To investigate the effect of the copper anode after electrolysing it in Copper Sulphate solution for certain periods of time
ELECTROLYSIS OF COPPER
Aim: To investigate the effect of the copper anode after electrolysing it in Copper Sulphate solution for certain periods of time.
Planning:
Introduction: Electrolysis is the process of splitting up compounds or substances by passing an electric current through it. The substance being electrolysed is called an electrolyte. An electrolyte is a substance that cannot conduct when solid but can when molten or dissolved. In electrolysis, various equipment are involved in order to make the experiment successful. First of all, a power pack is needed. This acts as a pump to push electrons through the circuit i.e.) it provides energy. There are two electrodes involved. An electrode allows electrons to enter or leave a solution. It completes an electrical circuit. One electrode is called the Cathode, and one the Anode. The cathode is the negative electrode thus attracting the positive ions called cations. Electrons are drawn TOWARDS it. The anode on the other hand is the positive electrode thus attracting the negative ions called anions. Electrons are pulled AWAY from the anode.
Here is a diagram showing the set up of this experiment:
In this experiment there are several ions in solution. Copper 2+, Sulphate 2-, Hydrogen- and Hydroxide- ions. My electrodes will both be copper. Copper, being extremely low down in the reactivity series is discharged at the cathode before all ions except silver. So in this experiment copper will be discharged. Since the anode will be copper as well, both the copper in the solution and from the metal will be attracted towards the negatively charged cathode. "During this process, the impure anode loses mass because the copper atoms lose electrons and become copper ions.
Copper atoms copper ions + electrons
Cu (s) Cu 2+ + 2e-
The electrons released at the anode travel around the external circuit to the cathode. There the electrons are passed onto the copper ions from the CuSo4 solution and the copper is deposited on the cathode.
Copper ions + electrons Copper atoms
Cu 2+ + 2e- Cu (s)"
(GCSE Chemistry: B. Earl & L.D.R. Wilford)
Factors that affect the experiment:
Theory says that the mass lost at the anode is the mass gained at the cathode. I have proved this theory in my preliminary experiments.
The amount of time that the electrodes remain in the solution while being electrolysed makes a significant difference to the mass lost at the anode and the mass gained at the cathode. The current also affects the mass lost at the anode. Increasing the current increases the amount of electrons passing through the circuit. Voltage is directly proportional to current. You may either keep the current constant and vary the time or you could vary the time and keep the current stable. The concentration of the copper sulphate solution also affects the experiment. A few other things that affect the experiment other than the ones I've mentioned are:
* The volume of a solution
* Distance between the electrodes
* Mass of electrodes
Prediction:
I predict that the longer amount of time that the anode stays in solution the more mass it will lose.
Fair Test:
In order to make this experiment a fair test, there are various factors that need to be kept constant as accurately as possible. The current that I will be using is going to be 1.5A. This current is allowed to drop or rise by 0.1A this way you can make sure that your results ...
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* The volume of a solution
* Distance between the electrodes
* Mass of electrodes
Prediction:
I predict that the longer amount of time that the anode stays in solution the more mass it will lose.
Fair Test:
In order to make this experiment a fair test, there are various factors that need to be kept constant as accurately as possible. The current that I will be using is going to be 1.5A. This current is allowed to drop or rise by 0.1A this way you can make sure that your results are accurate otherwise, the results you will get won't be very useful. It is important to obtain accurate results in order to see a pattern in the graphs that will be drawn in the conclusion. One way in which you can monitor the current is by holding the electrodes with your hands and adjusting the distance between them until you get the accurate current. This is an effective way because it is easy and gives you the right results. Or you may find some other way to hold them. Another effective way to do this could be by holding the electrodes with retort stands and clamps. This keeps the electrodes at the same distance apart throughout the whole experiment therefore it will keep the current constant. You must use the same copper sulphate solution throughout the experiment. It is advisable to sandpaper the copper before using it to scrape off the tarnish on the surface of the copper to enable all the copper that comes off the anode to form on the cathode.
If these factors remain constant throughout the experiment, then I can ensure that I have accurate results.
Preliminary experiments:
In the very beginning, I was going to use aluminium electrodes in sulphuric acid and see whether the oxide layer that would form on the anode after electrolysing it would cause a significant increase in mass. After experimenting on this I realized first of all, that I couldn't dry the anode with a paper towel because the oxide layer would come off so I would have to wait till the anode dried to weigh it. Second of all, after weighing it, the difference in mass was something like 0.01 so there wasn't really a point in doing this because the points on the graph would have been plotted very close together and to see a clear pattern wouldn't be very easy, so I changed my whole experiment. I decided to purify copper using copper electrodes in copper sulphate solution. This was very effective but only after taking quite a while to figure out ranges and times. In the experiment shown in the appendix, the range of time that the copper was left in solution was from 1-5 minutes. This range was far to small to actually see a significant difference in weight so I decided to double it. I had the most problem with trying to figure out a current. At first I had tried out currents that were very small because I didn't know what to expect, so I was trying out currents like 0.3, 0.4 and 0.6. after a while I kind of realized that it wasn't really working because the decrease in mass was very small and it didn't really make a significant difference so I decided to increase it. The problem was that I had increased it too much and so the results that I was obtaining weren't stable and therefore not very reliable. In the end I tried out 1.5A of current. After electrolyzing the copper for one minute, there was a difference of 0.02 so I figured with a range of ten minutes, on average, the difference would be 0.20. this was a good and reliable range.
