Richard Coppock
How does concentration affect the rate of electrolysis of Copper Sulphate solution?
Aim:
In order to find out what factors affect the rate of electrolysis, we will change the concentration of the solution to find out which produces the highest current.
Background information:
Electrolysis is defined as:
”The breakdown of a chemical substance by electricity.”
The following is the formula for what happens during electrolysis of Copper Sulphate Solution:
2CuSO4 (AQ) + 2H2O (L) 2Cu (S) + O2 (G) + 2H2SO4 (AQ)
Reaction in the solution Copper + Oxygen + Sulphuric Acid
Variables:
The following is a list of variables that could effect the experiment:
Table 1
Along with the above, other factors such as the sunlight on the experiment and the color of the solution could affect the experiment and must be taken into account. These will be referred to in the evaluation.
Prediction:
This experiment shall focus on the concentration of the Copper Sulphate Solution and how it effects the current recorded. I predict that as the concentration increases the electrolysis of the solution will increase also, meaning that a higher current will be produced. This is because as the number of mol’s increases within the solution, the number of ions moving from the anode to cathode (and cathode to anode) will as well.
The ions will be moving between the two poles in order to acquire a full outer shell of electrons. This ...
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Prediction:
This experiment shall focus on the concentration of the Copper Sulphate Solution and how it effects the current recorded. I predict that as the concentration increases the electrolysis of the solution will increase also, meaning that a higher current will be produced. This is because as the number of mol’s increases within the solution, the number of ions moving from the anode to cathode (and cathode to anode) will as well.
The ions will be moving between the two poles in order to acquire a full outer shell of electrons. This is because the copper only needs two more electrons and therefore is desperate to fill its outer shell and moves around quite quickly.
CuSO4 -> Cu (++) + SO4 (-.-)
The positively charged copper ions migrate to the cathode, where each gains two electrons to become copper atoms that are deposited on the cathode.
Cu (++) + 2e(-) ==> Cu
Each copper atom loses two electrons to become copper ions, which go into solution.
O2 Cu
Oxygen from the solution is produced at the anode, and the Copper is generated at the cathode. The reaction is completed.
Plan:
Equipment needed:
- One 150ml beaker with markings showing 20ml, 50m, etc.
- One power supply to convert the power from the mains. Set at 9 volts.
- One set of graphite electrodes (so that they will not react with the solution over the course of the experiment).
- One ammeter – a meter that measures the flow of electrical current in amperes.
- Enough wires to link the electrodes, circuit box and ammeter together
- One thermometer
- One stop-watch
- About 1500ml of Copper Sulphate Solution
For this test, the following list details the actions that should be taken and in what order to provide the appropriate and expected results for the current generated in relation to the concentration of the solution:
- First, the two 7.5cm graphite electrodes (the anode and cathode) are connected up to the current transfer box, through the ammeter.
- Then 6.8cm of the electrodes are held in the Copper Sulphate Solution for 30 seconds. They are in a bracket so that they remain at a constant distance apart.
- During the 30 seconds, the ammeter is closely monitored in order to be aware of any abnormal discrepancies within the results and any huge jumps of current.
- After the 30 seconds is finished, the current will be switched off and the electrodes taken out. The electrodes are then washed in nitric acid.
- Then the results of the ammeter are recorded – the voltage of current in relation to the concentration of the Copper Sulphate Solution
This exact same cycle (the same concentration) is then repeated twice more in order to gather an overall average. After this, a different concentration of solution is added and once again, the above cycle will repeat. In order for the best possible accuracy, we will start off at full concentration and then gradually reduce the concentration by 0.2ml each time. For each experiment, the Copper Sulphate Solution will be reduced by 0.2ml and the water increased by 0.2ml to keep the change in concentration consistent.
After each recording the 7.5cm electrodes will be cleaned in concentrated nitric acid to remove the copper, and then doused in water to remove the nitric acid. The beaker containing the Copper Sulphate Solution is then emptied, washed out with water and refilled. This is to ensure that the beaker is empty of the previous concentrate and the electrodes are free of any copper from the previous experiment, thus providing maximum accuracy of results.
The beaker will not be washed out with nitric acid, as it would be unnecessary as no copper forms within the beaker during the experiment. Also, it would be a waste of nitric acid to wash out the whole of the beaker.
In order to be completely safe when doing the experiment, goggles will be worn at all times and the current will always be turned off when it is not needed. It must also be noted that Copper Sulphate Solution is poisonous, and the nitric acid is also very dangerous, therefore extra care will be taken around these two liquids.
Diagram:
Fig. 1
Results:
The following tables are the results gathered from the experiment. Table 2 is the initial test, and tables 3 and 4 are the ‘backup’ tests that are taken in order to acquire an average in case of any errors in one particular recording.
First experiment:
Table 2
For the following experiments, (table 3 and 4) the color was not recorded, as it looked the same as previously stated in experiment one (table 2).
