Copper(II) sulfate (CuSO4) is the most common , made by the action of on the . The form is a pale green or gray-white powder, while the hydrated form is bright blue. The electrolysis of copper sulphate solution is udeful because it can be used to purify copper. The purer the copper is, the better it is at conducting electricity so the better it is for electrical wires and connections
I have chosen to change the time and my aim in this investigation is to see whether as the time changes if there is a change in the mass added to the cathode.
Hypothesis
I think that the mass of the copper electrode at the cathode will increase because once a current is applied the copper and the sulphate ions are free to move from their ionic bonds so are attracted to the cathode. At the cathode the copper ions gain two electrons to become copper atoms, which is called a reduction reaction. The copper atoms will then cling to the pure copper cathode so it gets thicker. I don’t think that the colour of the solution will change as the copper ions become atoms at the negative electrode the same number of copper atoms become ions at the positive electrode. So the overall number of copper ions in the solution remains unchanged. I can predict the mass of copper that will be added to the cathode and think that it should be proportional to the charge using the principles of Faraday’s law. To calculate the mass added I know that at the cathode the copper ions pick up two electrons.
Cu²+ + 2eˉ Cu (s)
1 mole 2 moles 1 mole
If I was calculating for two minutes, you use the formula Q=IT in which the current for this experiment will be 0.5A and the time will be 120 as 2 minutes is 120 seconds.
Q= 0.5A × 120 = 60 C
Now we can see that the current is 60 C.
1 mole of electrons carry 96500C of charge (in Faraday’s law) and wee need 2 moles as the copper ion is gaining to electrons.
2 × 96500C = 19300C which produces 63.5g of copper.
Then to find 300C for our relevant time you do 60 × 63.5 = 0.02g.
193000
SO after 2 minutes the cathode gains 0.02g.
We can calculate the other masses added by just replacing 120 seconds with the relative time in Q=IT then following the same method.
Here is my results table for predicted results.
By looking on the predicted graph results I also predict that the results of mass gained will be proportional to the current as stated in Faraday’s law.
Diagram
Apparatus list
- 200cm³ copper sulphate
- wires
- variable resistor
- ammeter
- power pack
- 2 copper electrodes
- 2 crocodile clips
- stopwatch
- scales
- sandpaper
Safety Precautions
It is important to conduct the experiment in a safe way so overalls should be worn in case the copper sulphate solution spills onto clothes, goggles should also be worn just in case the solution comes in contact with the eyes. It is also important to wash your hands after handling the solution as it is prone to irritate the skin.
Fair test
To make this a fair test I must make sure that all the variables other than time must be kept the same to keep the results as accurate as possible. To do this I will keep the electrodes the same distance apart through out the experiment so it doesn’t change the distance the ions have to travel making them faster or slower, therefore affecting the mass of the electrodes. The same cathode will be kept throughout the experiment but just cleaned after every repeat set so that the area the copper ions will be attracted to will remain the same. I am going to try and keep the temperature of the solution the same so that the ions don’t gain kinetic energy and vibrate faster to move to the cathode quicker making the mass added higher. I will keep the current (0.5A) and the voltage (3V) of the circuit the same so that the solution doesn’t decompose quicker and move copper ions to the cathode quicker. The volume and concentration of the solution are also going to be kept the same so there are not more molecules in the solution therefore making the final mass of the cathode more because there is more of a chance that more of a reaction will occur.
I’m planning to take five results at intervals of two minutes for each set of data and then do two repeats so that the averages would be more accurate and hide any anomalies. Five results would give us more evidence when plotting the line on the graph so we would do it more accurately. A curve only needs five points so my results are sufficient and I am also doing 0 minutes as a control to show that without the current there is only a slight movement of the molecules which doesn’t affect the mass of the cathode.
To ensure that these results are accurate I am going to keep the other variables constant as referred to in fair test and by using a stopwatch to time as it is more accurate than a clock, only wiping the cathode slightly before weighing it so it doesn’t rub any atoms off. I will also use the equipment to help me because the variable resistor can keep the current constant giving more reliable results, the crocodile clips will hold the electrodes firmly in place and by putting them into a sheet of cardboard we can ensure that the electrodes stay the same distance apart. The scales give a more precise 2dp reading and by using propane to clean between each set of results it ensures that we start from fresh again. This method is a good way of carrying out the investigation because the only way the ionic bonds in the solution can be broken apart; i.e. with a current and by weighing the piece of copper before and after we can find exact gains.
I used the preliminary experiment as a way to find out the right conditions needed and found that 0.5A gave very precise readings that went up steadily. I also found to make it easier we could place a piece of cardboard on top of the beaker with holes in to put the crocodile clips through and this would keep the electrodes the same distance apart. Also by using tape we could tape the wires down to the table so they did not get in the way. It made my plan better and to do it I followed the same method as below.
Method
- Collect equipment and set it up as shown in the diagram above.
- Weigh both the anode and cathode and the anode and record the result.
- Connect them both to the circuit and turn on the circuit using the variable resistor to allow a 0.5A current to flow. Then place the electrodes in the solution and leave the current on for 2 minutes using a stopwatch.
