An investigation into the amount of copper deposited on the anode and cathode during electrolysis

It is important to keep the strength of the copper sulphate solution the same throughout because the stronger the solution the more copper ions there will be to take electrons from the cathode, therefore speeding up electrolysis. To keep my test fair I have decided to keep the strength of the solution at 0.5 moles each time.

I have decided to use voltage as my input variable. The voltage is directly proportional to the amount of mass lost and gained at the anode and cathode because with a greater charge, the copper at the anode has to decompose more to give more electrons. I need to decide on a range of measurements. I conducted a preliminary experiment to make sure that my method worked. For this preliminary experiment I had the power pack set at six volts. The experiment was a success and the results were consistent. I concluded from these results that if the power pack was set too low the results would not have enough variation. I need five different voltages so as to create a good and variable set of results. Knowing this I have decided to use the voltages of 4, 6, 8, 10 and 12.

Apart from controlling these variables there are other ways in which I am maintaining that the experiment is fair. I am going to sandpaper the electrodes before using them in order to remove the layer of oxide that could be there. I will also use the same electrodes for each experiment so that there are no variations. I will also keep the power pack the same as if there were any minor faults on it at least they would be constant throughout the experiment.

Method

Upon choosing voltage as my input variable I must now decide how to carry out my experiment. This is how it will be conducted:

I will collect two copper electrodes of roughly equal size, this would be fairer if they were exactly equal but I cannot achieve this level of accuracy due to limited materials. The electrodes will then be sandpapered in order to remove the layer of oxide before weighing them on electronic scales. I will then attach the electrodes to a power pack with cables and crocodile clips. The power pack will be set at DC as if it was on alternating current the experiment would not work because the current would keep changing direction and this would effect which electrode was the anode and which was the cathode. Therefore the electrode would lose mass but then gain it back when the current changed. I will then fill a beaker with 200mls of copper sulphate solution (strength 0.5 moles) and place the electrodes in it. The power pack will then be switched on and left in the solution for ten minutes. After ten minutes I will remove the electrodes from the solution and place ethanol on them in order to take away the water and make them easier to dry. The electrodes will then be blow-dried in order to be sure that I have removed all excess solution. With the electrodes completely dry I will weigh them on the electronic scales, noting the increase/decrease in mass at the anode and cathode. This process shall be carried out fifteen times, three times each with a voltage of 4, 6, 8, 10 and 12, keeping all the other variables the same. Below is a diagram of my experiment

This is the equipment which I shall require:

250ml plastic beaker

A bottle of copper sulphate solution

Two copper electrodes

power pack

pipette

piece of sandpaper

A supply of ethanol

Electronic scales

A hairdryer

2 electrical wires in order to connect up the circuit.

Prediction

From research and previous experience I know that the anode will loose mass and the cathode will gain mass. I believe that the voltage will be directly proportional to the amount of mass gained and lost, as the more voltage there is the more current there will be flowing through the circuit and the faster electrolysis will take place. I predict that as the voltage increases so will the amount of mass lost at the anode and gained at the cathode as the more voltage there is the more current there will be flowing through the circuit and the faster electrolysis will take place.

I am now going to attempt to predict what my graph may look like. I think it will look something like the drawing below, I think this because whatever the anode looses the cathode will gain and so the graph should look something like the one I have predicted.

Obtaining evidence

I collected a good and wide ranging set of results. The following table shows my results

Voltage Cathode (grams) Anode Cathode After Anode After ChangeC Change A

4 116.8 105.9 117.2 105.5 +.4 -.4

116.9 105.8 117.2 105.4 +.4 -.4

116.8 105.8 117.3 105.4 +.5 -.4

6 116.4 108.1 117.1 107.6 +.7 -.5

116.5 108 117.2 107.5 +.7 -.5

116.5 108.1 117.1 107.5 +.6 -.6

8 116.5 107.5 117.3 106.9 +.8 -.6

116.5 107.5 117.2 106.8 +.7 -.7

116.6 107.6 117.3 106.9 +.8 -.7

0 116.9 106.8 118.0 105.8 +1.1 -1

116.9 106.8 118.1 105.8 +1.2 -1

116.9 106.7 118.1 105.8 +1.2 -.9

2 117.0 105.5 118.5 104.3 +1.3 -1.2

17.1 105.4 118.5 104.3 +1.2 -1.1

117.0 105.5 118.4 104.2 +1.4 -1.3

The results were very pleasing as all repeat results were fairly consistent. As the information on this table is quite hard to read at a glance I am now going to create a summary table. On this table I have used the averages from my results.

