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When an electric current is forced to pass through an electrolyte or electrolyte solution, chemical reactions take place both at the anode and at the cathode. The stoichiometry of these reactions obeys Faraday's laws of electrolysis. However, when several different reactions are possible at an electrode of an electrolytic cell then the process, which actually does take place, will be determined by the potential of the electrode. The reactions, which take place in electrolytic cells, are those reactions, which require the least potential difference between the two electrodes.

Introduction

Principles of electrolysis

When an electric current is forced to pass through an electrolyte or electrolyte solution, chemical reactions take place both at the anode and at the cathode. The stoichiometry of these reactions obeys Faraday's laws of electrolysis. However, when several different reactions are possible at an electrode of an electrolytic cell then the process, which actually does take place, will be determined by the potential of the electrode. The reactions, which take place in electrolytic cells, are those reactions, which require the least potential difference between the two electrodes.

Electrolysis is an electrochemical process by which electrical energy is used to promote chemical reactions that occur at electrodes.

Electrolysis often involves metals that are capable of being reduced at the cathode to metallic atoms. These atoms can then be deposited on whatever is serving as the cathode surface. This kind of electrolysis is known as electroplating, and it is the usual process for producing jewellery that is silver, chromium, platinum, or gold plated objects. Copper plating usually involves a copper salt that serves as an electrolyte. The electrolyte is a solution that allows electrical current to move through it and is essential to any electrolytic process.

If one passes a constant electric current through an aqueous copper sulphate solution the passage of ions through this solution results in copper atoms being dissolved into the solution from the anode, whilst positive copper ions (cations) are discharged at the cathode. Normally, anions are discharged at the anode.

The experiment that I will carry out will be aimed to monitor the quantity of Copper (Cu) metal deposited during the electrolysis of Copper Sulphate solution (CuSo4) using Copper electrodes.

Possible factors affecting the deposition of copper on the cathode, that I can consider could be time, current, temperature, concentration of solution, quantity of solution, size of electrodes, distance between the electrodes, the surface of the electrodes. I have decided to choose current because it is an easy quantity to measure and record, whilst at the same time I will maintain the other variables at a constant level.


Aims and Predictions

Aim

My aim will therefore be to investigate what effect current size has on the amount of copper deposited on the cathode during the electrolysis of copper sulphate solution.

Prediction

I think that the larger the current the more the copper will be deposited on the cathode. Also there will be a greater loss in mass at the anode at the highest current.

Faraday discovered that the amount of chemical change that occurs during electrolysis is directly proportional to the amount of electricity that is passed through an electrolysis cell.

'To deposit one mole of metallic copper requires two moles of electrons. To deposit two moles of copper requires four moles of electrons, and that takes twice as much electricity.'

Scientific reasons to support my prediction

Electrolysis of an Aqueous Copper Sulphate Solution using Copper Electrodes

The electrolysis of an aqueous solution of copper sulphate using copper electrodes (i.e. using active electrodes) results in transfer of copper metal from the anode to the cathode during electrolysis. The copper sulphate is ionised in aqueous solution.

CuSO4 >> Cu + SO4

The positively charged copper ions migrate to the cathode, where each gains two electrons to become copper atoms that are deposited on the cathode.

Cu + 2e >> Cu

At the anode, each copper atom loses two electrons to become copper ions, which go into solution.

Cu >> Cu + 2e

So therefore; Loss in mass at anode = gain in mass at the cathode

The sulphate ion does not take part in the reaction and the concentration of the copper sulphate in solution does not change. The reaction is completed when the anode is completely eaten away.

When copper sulphate solution is electrolysed using copper electrodes the mass of both electrodes changes. The metal will form at the cathode, which will be copper in this case, so therefore the cathode will get heavier, whilst at the Anode, it loses mass.

The electrolytic process was investigated by Michael Faraday who worked out the Laws of Electrolysis.

Faraday's first law of electrolysis states that:

'The mass of a given element liberated during electrolysis is directly proportional to the quantity of electricity consumed during the electrolysis'

A coulomb is a measurement of electric charge. The quantity of electricity flowing through an electrolysis cell is measured in coulombs. It is the product of the current (Amperes) and the time (seconds). If one ampere is passed for one second, the quantity of electricity is said to be 1 coulomb.

Quantity of electricity = current x time

One faraday of current (96,500 coulombs) will deposit one gram equivalent mass of a substance. Two faradays of current will deposit two gram equivalent masses of substance and so on. A gram equivalent mass of a substance is equal to the Formula Mass of the substance divided by the number of moles of electrons passing through a cell. For example, if Cu+2 ion is reduced at a Cathode according to the following half reaction:

Cu + 2e >> Cu

Then the gram equivalent mass of Cu is:

Atomic mass of Cu =

31.75 grams

 

2 equivalent mass

So if one faraday of current is passed through the electrolytic half cell then 31.75 grams of Copper will be deposited. Two faradays of current will deposit two equivalent masses or 2 (31.75). According to the Laws of Electrolysis developed by Michael Faraday:

Weight deposited= Current(amps) x Time(seconds) x Equivalent Mass(grams)
  96,500

Method

Method

To ensure a fair test I will ensure that:

Equipment


Results

Experiment Results

All experiments were done over a three minute time period.

