Copper ions then move towards 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 weight. At the anode the hydroxide ions in the solution are also decomposed and go to the anode. They each give up 4 electrons to become oxygen and water atoms in the solution:
4OH – 2H2O + O2 + 4e-
The oxygen bubbles off and is released as gas. The diagram below shows a more detailed picture of the electrolysis process and procedure.
The experiment requires energy to make it work, therefore a large current is needed to be passed through the circuit. The current is the energy that is contained in the electricity from the power supply. The current acts as a pump which pumps the charged electrons around the circuit.
Each electron has a charge of:
1.6 * 10 -9
In one mole of a substance or compound there is 6 * 10 23 atoms. Therefore charge in one mole of electrons will be:
(1.6 * 10 -9) * (6 * 10 23) = 9.6 * 10 4 or 96000 coulombs.
96000 Coulombs are equal to one faraday.
Therefore the formula for calculating the actual mass lost is:
Current * time in seconds * RAM
Charge of ions * 96000
Aim:
The experiment I carried out aimed to monitor the quantity of copper meta deposited during the electrolysis of Copper Sulphate Solution (CuSo4) using copper electrodes, when certain variables were changed. It was considered that the following factors could affect the deposition of Copper metal on the cathode.
- Time
- Current
- Temperature
- Concentration of Solution
- Quantity of Solution
- Size of Electrodes
- Distance between Electrodes
- The surface of the Electrodes
The time factor was chosen because it is an easy quantity to measure and record in the laboratory whilst at the same time maintaining the other variables at a constant level. The other factors could be doubling the charge.
Prediction:
It is possible to predict that the relationship will be directly proportional between the time the current flows and the mass of Copper deposited and withdrew from the two electrodes. I can therefore predict that if I double the time of the experiment, I will therefore be doubling the charge, and hence double the amount of copper deposited and withdrawn.
Hypothesis:
It is known that by passing a constant electric current through an aqueous copper sulphate solution that the passage of ions through this solution results in copper atoms being dissolved into the solution from the anode while positive copper ions (known as cations) are being discharged at the cathode. Normally anions are discharged at the anode.
My Prediction can be supported by both of Faraday’s Laws.
Faraday’s First Law of electrolysis states that:
“The mass of any element deposited during electrolysis is directly proportional to the number of coulombs of electricity passed (the current in our case).”
Faraday’s Second Law of electrolysis states that:
“The mass of an element deposited by one faraday of electricity is equal to the atomic mass in grams of the element divided by the number of electrons required to discharge one ion of the element.”
Apparatus:
- D.C Power Supply
- Ammeter
- Copper Electrodes
- Circuit Wire
- Crocodile Clips
- Beaker
- Copper Sulphate Electrolyte
- Top pan balance
Variables:
Independent: Time of current flowing through circuit
Dependant: Mass of electrodes
Control: Current
Initial Size/Mass of electrodes
Amount of electrolyte used
Temperature
Method:
Measurements used: 0min, 5min, 10min, 15min, 20min, and 25min.
Readings used: weight of electrodes
Current: 5v
Test Repeated: 2 readings per strip (2 strips were used).
The electrolyte (50cm3 of Copper Sulphate Solution) was poured into a small beaker. The 2 copper electrodes were thoroughly cleaned. The electrodes were weighed, their masses recorded and then they were placed into the beaker containing the electrolyte.
The surrounding temperature was kept roughly constant, and the current was constant as well.
I used 2 electrodes, Strip A which was positively charged, and Strip B which was negatively charged. Both of the electrodes were connected to the power pack and ammeter, and a steady current was passed through the circuit. The experiment was stopped at definite times (i.e. 5, 10, 15, 20, and 25 minutes).
At these times the current was switched off and both electrodes were removed from the solution. They were then dried, and once dry both electrodes were carefully weighed and their subsequent masses recorded.
A table of results was drawn up as showed below.
Results:
I was able to obtain reasonable good results... showing a trend which backed up my prediction and proved the hypothesis…
I was also able to draw up graphs of my results, so that I would be able to identify the trends in my results easily…
Analysis/Conclusion:
From the results obtained and the graphs, it is clear that the mass of copper deposited at the cathode is directly proportional to the current flowing through the circuit, hence showing that the mass of copper is directly proportional to time. However during the experiment we realized that it was extremely hard to clean the cathode, because as we wiped it and washed it clean pieces of the copper plating was coming off, however the washing process was important as it removed the remaining crystals from the experiment. The results would have been better if the experiment was left on for longer.
The anode can be measured more accurately since the copper does not drop off when cleaned and dried.
This showed that my results were correct, and I was able to base a strong conclusion on my results, which backed up both of Faraday’s laws, my hypothesis, and proved my prediction to be correct, being that I double the time of the experiment, I will also double the charge, and the weight gained or lost on the electrodes, therefore showing that all three are in proportion.
Safety:
There were certain precautions that had to be taken during the experiment because I was dealing with electricity, water, and reactive metals.
- The electricity is dangerous therefore we had to be careful while connecting the apparatus. The power supply was always checked to make sure it was turned off.
- Too much electricity would have been dangerous, because it may result in the apparatus defecting. Therefore we used lower voltages for the current.
- Safety goggles were worn, along with protective gloves and lab coats.
Evaluation:
Although this was a successful experiment, and all went well obtaining excellent results, there were some factors of the experiment, which could have been improved to make it even more successful.
One of these factors could have been the electrodes, which even after a good clean were still quite dirty and obviously had irremovable substances from previous experiments still attached to them.
However this was a problem, because either the electrodes were not cleaned well enough, or if they were cleaned too much parts of the newly attached copper started to fall off and so it was hard to clean it enough to remove all of the previous substances on them.
This problem only affected the cathode as this gained the mess, for the anode it was simple to clean the electrode very thoroughly because there was no loose parts to fall off if it were rubbed and cleaned vigorously.
If I was to repeat this experiment for a second time, in need of greater accuracy it would be imperative to have a new pair of electrodes which have never been used before, so that it would be cleaner than the present ones being used, and more mass would have been gained and lost due to the fact that they are being used in an electrolysis reaction for this first time.
Another factor which may of 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 Copper Sulphate Solution being used as electrolyte 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 electrodes, the same electrical top-pan balance should have been used as there may have been slight differences between the two balances. This is what could explain any anomalous results.
In future experiments I would like to investigate and go further into electrolysis, and look at and test other factors including the current, temperature, concentration, quantities and sizes, and distances between electrodes that affect the rate of electrolysis.
An interesting test would be the test for different currents, and for different temperatures, as these are fairly simple and can be conducted easily in my school laboratory. This would allow me to investigate further into electrolysis, and be able to identify the proportions and similarities that the rate of electrolysis depends on.
I found the investigation to be very interesting, and I am looking forward to investigating the other variables and factors for the experiment.