Second experiment:
Table 3
The yellow highlighted results shown in tables 3 (above) and 4 (below) are where different electrodes were used. (See notes on the experiment)
Third experiment:
Table 4
Average:
Table 5
Graph of Average results with sloping line:
Fig. 2
Graph of Average results, with margin or error and best-fit line:
Fig. 3
Note’s on the experiment:
- As stated, the boxes shown in table 3 and 4 highlighted in yellow were recorded with a different set of electrodes. This was to save cleaning time during the experiment.
- The error range of the thermometer was +/- 0.5ºC.
- The error range of the measurement for the concentration was +/-1ml.
- The circuit box was set up on 9 volts.
- The electrodes were 4.6cm apart and were 6.8cm of the electrodes was held in the solution. Both pairs of electrodes used in the experiment were the same.
- The electrodes weighed 23.25g before the experiment. As they were made from graphite they did not lose any mass during the experiment and therefore weighed the same at all times.
- Apart from the change of electrodes noted, the only other change of equipment within the experiment was the change of wires. Other than that, the same equipment was used throughout the experiment.
Conclusion:
Through the results, (primarily the graph Fig. 2) it can be seen that my initial prediction was correct. As the concentration increases, so does the current. This is due to two things that happen as the electrolysis occurs. First, bubbles form at the anode, meaning that the oxygen gas is being produced and the ions are being transferred. Then copper is produced at the cathode meaning that the ions have successfully transferred and now have a full outer shell of electrons. As the concentration of the solution increases, so do the ions within the solution. This means that more and more copper ions are moving around within the solution and travelling to the cathode to receive two electrons and complete their outer shell.
Also, as shown through the tables of results, (table 2, 3, 4 and 5) the temperature slightly increased during the experiments. This could mean that the electrolysis of Copper Sulphate Solution is an exothermic reaction. This is unlikely; instead, it is more probable that the current flowing through the circuit and the electrodes produced resistance that caused the slight rise in temperature.
From the graph it can be seen that whilst there is no specific formula or ratio for the current generated in relation to the concentration (e.g. the current of 1 mol/dm³ is not double that of the current of 0.5 mol/dm³). The overall incline of the graph is upwards. This indicates that as the concentration increases, so does the current recorded.
Through looking at the gradient of the graph (fig 2), it can be seen that initially it is steeper but then (after the anomaly that occurred at 0.8) the gradient decreases. This may have been effected through the different electrodes that were used twice in the concentrations of 0.9 and 1 (shown in tables 3 & 4) although other factors may have had an influence as well.
If the solution were to be left for excessive periods of time, then eventually the current would reach zero. This is due to all the molecules within the solution getting their outer shells and either turning into copper, oxygen or Sulphuric acid. As there would no longer be any movement of the molecules, no energy would be transferred. This is unlikely to be a cause of the drop in the current generated at 0.9 and 1 concentrates however, as not enough time was given.
Evaluation:
As can be seen through the graph (Fig. 3), there is an anomaly within the results. This means that there is one particular result that has gone outside the margin of error. This anomaly may have occurred due to several different factors. As stated, different electrodes were used on some of the experiments and this may have caused some different results. Human error is also a present factor that must be taken into account. In addition, different wires were used meaning that there could have possibly been a different resistance on one set of results to another, altering the average. Plus the fact that the experiments were taken on different days to each other, these are the key factors that could have possibly altered the results of the experiment.
In order to solve the errors in any future experiments, I could further investigate around the anomaly by focusing in more detail on the different concentrations. By using 0.725, 0.75, 0.775 (mol/dm³) and so on, I could go into more detail around the anomaly and in turn, ensure it is erased. Another method that could be used would be to create the solution by myself from scratch, instead of using pre-made Copper Sulphate Solution.
Also, for further investigation I could have weighed the electrodes directly after the experiment, to ensure that the only mass gained was through the extra copper.
In addition the final weight of the electrodes could be measured at the end of each experiment, to attempt to determine the weight of copper deposited.
Also, I could change the voltage on the power supply instead of keeping it at a constant 9 volts. This would give a more varied set of results that I could gather the average of. Finally, I to ensure better results I could try and use the exact same wires and the same electrodes along with keeping up with all the constants that are already in the experiment (such as the size of the beaker).
Overall, the best-fit line (shown in Fig. 3) shows that this experiment was a success. The results indicate an upward trend between the concentration of the Copper Sulphate Solution and the Current recorded. Therefore showing that this experiment was a success. It is likely that even if this experiment was to be repeated the results would be similar and the best-fit line generated would be very similar, if not identical due to the fact that the anomaly shown is only just outside the margin of error.
Through this experiment, it has been determined that an increase in the density of the concentration of Copper Sulphate Solution results in a higher rate of electrolysis, or an increase in the recorded current.