Current (Amps) 1st Anode (g) 2nd Anode (g) Anode (g) 1st Cathode (g) 2nd Cathode (g) Cathode (g)
0.25 2.15 2.14 - 0.01 2.35 2.37 + 0.02
0.50 2.14 2.13 - 0.01 2.37 2.38 + 0.01
0.75 2.14 2.12 - 0.02 2.38 2.42 + 0.04
1.00 2.12 2.09 - 0.03 2.42 2.45 + 0.03
1.25 2.18 2.24 + 0.06 2.37 2.28 - 0.09
1.50 2.24 2.19 - 0.05 2.28 2.32 + 0.04
1.75 2.19 2.14 - 0.05 2.32 2.39

+ 0.07

I expected that the mass of copper to be directly proportional to the current passing through the solution. According to the ionic equation of the reaction at cathode:

Cu (aq) + 2e >> Cu(s)

Every copper ion requires two electrons to become copper metal thus one mole of copper ion plus two moles of electrons give one mole of copper metal. The current is a measure of how many electrons flow through the circuit in a given time, so if the time is kept constant, the current only affects the number of electrons passing. Therefore, the current would, only affect the number of copper atoms and their combined mass. In fact, if the mass of copper required is doubled, the current must be doubled to allow double the number of electrons to pass through in the same amount of time, this giving twice the number of copper atoms as well as twice the overall mass.

I can see from my results that I have obtained from my experiments some evidence that there is a decrease in mass at the anode when the current is increased (at 0.25 Amps there is a loss of 0.01g whilst at 1.75 Amps there is a loss of 0.05g). There is some evidence that the opposite effect has occurred at the cathode with an increase of 0.02g at 0.25 Amps and an increase of 0.07g at 1.75 Amps.

There appears to be an error in the readings of 1.25 Amps as these results show an increase of 0.06 g at the anode and a decrease in mass at the cathode of 0.09g. These results are completely the opposite of what I predicted and expected to find.

From the results obtained and the graphs, the lines of best fit show that the mass of copper deposited at the cathode is directly proportional to the current. However, during the experiment I found that it was extremely difficult to measure the gain of copper on the cathode and the loss of mass at the anode.

The results would have been better if the experiment was left on for longer, say, five minutes, to compensate for any errors. It was found that the mass of copper lost at the anode should be exactly the same as the mass of copper gained at the cathode, since the concentration of Cu2+ ions in the electrolyte does not change. For every copper ion reduced at the cathode, one copper atom must be oxidised at the anode to keep the electrical charge balanced.

This made it possible for the loss of copper at the anode to be used as a part of the results. The loss at anode can be more accurately measured since one cannot alter the mass of the anode significantly if it had been dried properly, since the copper does not drop off.


Conclusion and Evaluation of the Experiment

Conclusion

The results of my experiments show that the current is directly proportional to the mass of copper deposited at the cathode.

Evaluation

I am not sure how much effect time has on the rate of electrolysis, I perhaps should have considered this factor as a way of developing my experiment further. I think that the time allowed for the electrolysis to run would obviously have an effect on the mass of copper deposited, since the deposition of copper is a continuous process, and providing there is a fixed current it happens at a fixed rate. Therefore, the longer the time, the more copper atoms would be allowed to deposit and the bigger the combined mass.

Whilst, I do not think that the concentration of the solution would have any effect on the process, since the concentration of ions do not make it any easier for the electrons to flow. However, at very low concentration, the resistivity of water increases due to the lack of dissolved ions, and this may have had an effect on the current and therefore had an effect on the mass of copper deposited indirectly.

When I left the experiment for the intended three minutes I would find that there would be fluctuations in the current, so that it was possible that there was never a constant supply of electricity. This would have affected the experiment by making greater or fewer electrons to be displaced when the current changed in this way.

The electronic balance may have also made my results more inaccurate by only giving a the measurements to two decimal points.

The hair drier that we used for drying off the copper sulphate solution from the electrodes was not as efficient as I would have expected. After many minutes of drying, remnants of the solution were still be visible on the surface. This excess weight would have pushed up the weight of the electrodes and would have given inaccurate results. Waiting for the hair drier to do its job took considerably longer than I expected and did not leave me enough time to complete as many experiments as I would have liked.

To further increase the efficiency of my experiments I should have after measuring the electrodes been more careful to ensure that all deposited copper was removed before I re-measured the electrodes for the starting weight of the next experiment.

When I had finished all my experiments and came to clear out the pot which had all the copper sulphate solution in, I discovered floating pieces of copper. These had obviously become detached from the anode during my experiments and would have affected that particular experiment when it became detached and the results of all subsequent experiments because copper may have been deposited without electrolysis even taking place.

After each experiment the electrodes were not always put back into the same position which may have affected the results because the ions may have a shorter or longer distance to travel through the solution and therefore more or less copper would be deposited or detached.