Information gained from previous Experiments
Used to inform this plan
In previous experiments, We found that using the level of voltage as a variable was the most accurate and practical method of achieving a satisfactory result.
We found, that when trying to use the distance between the electrodes as a variable, it was difficult to ensure that the distance between the electrodes remained the same between taking measurements.
It was impractical to use the size of electrodes as a variable, as we did not have access to enough copper to have a good range of different sized electrodes. A good range is vital when carrying out an experiment.
We did not use the molarity as a variable because I found in previous experiments that it had no effect on the rate of electrolysis.
We decided not to use temperature of electrolyte as a variable as it would take too much time to heat or cool the electrolyte.
A process of elimination brought us to the conclusion that the level of voltage would be the most appropriate variable for our experiment.
Predictions
- I predict that pure copper will form on the cathode and that the cathode’s weight will increase. Also, I predict that the anode will dissolve and its weight will decrease.
- I predict that the anode’s weight will decrease in direct proportion to the increase of the cathode’s weight.
- I predict that the greater the voltage, the greater the change in weight over the same period of time (5 minutes). In other words, electrolysis occurs at a faster rate when a greater voltage is passed through the solution.
Hypotheses
1) In the solution there are Copper ions (Cu2+). The positive copper ions are attracted to the negatively charged cathode, because it lacks 2 electrons and at the cathode there is a flow of electrons. When these copper ions (cations- as they are attracted to the cathode) “grab” the electrons they become atoms and then plate the cathode.
The anode dissolves because its atoms give up electrons (which go into the copper anode and round the circuit to the cathode) to form cations in the solution which are then attracted to the cathode as detailed above.
2) Therefore the mass of copper gained by the cathode exactly equals that lost by the anode.
3) Electrolysis occurs at a faster rate when a greater voltage is passed through the electrolyte because, if there is a higher voltage that means that electrons are passing through the circuit at a greater speed, so electrolysis reaction can occur faster and the whole process is speeded up.
Method
Our experiment went ahead as detailed in my plan.
Results Table 1
Results Table 2
1st Results 2nd Results
Results Table 3
Results Table 4
Results Table 5
Results Table 6
Conclusion
As I had predicted, my results show that during the electrolysis of copper sulphate, the weight of the cathode increases and the weight of the anode decreases. I think this because the positively charged copper cations (Cu2+) in the copper sulphate solution are attracted to the negatively charged cathode where there is a flow of electrons. At the anode Copper atoms (Cu) lose two electrons and become cations; the electrons they lose go into the circuit and round to the cathode, therefore the weight of the anode decreases.
Secondly, I found that the anode’s weight decreased in direct proportion (As shown on the graph), to the increase in the cathode’s weight, this was my prediction. I think this is the case because during the reaction no copper cations are released into the atmosphere, this is because they are strongly attracted to the cathode and are not in a gaseous state.
I also found that the greater the voltage, the more copper was plated onto the cathode and the anode dissolved more over a period of 5 minutes. I think this is because when the voltage is greater, electrons are pushed around the circuit at a greater rate, therefore electrons become available at the cathode more frequently so the reaction can take place faster.
Evaluation
All in all I think that my experiment was a success. I planned it carefully (using past experience of preliminary experiments) including detailed predictions and hypotheses. I conducted my experiment accurately and safely as I had planned to. From my experiment I obtained accurate results with a good range and also obtained repeat results. I have drawn several line graphs to illustrate my results. Finally, I have been able to draw several conclusions (above).
Although my experiment was a success, there are some refinements I could make:
- Before weighing the electrodes we dried them off, while doing this, copper atoms may have been rubbed off, therefore causing our results to be slightly inaccurate. This could be avoided by leaving the electrodes to dry naturally. However, due to time restrictions this was not practicable.
- It would be preferable to use 100% pure copper electrodes, and we cannot be sure that this was the case. It is likely that the copper used in our experiment was not pure and therefore this would account for the very slight anomalies in the results. (i.e. direct proportion was approximate)
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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 slight anomalies in the graph.
Despite the fact that my results seem to be quite conclusive, there is not enough evidence to prove my predictions are absolutely plausible. For this to be possible, one would need to repeat the experiment hundreds of times. It would also be necessary for other people to carry out the experiment and achieve the same pattern of results.
I did not find any major anomalies in my results; this can be seen by the uniformity of my graph lines.
