- Discussion:
When doing the experiment, there some precautions taken to minimize the number of possible errors. First of all, the copper foil cathode and anode were rubbed with sand paper to remove copper oxide which was formed by the reaction of copper and atmospheric oxygen on their surface. If the electrodes were not treated like this, the layer of copper oxide would hinder the electrolysis of copper. It would turn out to be the electrolysis of copper oxide in the first stage. So the molar mass of the electrode should be changed to the molar mass of copper oxide.
After the electrolysis and disconnecting the circuit, the copper electrodes were rinsed with water to remove the impurities formed on them. The copper anode should even be cleaned by a strong jet of tap water. This was to remove all the loss in copper which had already turned into copper (II) ions, since part of the copper (II) ions might not be very soluble at that moment.
After that, the copper electrodes were rinsed with propanone and heated under a Bunsen flame. Since propanone can remove grease or oil, this treatment could leash off the sebum which we have left on the electrodes when transferring them. Of course, when rinsing them by propanone, a pairs of forceps was used. After rinsing, the copper electrodes were warmed high on a Bunsen flame to evaporate propanone more quickly. However, they should be placed high above the Bunsen flame because propanone is flammable and a Bunsen flame can oxidize the copper plate, forming copper (II) oxide on their surface, which will finally lead to a heavier weight and smaller Faraday Constant.
The value of the Faraday Constant was calculated to be smaller than the literature one (96500 Cmol-1) due to a number of errors. For instance, it was inevitable that the electricity supply was not exactly 0.19A in the time interval 20 minutes. Sometimes when the set-up was moved a little bit, a slight change in ammeter reading would be observed. This would lead to a change in the quantity of electricity supplied, thus deviating the calculated value of the Faraday Constant.
Moreover, it was assumed that there was only copper and copper(II) ions involving in the electrolysis. However, the truth is different. Hydrogen ions and hydroxide ions would also take part in it, liberating hydrogen gas and oxygen gas at the cathode and the anode respectively. Thus, the calculated gain in weight in copper cathode was smaller than the ideal one. That’s why the gain in weight of the cathode was not taken in account when calculating the Faraday Constant. The loss in weight in anode should always be larger than the gain in weight of the cathode.
On the other hand, during electrolysis, it couldn’t be confirmed that the copper metal deposited around the cathode would firmly attach onto the cathode. Copper metal formed from the copper solution might sink to the bottom of the solution, giving a smaller gain in weight of the cathode. This can explain why the gain in weight of the cathode and the loss in weight in the anode were not the same. So if the gain in weight of the cathode was taken into consideration, the calculated value of the Faraday Constant would be larger.
After finding out the value of the Faraday Constant, the thickness of the metal plating can also be calculated. According to the formulae Density = Mass / Volume and Volume = Thickness * Surface Area, since the density of copper (8920kgm3), the surface area, the mass of copper formed are all known, the thickness of the plating can be known. It is actually calculated by: Mass / Density / Surface area.
Such electrolysis process performed in this experiment is useful in electro-plating. It can plate a certain object by placing it on the cathode, and the metal to be plated onto the object will be put on the anode.
- Conclusion:
The Faraday constant was calculated to be 161000 C mol-1.
