It is found experimentally that 96,500 coulombs are required to charge one mole of monovalent ions such as H+, Na+, Ag+, Cl-, and Ho-. This quantity is often known as the Faraday constant and it is defined as the quantity of electricity required to liberate one mole of silver during electrolysis. A faraday is the equivalent of one mole of electrons.
An example of this is:
1 faraday is required to liberate one mole of Na+ ion.
2 Faraday’s are required to liberate one mole of Mg2+ ion.
3 Faraday’s are required to liberate one mole of Al3+ ion.
Another piece of scientific theory we can use to support my prediction is:
Reaction accruing at the anode (+):
Cu Cu2+ + 2e- (oxidation)
Reaction accruing at the cathode:
Cu2+ + 2e- Cu (reduction)
So for every two electrons passing through the circuit, one copper ion should be formed at the anode and one copper ion discharged at the cathode. So for every two electrons, at the cathode one copper ion is discharged, whilst at the anode, one copper ion is formed. This can be explained with the ionic theory, which basically states that the electrons flow away from the cathode, to the anode where the Cu2+ ions take 2 electrons from the negative electrode and become Cu atoms, making mass loss at cathode = mass gain at anode.
Apparatus
To carry out this experiment I am going to need the following equipment:
- Power pack,
- Beaker,
- Ammeter,
- Wires,
- Crocodile clips,
- Two copper electrodes, (an anode (+) and a cathode (-)),
- CuSO solution,
- Stopwatch,
- Goggles,
- Scales,
- Paper towels.
I will use a power pack to supply the power for the experiment. The beaker is to hold the copper sulphate solution. I will use the ammeter to measure the amount of current going through the circuit. The wires are to connect the apparatus to the power pack and the crocodile clips I will attach to the electrodes. Copper sulphate solution for doing the electrolysis experiment. I will use a stopwatch to check how log I have left the electrodes in the solution. I will have to wear goggles as a safety precaution. I will use paper towels to wipe the anode (+) electrode and then weigh it to see how much copper has been lost.
Method
First I set up all the apparatus, as shown in the diagram. Then I will check that both electrodes are clean, when they are clean I will weigh the positive electrode. Once I have done that I will connect the electrodes using the crocodile clips and place them in the copper sulphate solution, I will turn on the power pack and set the current and the stopwatch. Then I will time for five minutes, during this time I will be monitoring the ammeter as the current could fall or rise. After the five minutes are over I will switch off the power pack. Then I will wipe the positive electrode clean and weigh it. I will record the results down and connect the positive electrode as before. I will then do the same but change the current. The currents I will be using are: 0.2, 0.4, 0.6, 0.8, 1,0, and 1.2.
Fair test
To ensure that this experiment is fair I will only change one of the variables. The variables to choose from are:
- Time,
- Current,
- Distance between electrodes,
- Area of electrodes in solution,
- Concentration of CuSO solution,
- Volume of CuSO solution.
I have chosen to vary the current as it is quiet easy to change, I will change it using an ammeter. To ensure that the others stay the same I will always time for five minutes, and I will use the same solution each time. I will also clip the electrodes to the beaker so that they stay the same distance apart.
Diagram
Trial Test
From the trial test I did, I have decided to use the current: 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2. I decided to do these because when I tried to do 1.5 amps the current would only stay at 1.5 for a minute or two and then it would keep changing and was very hard to keep it at 1.5 amps. I have also decided to use 200ml of copper sulphate solution and I will time for five minutes. I have decided that I will weigh the positive electrode because otherwise if I weigh the negative electrode when I am wiping it clean I will wipe away most of the copper gained. Hopefully with the currents I have chosen I will be able to notice a relationship.
Results
Analysis
From my graph I can see that the mass collected is directly proportional to the current. It is a straight line that goes through the origin showing that if the current is doubled so is the mass of copper plated. Proof of this can be seen in the obtained results:
With a current of 0.2a 0.010 grams of copper is plated.
With a current of 0.4a 0.020 grams of copper is plated.
0.020g is exactly double 0.010g. This proves that the prediction I made of, the bigger the current the greater the mass of copper plated and if I double the current the mass of copper plated will also double, is accurate.
It also shows that both of Faraday’s laws are correct:
Faraday’s first law of electrolysis sates:
‘The mass of a substance liberated during electrolysis is proportional to the quantity of electricity passed.’
Faraday’s second law states:
‘When the same amount of electricity is passed through different electrolytes, the amount of different substances deposited or liberated are directly proportional to the equivalent weight of the substances.’
Evaluation
I think that this experiment went very well and was accurate as all my results supported my prediction and followed the line of best fit on the graph. I can tell that my results are accurate because of them being proportional.
I think that my experiment was fair as I kept the same solution throughout the experiment so it stayed the same concentration and volume, the current stayed at the write current and I timed all experiments for five minutes. The only thing I changed from my trial test was the current as I did them too high.
To improve my experiment I would have been the electrodes, which, even after I cleaned them were still a but dirty from previous experiments, so if I were to do this experiment again I think I would either have cleaned them with some stronger cleaning substances rather than just a paper towel or by using a new pair of electrodes.
To investigate this experiment further I could change one of the other variables for example time. I could do exactly the same experiment but changed the time from maybe five minutes to two, and see whether this affected the amount of copper collected. Or I could investigate temperature, to see whether the temperature will affect the time in which electrolysis happens.