Another thing about this experiment was that it was good because I could actually wipe the anode and remove any of the tarnish or bits that were coming off it, so weighing it was quick and easy.
I think that I finally have everything set and my ranges and current are reliable and will give me accurate results.
Final Method:
Apparatus:
* 2 copper electrodes cut out from a copper sheet.
* Weighing scale/ balance
* 3 wires
* 1 power pack
* 1 ammeter
* one 250ml beaker
* Copper sulphate solution
* 2 crocodile clips
stop watch/clock
Diagram:
Method:
Weigh the anode and record weight.
2 Set up the circuit as shown above.
3 Fill the beaker with 150ml of copper sulphate.
4 Get someone to hold the electrodes while you turn the power pack and stop clock on. adjust the electrodes until you get the current to 1.5A (this can also be done by increasing the voltage on the power pack)
5 Wait for one minute and then turn the power pack off.
6 Dry the anode with a paper towel and ensure that all the tarnish comes off.
7 Weigh the anode and record it. Write down the weight difference of the anode before electrolysing and after electrolysing.
8 Sand paper the cathode to scrape away the layer of copper added during electrolysis.
9 Repeat the experiment using the same anode. (you may choose to change it, but if so you must weigh it at the beginning of each experiment).
0 Repeat the experiment 4 more times but for the next one keep it in for 2 and a half minutes and the for 5 minutes, then 7 and a half minutes, then 10 minutes.
1 All the experiments must be repeated at LEAST 3 times in order to obtain accurate results.
Safety points:
* You are dealing with electricity here so be careful not to touch any of the equipment with wet hands otherwise you may get electrocuted.
* Be careful with the electrodes don't ever let them touch while the power pack is on because you might short circuit the power pack.
Results (obtaining evidence):
As you can see from this table, only the first set of results has the copper remaining in solution for 12:00 minutes, this is because I wanted to try out what would happen at that length of time. After trying this, the mass lost was enormous and I realized that there was a significant structural change in the copper, bits of copper were coming off. This is why the rest of the experiments are left at 10:00 minutes. The beaker was also getting hot. This was because of the amount of electricity passing through the circuit.
Analysing evidence and drawing conclusions:
'I predict that the longer amount of time that the anode stays in solution the more mass it will lose' was my prediction at the very start of this write-up. It so happens that I have proved my prediction right. It turns out that the copper anode does lose more mass the longer that it stays in solution.
The shorter the time in which the copper is being electrolysed, the less mass the anode loses. Therefore increasing the length of time that the copper stays in solution increases the mass lost at the anode.
I think, judging from this table, that the heavier the anode is at the beginning, the more mass it loses because in the third set of results the difference in weight is much greater than those in the first and second set of results.
Here is a graph to show the first set of results:
In the first graph, the points are not very close to the line, this suggests that the results are not extremely accurate but still relatively accurate. However, they do show some pattern which is that the change in mass is increasing as the time increases.
This next graph is showing my second set of results.
As you can see, this second set of results are a little more accurate as the points lie much closer to the line than the previous set of results. I think that this might be due to the fact that I was getting used to the equipment and becoming more familiar with it so the experiment worked a little better.
Here is a graph for the third set of results.
Looking at these three graphs, I can clearly see that the change in mass is proportional to the time. There was something that I had noticed, to do with the second and third set of results. I realized that the last point curved slightly and didn't really follow the other points. This is something that could be investigated. I think that this could be due to the fact that after a certain point, because the mass of the anode had decreased so much, it started to decrease slower at 10:00 minutes. At the anode I noticed that there was some fizzing occurring. This was due to the hydroxide ions in solution.
I have found out that the longer the anode remains in solution, the more mass it loses. This is because when the amount of electrons increase there will be more reactions taking place with the copper ions, thus leading to more copper being deposited at the cathode. This also leads to the anode decreasing in mass, mainly because the electrons are leaving it, but also because of the copper ions being deposited at the cathode.
Charge is directly proportional to current x time ( Q = IT ). We need to find the charge in this experiment because when the copper splits up, the electrons are gaining charge. I will now draw a graph using my results to prove this:
Here are the calculations for the charge in my results:
.5 (A) x 60 (secs) = 90 coulombs
.5 (A) x 150 (secs) = 225 coulombs
.5 (A) x 300 (secs) = 450 coulombs
.5 (A) x 450 (secs) = 675 coulombs
.5 (A) x 600 (secs) = 900 coulombs
I will plot the graph of charge in coulombs against time in seconds.
My times are 60, 150, 300, 450, 600 seconds.
Michael Faraday, an English scientist, studied the reactions which take place at the electrodes of electrolytic cells. He recognized that the mass of an element discharged at an electrode is proportional to the amount of electric charge passed through the electrode. He also found out that if the same amount of electric charge is passed through several electrodes the mass of the element discharged at each will be directly proportional to both
a) the atomic mass of the electrode
b) the number of moles of electrons required to discharge one mole of the element from whatever material is being discharged at the electrode.
This is how charge fits into this whole experiment and that's why I drew the graph above showing that Q is proportional to T and that the gradient represents the constant, which is what I investigated, the current.
Evaluation:
Looking at my experiment overall, I would say that it was very successful. Although the first graph that I drew wasn't very accurate, I think that the rest of the repetitions were pretty accurate. The reason my results were accurate was because I had repeated the experiment 3 times and so spotting any anomalies would have been much easier. There were no anomalies in this experiment. The fact that the factors affecting the experiment had remained constant also contributed to the fact that my results were reliable.
Hana Holdijk Chemistry SC1 January 2001
Electrolysis
