Copper ions are formed at the cathode:
Cu2+ + 2e- Cu
The anode decomposes forming copper ions:
Cu Cu2+ - 2e-
The result is that the anode wears away while cathode gains mass. When the copper anode decomposes it loses two positively charged copper ions and electrons. Copper ions then move toward the cathode, which has a negative charge. At the cathode, the copper ions each receive 2 electrons and become copper atoms:
Cu2+ + 2e- Cu
The copper atoms collect together on the cathode, which results in it gaining mass. The hydroxide ions in the solution decompose and are attracted to the anode. They each give up 4 electrons to become oxygen atoms:
4OH - 2H2O + O2 + 4e-
Oxygen is formed and bubbles off. The following diagram shows this.
Resistance is the hindrance to the conduction of electricity. When there is a potential difference V across a conductor, this causes a current I to flow. The resistance of the conductor is defined to be the ratio R = V/I.
Prediction – My prediction of the outcome of my main experiment.
My prediction, based on scientific knowledge, is that the stronger the concentration of the solution, the lower the resistance. This is because, the stronger the solution, the easier it is for the electrons and ions to move through the solution to the cathode.
Apparatus – A list of the apparatus I will be using for these experiments.
- Power Pack – D.C. power supply
- Ammeter – For measuring current
- Voltmeter – For measuring the voltage
- Leads - For connecting the apparatus to the Power Pack
- Crocodile Clips – For attaching the power supply to the electrodes
- Copper Electrodes – Anode (+) and Cathode (-)
- Thermometer – To regulate solution temperature
- Beaker - For containing the Copper Sulphate solution
- Measuring Cylinder – For accurate measurement of the solution
- Sand paper – For cleansing of electrodes in between experiments
- Card – For securing the electrodes in the solution
- Scissors – For cutting the card and the electrodes
- Goggles – See “Safety Precautions”
Safety Precautions – A list of precaution which should be taken while conducting experiments.
- Electricity is dangerous is dangerous, and therefore it is important to take care when conducting experiments involving electricity. When the power pack is not in use, ensure that it is switch off, and that it is switched off while setting up or dismantling the apparatus. It is also important to take care not to have to higher voltage or current, which could result in a potentially dangerous situation.
- When handling any chemicals it is always necessary to wear goggles t protect ones sight. If we were using a Bunsen Burner, goggles would also be necessary.
- Precaution may need to be exercised when handling copper, which may be sharp. When rubbing down, with sandpaper be sure to watch fingers.
- Other common sense precautions are always necessary. Do not run in class, which may result in an accident. Tie back loose hair or dangling clothing to reduce a fire risk. Tuck stools under chars while conducting an experiment, so that nothing is in the way.
Method
Electrodes of the same size will be cut out with scissors from sheets of copper. The electrodes will first be rubbed with sand paper, and then swilled under a tap to remove their layer of copper oxide, and any other substances which may be present and would effect electrolysis. Once they have dried, they will be places through a piece of card, the purpose of which will be to keep them precisely 2cm apart. The current and voltage will be set at the required level for the experiment taking place. The solution will then be measured to precisely in the correct proportions, using a measuring tube, into a beaker. The crocodile clips will be played on the end of the card, which will have the electrodes bent around it. The temperature of the solution will be taken (room temperature). As soon as the electrodes are placed into the solution, the power pack will be switched on. N the assumption that everything will be as it should be, which of coarse it will, electrolysis will then begin. Readings of the current and voltage will be taken four times per experiment and at 30-second intervals, i.e. after 30 seconds, 1 minute, 1 ½ minutes and 2 minutes. In the main experiment, each concentration will be repeated three times. In between experiments, the electrodes will go through the same cleaning process as was previously stated. Please take note that the electrodes will slowly loose mass as the experiments go on, but due to it being an in-school experiment, we will be having to re use electrodes. When the temperature is back to room temperature (if it will have changed at all), the electrodes will be placed back into the solution and the process will begin again.
Preliminary
It was necessary to do a preliminary investigation in order to discover the ideal voltage for the experiments. Pure copper sulphate was used, and six one time only results were obtained for the resistance when the power pack was set at 2v, 4v, 6v, 8v, 10v and 12v. The results are as follows.
It was decided that 8v should be used as there was only a difference of 1.9Ω in the top five. 8v is the median number.
Main Experiment
For the main experiment, and voltage of 8v was used, and the concentration of the solution was changed. Four results were taken from each experiment. 5 experiments were done with concentrations of 1/5, 2/5, 3/5, 4/5 and 5/5 = 1. All five were repeated three times and the averages from all were found. Results were taken and Averages calculated to two decimal places. I feel this is the maximum possible accuracy, and is therefore a good accuracy to use. The overall averages were as follows.
None of the results appear to be anomalous, either on the individual tables, or this final Average table. Therefore I will no be doing any repeat results.
Analysing Evidence and Drawing Conclusions
Looking at the graph, which I have drawn, I have deducted hat my results were as accurate as they can have been expected to be. Both lines, for CuSO4 and H20 follow pretty closely the line of best fit. If anything, it is probably the line of best fit, which is incorrect. The graphs are very slightly bent and pointing in opposite directions, which is expects. As one number goes up the other comes down. A small amount of bowing in the graph would be expected to show how line may level off eventually, as the concentration gets to the point where it has no effect on the resistance. What the graph does show is that the stronger the concentration of the copper sulphate, the lower the resistance. This is as I predicted would be. The strong solution is far better at carrying the ions than a weaker one.
Evaluating Evidence
My graphs go to well with the line of best fit to really say that there were anomalous results If it were really necessary to find a fault ten 15.83Ω, with a concentration ratio of 4/5 is not perfect, but as I mentioned before, I feel that there should be some bow in the line. The reason for this may have been a miss calculation, although I did d all calculation twice o make sure, or if it were not a miss calculation, then it must have occurred earlier on, perhaps when I was collecting my data. Ways to avoid such mistakes in the future, could include getting a second opinion on my calculations, which would severe reduce the possibility of a strait miscalculation. Taking more sets of results would have done one of two things, it would have hopefully made it clear that there was a possible anomalous result earlier on in the investigation, or, on the assumption that the extra results would be right, they would help bring the average a little closer to the true number. More scientific circumstances do always help, and mistakes are unavoidable in a classroom environment. There were a fairly large number of possible affecting factors in this experiment, which would therefore have increased the possibility of anomalous results. Taking this into account, I would say that my results were quiet good if they don’t have any, or at the most have one anomalous result.
If was to do this experiment again, I would like to have a calmer environment and more time in which to do the experiment. Having fresh electrodes for every experiment which had been more accurately cut would irradiate any potential problems that we had there. F new ones were not possible, then a more thorough clean process would have increased the accuracy as well. A more accurate device for keeping the electrodes in the correct places would also have been useful. Fresh chopper sulphate for every experiment would also be good, as changes would have occurred to the copper in-between the first and last experiment being done. I attempted to perfect the temperature as much as possible during the experiment, however with an analogue thermometer, that is not always possible. Using a digital thermometer, or even having the experiment in a temperature-controlled environment would have increased the accuracy f the results.
For further extension work I would think that looking into the other affecting factors would be a good way forwards. Once data had been collected on all of the affecting factors, a perfect condition could be created based on the results. It would then e interesting to try electrodes of different metals, to see if electrolysis is at its most efficient with them, the same as it is with copper.