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=IT
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.
I did some preliminary work to see which current values, and for how long to time. The results of this are in the tables below:
Electrode-13V Mass before (g) Mass after (g) Mass change (g)
Anode 1.38 1.30 -0.08
Cathode 1.35 1.65 +0.30This was done for 10 minutes. The mass lost at the anode should equal the mass gained at the cathode, which this doesn't, it has a percentage inaccuracy of 0.22¸ .30x100= 73% which is very inaccurate, This may be due to the voltage being too high, so the copper does not all transfer properly, but lies on the bottom of the beaker, therefore a lower voltage must be used, as in the table below:
Electrode 3V Mass before (g) Mass after (g) Mass change (g)
Anode 1.42 1.35 -0.07
Cathode 1.16 1.21 +0.05This was also one for ten minutes, and shows much more accurate results, as the percentage inaccuracy is only 0.02¸ 0.07x100=29%, which is still inaccurate, but is a lot better . This could be due to the voltage value being to low, so I will take a range of 5 results from 4V to 12V at 2V intervals. Each electrolysis will last 10 minutes, and each will be repeated twice so that a more accurate average can be taken.
Variables
* Temperature of the electrolyte
* The concentration of the electrolyte
* The separation of the electrodes
* The size of the electrodes
* Voltage
Only the mass or size of the electrodes, and the current are being investigated, therefore in order for this to be a fair test, the other factors must be kept constant. The temperature was monitored during the preliminary results, and the higher the voltage the higher the temperature change, which in the 1V reading was 20º C. There will be a thermometer in the electrolyte so that the temperature can be monitored. The same CuS04 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 experiment to experiment.
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. During the electrolysis, the voltage is controlled and maintained at a constant value. Five voltage values in the range 0-13V are used, each for a period of 10 minutes, repeating each value two times to improve the accuracy of the results.
* scrub copper electrodes with wire wool
* rinse in distilled water
* weigh and record anode and cathode
* put into circuit ,set voltage value, with crocodile clips, making sure the clips are not touching the copper sulphate.
* time for ten minutes
* remove and dry, weigh and record result
Diagram
Obtaining evidence
Anode
Cathode
Averages
Analysis
There are two straight lines of best fit through the origin , the red one is the mass gained at the cathode, and the grey 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) = Cu² +(aq)+2e- (oxidation)
during the electrolysis of a copper salt is the reverse of the cathode reaction:
Cu² +(aq) + 2e- = 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 voltage is 4V, the mass lost at the anode is 0.035g, and the mass gained at the cathode is 0.04g, and when the voltage goes to 6V, the mass change also goes up 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 = IT
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 voltage, 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 voltage by 10° C This would cause less ions to turn to copper at the anode, and make the voltage 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. As the amount of copper decreases the resistance decrease and so the voltage increases. Other errors could have been caused by the apparatus the scales, which only show the mass to 2 decimal places and the power supply which you could not tell if they gave the right voltage you wanted. The rest are cut off with out rounding. Therefore this experiment could have been made more accurate by using lower voltage values, with the same size and separation of electrodes, controlling the voltage so that the temperature is constant, and the voltage more accurately controlled, and using a more accurate balance which rounds the other decimal places and a more accurate power supply. has the effect of making less ions sticking to the cathode. The anomalous result for in the 6V 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 4V to 12V. 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 very accurate power supply, a balance with more digits 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 2V to 24v 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 the electrodes, and the size of the electrodes.