- After the period of time is over dry the electrodes and weigh them both recording the both.
- Re-connect the circuit, put the electrodes in the solution and leave it for 2 more minutes and then record the results again.
- Keep doing this adding 2 minutes each time until you finish 10 minutes recording each set as you go.
- Repeat the whole experiment twice so you have three sets of data, sandpapering the cathode and cleaning it with propene between each set and replacing the anode with a new piece of copper after each set.
To help me I used the Chemistry for Higher Tier text book, Wikipedia, Encarta and google. I also used a preliminary experiment to help set the apparatus so I could familiar myself with it and get the best possible results from them.
Results
Colour before experiment –electric blue
Colour after experiment –electric blue
(Not needed for my analysis and evaluation but was raw data)
See attached sheets for graphs.
Analysis
From my graph and results table of the cathode I can see that as the change in mass at the cathode increases then so does the charge. The graph also shows that the change in mass is also proportional to the charge however it is slightly off because the results are 0.01g out than they should be. The positive correlation is due to the fact that as the copper ions in the solution are broken from their ionic bonds with the current they attract to the cathode because they are positive and opposites attract. Then at the anode the copper atoms lose two electrons to become ions, a reduction reaction.
Cu – 2eˉ Cu²+
The copper ions dissolve into the solution and at the cathode the copper ions fain two electrons to become atoms, a reduction reaction.
Cu²+ + 2eˉ Cu
The copper atoms are then attracted to the cathode and cling to it making it pure copper and thicker, therefore it gains mass. It is almost proportional because it is following Faraday’s law stating that the mass of an element discharged at an electrode is directly proportional to the amount of electrical charge passed through the electrode. Also by observing the colour throughout the experiment I saw that it didn’t change. This was because as the copper ions became atoms at the negative electrode the same number of copper atoms became ions at the positive electrode so the overall number of copper ions in the solution remained the same.
I did not use a line of best fit because from 60C to 300C on the graph the points were a perfect straight line and could be joined from dot to dot and the little curve to 60C shows that the results were slightly abnormal and we can see clearly it isn’t proportional whereas a line of best fit would have been harder to look out.
My results did turn out the way I thought they would and matched my prediction. This is because I predicted that the mass of the copper would increase which both my graph and the results table reinforce. I found that the scientific explanations I thought were correct and also the colour of the solution prediction matched. I predicted the mass that was to be gained went up in 0.02g steps after every two minutes and it matched from 2 – 10 minutes however the first result didn’t match because it went up by 0.03g instead of 0.02g. Then because of this result, my prediction of the mass gained was not proportional to the charge which it should have been according to Faraday’s law. But other than this my prediction matched quite well.
Evaluation
I think that the experiment worked well because it was efficient and accurate, which our results can reflect as they go up in almost equal stages. For this investigation I think that the results were quite accurate even though the graph curved slightly at the beginning. We can see that this is not right because it is supposed to be proportional to the charge so should have been a straight line. However as this is only an experiment at school being 0.01g out is accurate enough however if this was on a national scale it wouldn’t be.
There were no anomalous results in the change in mass for the cathode results however the first result didn’t fit the pattern as it was 0.01g to much compared to the rest of them and what it should have been according to Faraday’s law. This may have happened because it had more time due to a bigger reaction time with the stopwatch, the cathode may not have been dried properly so it had a little bit more weight or the cardboard may not have worked on this occasion and the electrodes were closer together so the copper didn’t have as far to travel making it accumulate quicker.
I think that that we have enough results to draw a conclusion because 5/6 results were accurate which is a substantial proportion and as I took 3 repeat readings which were almost the same a firm conclusion can be drawn. But we could have taken more results given the time, to give a better average reading that would have made a more meaningful conclusion. I think this because then any unordinary or slightly odd
results can be reduced by the other sets of results making it more accurate.
I think that this method was the best way to extract the information because the only way to separate the copper sulphate ionic solution is with a current and if the solution didn’t separate then the ions wouldn’t have been free to build up on the cathode. Also as we were trying to find the weight gained after each interval the best way to do that quickly and efficiently is to weigh it before and then after to find out the difference. However if I was to do this experiment again I would improve the method by making sure that the same ammeter was used each time we tested the different concentrations to make it more accurate, to sandpaper each cathode for the same amount of time to ensure that I have cleaned them all the same and as the electrodes were moving around a lot even with the cardboard I would take the crocodile clips to the edge of the beaker so they stayed in the same position. Other than this the method was good because it fitted what I needed to find out in the aim.
I think that our method gave reliable results and were almost correct because it almost followed Faraday’s law and my predicted results ( going up by 0.02g) as it was almost a straight line. The graph showed a strong, clear pattern so the conclusion was reliable. All the repeats were almost exact to each other so the averages must have been quite reliable. However these results could not always be counted on as correct because just because the pattern was correct it doesn’t mean that every experiment scientists do are correct. They have to redo them several times before they get satisfactory results. If we had better equipment like the professional scientists do or we knew also the factors about the experiment e.g. fair test then our results would be more accurate. Reliable means that the results are dependable whereas accurate means correct results. There is a fine line between them so reliable results are not the same as accurate results.