48 0.08
0.40 1.20 1.18 -0.021.47 1.57 0.10
0.40 average 1.25 1.19 -0.062.87 1.35 0.09
0.60 1.01 0.91 -0.100.98 1.11 0.13
0.60 0.98 0.89 -0.090.92 1.06 0.14
0.60 average 1.00 0.90 -0.100.95 1.09 0.14
0.80 0.91 0.75 -0.161.11 1.27 0.16
0.80 0.72 0.57 -0.151.25 1.41 0.16
0.80 average 0.82 0.66 -0.161.18 1.34 0.16
1.00 0.70 0.54 -0.161.20 1.03 0.17
1.00 0.68 0.55 -0.131.18 1.35 0.17
1.00 average 0.69 0.55 -0.151.19 1.19 0.17
The Electrolysis Of Copper Sulphate Solution Using Copper Electrodes
Analysis
There are two straight lines of best fit through the origin , the red one is the mass gained at the cathode, and the pencil one is the mass lost at the anode. The lines are nearly as they should be, which is equal, as the mass lost at the anode should equal the mass gained at the cathode. This is because as explained in the planning, the reaction occurring at the anode,:
Cu(s) (r) Cu2 +(aq)+2e- (oxidation)
during the electrolysis of a copper salt is the reverse of the cathode reaction:
Cu2 +(aq) + 2e- (r) Cu(s) (reduction)
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. 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 This can be explained with the ionic theory, which basically states that the electrons flow away from the cathode, to the anode where the Cu2+ ions take 2 electrons from the negative electrode and become Cu atoms, thus mass loss at cathode = mass gain at the anode. This does support the prediction, as the two lines are at most only 0.018 grams apart, or 10% inaccurate, using the formula difference ¸ theoretical X 100. The other pattern is that the mass change µ current, This is shown by the construction lines on the graph, which show that when the current is 0.2A, the mass lost at the anode is 0.035g, and the mass gained at the cathode is 0.04g, and when the current doubles to 0.4A, the mass change also doubles as the mass lost at the anode is 0.07g, and the mass gained at the cathode is 0.078g. This is because, as explained in the planning section, 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
As t is a constant at 10min, then q µ i. My results support this as the greatest error was only 0.01g, or 12.5%.
Evaluation
There were several sources of error in this experiment as none of the results were 100% accurate. These error could have been caused by the fact that not all the ions "stick" to the anode, and so end up at the bottom of the solution. This happens most at higher levels of current, and causes the mass lost at the cathode to be greater than the mass gained at the anode. Also the temperature of the solution raised at higher currents by 5° C This would cause less ions to turn to copper at the anode, and make the current more, as there is less resistance. The size of the electrodes was also never exactly the same, as they were reused, so the amount of electrolysis differed from experiment to experiment. The separation of the electrodes was a small source of error, as they were not always exactly the same distance apart. The current which was controlled with the rheostat was not always the same, as the amount of copper decreases, so does the resistance, and so the current increases. Other errors could have been caused by the apparatus, such as the ammeter, which is quite old, and may not be perfectly calibrated, and the scales, which only show the mass to 2 decimal places. The rest are cut of with out rounding. Therefore this experiment could have been made more accurate by using lower current values, with the same size and separation of electrodes, controlling the current so that the temperature is constant, and the current more accurately controlled, and using a more accurate ammeter and a balance which rounds the other decimal places. My results showed many inaccuracies, shown by the accuracy bars on the graph (green for anode, and red for cathode). Which show the highest value and the lowest, with the average in the middle. This shows that for the 0.20A reading, the anode difference is 0.01A, and the cathode difference is 0.02A, both very small variations. For the 0.40A reading, the anode difference is 0.07A, a much greater difference, and the cathode variation was smaller, at 0.02A. The 0.60A anode difference was only 0.01A, and the cathode was the same. The 0.80A anode and cathode variation were also 0.01A. The final reading, 1.00A anode difference was 0.03, and the cathode variation was 0A. This nearly fits the pattern of the greatest variation being at the top, except for the 1.00A cathode variation of 0A. This increasing variation is caused primarily by two things, firstly the temperature of the solution increases more at higher current values, so the ions travel faster, and so do not stay on to the anode as well, and secondly the increased current itself has the effect of making less ions sticking to the cathode. The anomalous result for in the 0.40A value for the anode was probably caused by one or both of the crocodile clips touching the solution, so less electrons flow through the copper, and so less are transferred to the cathode.
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.
Factors which will effect the mass change of the electrodes are:
* Temperature
* Concentration
* Distance between electrodes
* Size of electrodes
These factors may alter the resistance of the circuit, so they must be kept constant to keep the experiment a fair test.
Safety
* Copper sulphate solution is poisonous, so must not be taken internally, or come in contact with the eyes.
* Propane is highly flammable, so must be kept away from flame. Damages eyes and skin, so safety glasses must be worn.
Method
Copper sulphate solution is electrolysed using clean copper electrodes which are weighed before and after use. To make sure that copper are dry and clean after use, they are rinsed in distilled water, and then propane. During the electrolysis, the current is controlled and maintained at a constant value by a rheostat in the circuit. Five current values in the range 0-1.25A are used, each for a period of 10 minutes, repeating each value three times to improve the accuracy of the results
