Prediction
I predict that the higher the voltage, the faster the ions in the copper sulphate solution will move and so deposit more copper at the cathode in the given time period. If I decrease the voltage less copper will be deposited. Below is some scientific theory to back up my prediction.
Scientific Theory
Electrolysis is the process of decomposing compounds by electrical energy. An element is produced at each electrode. In my experiment the copper sulphate will start to decompose. At the cathode copper will be formed. Oxygen will be formed at the anode. This causes the anode to wear away while the cathode gains mass.
According to Faraday’s second law of electrolysis, “The quantity of a substance that is electrolysed is proportional to the quantity of the electricity used.” It follows that the speed at which a substance is electrolysed is directly proportional to the quantity of electricity used. When you increase the voltage, the current of electrodes speeds up. This speeds up the ions in the solution and so more of the substance is deposited, and in a smaller amount of time. The positively charged copper will be deposited on the cathode and the negatively charged sulphate on the anode.
Plan
Method
First I will weigh the anode. Then I will place both (copper) electrodes (211.2cm³ each) in a beaker, 5cm apart. I will add 200ml of copper sulphate (CuSO4) solution (until 84cm³ of each electrode is covered). I will attach a power pack and an ammeter and set the voltage to 1 volt. I will then simultaneously start a timer and turn on the power. After 1 minute I will turn off the power and remove the negative electrode, use acetone to help dry it then dry it fully in an oven. Next it will be weighed again and it’s current weight taken away from its weight before electrolysis. The result will tell me how much copper has been deposited (in grams) after 1 minute and at 1 volt. I will repeat this whole process but with 2 volts, 3 volts, 4 volts and 5.
Apparatus
D.C. power supply - for providing the power for the experiment.
Ammeter - for measuring the amount of current flowing though the circuit.
Electrodes - Anode (+) and Cathode (-).
Wires - for connecting the apparatus to the power supply.
Beaker - for holding the solution.
Copper sulphate solution.
Acetone - for drying the cathode.
Fair testing
I will do the entire experiment (1-5 volts) three times to ensure that my results are accurate and I have performed a fair test.
Safety
I will wear goggles during the experiment to protect my eyes. At the end of the experiment I will take care to wash my hands thoroughly to be sure that there is no copper sulphate left on them. This is done because I could, for example rub my eyes and since the copper sulphate is an irritant.
Results
(g=Grams)
Experiment 1
Experiment 2
Experiment 3
Average Results
Graph – Average Results
Conclusion
The results tables and the graph show that my prediction was correct – the higher the voltage, the more copper was deposited.
Evaluation
I think that the evidence that was collected was easily sufficient for a firm conclusion to be drawn. If I needed to further test my results I could check it against the results of others who had done the same experiment and used the same variables. The results may have not entirely been accurate – some of the ions would not have “stuck” to the cathode and would have fallen to the bottom of the solution. The surrounding temperature would have changed during the experiment and the electrodes should have been cleaner (they had irremovable “dirt” from other experiments on them) if the experiment was to be entirely accurate. However, I can only see two feasible changes which could have improved the procedure; firstly, I should have kept, or found a way to keep the electrodes more stable as their movement could have affected the results. Secondly, I should have taken a little more care with the electrode during transport as the anomaly below shows:
Anomaly:
At some point in the transportation of the negative electrode which had been electrolysed at five volts
in the third experiment some of the deposited copper seems to have been lost or disturbed; the amount of copper had gone down from when it was tested at four volts. I would have expected to get results of between 0.21 and 0.26 grams.
Bibliography
Visual Chemistry……TONY DEMPSEY
The Essentials of AQA – The Tested Modules……LONSDALE (MARY JAMES)