Voltage (V)

Mass lost at anode (g)

Mass lost at cathode (g)

4

0.43

0.40

6

0.66

0.53

8

0.76

0.66

0

.16

0.96

2

.30

.20

As you can see my results are very accurate and reliable. It is easily noticed that the amount of mass lost at the anode is quite similar to that of that the mass gained at the cathode. This is because the copper lost by the anode is deposited on the cathode. It is also clear that the amount of mass lost/gained increases with the voltage.

The results are very consistent and so it can be said that the equipment was used with precision and now through knowing this fact it can be seen that the results are reliable and firm conclusions can be drawn.

Also throughout the experiment all normal laboratory safety rules were followed to ensure that the experiment was carried out with minimal risk to myself and those around me.

Analysing

My graph clearly shows that the amount of mass lost at the anode is directly proportional to the mass gained at the cathode. I have drawn lines of best fit for both the anode and the cathode in order to make the results clear and easily understandable. It also evident when looking at my graph that my results were very accurate as there were no anomalous results which did not fit in with my line of best fit.

I have not drawn another graph to compare repeat sets of data as all my repeat results were very consistent and so the graph to prove their reliability is not needed as this can already be seen from my result tables.

I have found out in this experiment that when copper sulphate is electrolysed the amount of mass lost at the anode and gained at the cathode increase with the voltage. I have also discovered that the loss from the anode is usually very similar to the gain on the cathode.

The pattern in the results is that as the voltage increases the amount of mass lost and gained by the anode and cathode increases. The other pattern is whatever the anode looses is deposited on the cathode.

The relationship between my input variable (volts) and my output variable (mass) is that as the voltage increases the mass lost and gained increases.

As shown by my results the prediction which I made at the start of this investigation is correct. As I predicted the amount of mass lost and gained did increase with the voltage, this can easily be proved by my results. For 4 volts the anode lost 0.43g and the cathode gained 0.4g. With the power pack set at 12 volts the anode lost 1.3g and the cathode gained 1.2 grams. This clearly shows that the amount of mass lost and gained increased with the voltage. I also predicted that the amount gained by the cathode would be almost equal to the amount of mass lost by the anode. The result that I obtained for 4 volts, the anode lost 0.4g and the cathode gained 0.43g showed this. I believe that if I had had better more accurate equipment the differences would have been exact rather than just very similar.

In conclusion I now know that when a copper sulphate solution is electrolysed the voltage is directly proportional to the mass in that as one increases so does the other. I also know that the amount of mass lost by the anode is the same/very similar to the amount gained at the cathode.

Evaluation

I believe that my results prove that my test was quite fair. However I could have made it fairer by using chemicals as well as sandpaper to clean the electrodes. I could also have measured how much surface area of the electrodes was covered by copper sulphate solution and kept the amount covered permanent throughout the experiment rather than keep it roughly the same each time.

I believe that I used quite accurate equipment however it could be improved. I could of used a stopwatch rather than a clock, but as the time span was 10 minutes I didn't think that a few seconds extra would affect the experiment in any way. I could also have used a beaker with more precise measuring units. The beaker I used had approx values and so the amount of copper sulphate solution put in may have been a few millilitres out for some of the experiments.

I think that my measurements made were very accurate. I can tell this because all the points on my graph fitted in with my line of best fit and I had no anomalous results. I can further prove their accuracy because of the relationship between the amount of mass lost by the anode and gained by the cathode. The pattern was constant and so this further proves their accuracy. I can prove their accuracy even further by the fact that I repeated each result three times. This built in statistical accuracy as by doing each one three times if I had done one wrong or there had been a mistake made it would have been highlighted by the repeated results. By repeating I could also calculate averages which led to increased accuracy.

I also believe that my measurements were very reliable. I can immediately prove this due to the fact that my repeat results were always very similar to each other. For example when investigating 8 volts the anode lost 0.6g the first time then 0.7g and 0.7g in the repeat results. This shows how similar and reliable the repeat measurements were. I also can prove their reliability as if I did this experiment again I am fairly certain that the results would be very similar to the ones I have gained. The fact that I have gained no anomalous results further proves this fact.

If I were to do this experiment again there are certain improvements which I would make. Firstly I would measure the current as well as the voltage so that I could use faradays law to explain and prove my theories and results. I believe that my method was quite good and that it was an accurate and quick way of finding results. If I were to change it any way I would measure the amount of the electrode covered by solution in order to gain greater accuracy of results. I would also use a precise and accurate measuring cylinder rather than one that has approximate values in order to achieve an even higher rate of accuracy.