Test whether the length of time affects the amount of copper deposited on a cathode electrode.

Scientific knowledge

Electrolysis is a process of splitting a compound by passing electric current through it. The substance that is split up is called the electrolyte. The electricity is carried through the electrolyte by ions and is then converted into an atom by a process called discharge, which means loosing charge. In this case the copper is a charged particle, which is why it’s called an ion, it’s written as Cu2+. As these ions are deposited at the cathode electrode the reaction that takes place is Cu2+ + 2-        Cu. At the anode however copper particles are being charged and turning into ions, the ionic equation that takes place at the anode electrode is Cu          Cu2+ + 2e- . the reaction at the anode is just as important as at the cathode because if we measured the anode at every 2 minute intervals we would find that the anode decreases in weight, this is because it is being turned into ions at the anode and then its deposited at the cathode electrode where its then turned back into an atom. My circuit diagram and my 3d diagram show how my circuit will be connected together.

Prediction

I think that at the cathode electrode will increase in mass the longer you leave it in the electrolyte as more copper will be deposited on the cathode. As explained before more atoms will be turned into ions and deposited at the cathode to turn back into atoms. I think that if you plotted the results on a graph there will be a straight line going up suggesting that there is a relationship between the times that you electrolyse for. Using Faradays law I predict that that I will get an average current of around 0.5amps and since I will only electrolyse for  2 minutes, which is 120 seconds I predict that I will get a charge of  60 coulombs, 60 x 12 is 720 coulombs. I times sixty coulombs by 12 because that’s the number of times I will be doing the experiment for. So 1 faraday would make 32grams of  copper, that’s half a mole. Now if I do 720/96500 * 32 I get 0.24 that means that in the 12 minutes that I run the electrolysis for, I predict ( if my current is at 0.5) that I will get an increase of mass by 0.24g.  

Variables

Variables are things that can be varied during the experiment but for fair testing all variables must stay the same for each test. The variables include:

• The distance between the electrodes

I will control this variable by clipping the copper strip to the beaker so that the distance between the electrodes will always stay the same.

• The size of the electrode in the electrolyte

I will control this variable by measuring my electrodes to make sure that they are 5 cm’s in length and the end f the electrode will be in line with the top of the beaker.

This will affect the current flowing through the electrolyte and if the current is more or less the same I will have a more accurate result.

• The amount of current  flowing from the power pack.

Not touching the power knob on the power pack and just operating the on/off switch can solve this problem very easily.

• The temperature of the room.

The room temperature is very important as the room temperature will affect the temperature of the electrolyte further than slow flowing current through it, if the electrolyte heats up, the atoms will expand therefore causing more current to flow through to deposit on the cathode.

To make this experiment a fair test I will aim to have all the variables the same, the only thing that I will change is the time that the current is passing through the electrolyte. To make this experiment accurate I will clip the electrode to the beaker so that I get the same current flowing through to the cathode. To make the experiment reliable, I will repeat the experiment 3 times and if we get results that are fairly close to each other then I will know that I have reliable evidence to support my prediction.

Obtaining evidence

Analysis

From my experiment I have found out that the mass of the cathode changed after 12 minutes of electrolysis. This is because the current was flowing for a longer period of time therefore depositing more copper onto the cathode electrode, this can also be proven with my graph. My graph is not a straight line but a curve, the graph is curving upwards. This tells us that the amount of copper deposited at 6 minutes was 0.05g, and at 12 minutes it was 0.11. so from my graph I can tell that the amount of copper deposited over time isn’t exactly proportional . I think my graph is the shape it is because there are more electrons flowing through the electrolyte. Also I had noticed with some loose particles of copper, that they are attracted to the electrode, even when the power is turned off, when you take the cathode out, the loose copper sticks to the cathode and when you put it back into the electrolyte, it goes off.

I will now try to see if I can prove my prediction correct using Faradays law (Q=IT).

Our average current was at 0.54amps so Q= 0.54 * 720. Q is equal to 388.8 coulombs.

1 Faraday would make 32grams of copper, which is half a mole.

So (388.8 / 96500) * 32 is equal to 0.13 so according to Faradays law, my increase in mass should have been 0.13 grams whereas my result was 0.11grams. So I was only two hundredths of a gram off.

Evaluation

My experiment went well, I was able to prove my predictions correct. All my results were close to the line of best fit on my graph. If I were to do this experiment again, I would measure the mass of the anode electrode as well to show if there is a relationship between the masses of the 2 electrodes. Below is a brief plan for my new investigation.

1. Weigh the cathode electrode and record the weight in a table.
2. Take a 100 cm3  beaker and fill 75 cm3 with the electrolyte, which is copper sulphate.
3. put a multimeter coming directly from the power pack and set to 6 volts.
4. complete the circuit with the multimeter set to be an ammeter.
5. switch the power pack on and simultaneously start the stopwatch.
6. time for exactly 2 minutes and turn the power pack off.
7. take out the cathode electrode and the anode electrode then wipe carefully with the soft tissue paper.
8. weigh both the electrodes and record the results.
9. repeat steps 5-8 six times, so the total time comes to 12 minutes.
10. once the experiment is finished perform again twice but make sure to change the electrodes and get a dry piece of soft tissue paper.