Electrolysis using copper electrodes

Modification to procedure:

• The theoretical mass of copper to be deposited was calculated to compare the actual results obtained to the theoretical results.
• The current was varied too, while keeping the time and mass and concentration of copper sulphate constant, to see the change in mass deposited at electrodes.

 

Experiment 1: Theoretical calculations

Current = 0.1 Amps

Time = 60 × 15 = 900 seconds

Because: Cu²+  + 2e                    Cu  

Therefore:  2 × 96500 =  1   (moles of copper)

                   

∴                 96500  =  0.5

               900 × 0.1  =   ?

∴         0.5  ×  90     =   0.0004663212435

               96500        

∴         0.0004663212435 × 63.5  =  0.0296113989g

                                            ( the mass of copper that should be deposited at the cathode)

Because: 4 OH-                            O2  + 2H2O + 4e

   Therefore: 4 × 96500 =  1  (moles of oxygen)

                   

∴                 96500  =  0.25

               900 × 0.1  =   ?

∴         0.25  ×  90     =   0.0002331606218 (moles of oxygen liberated)

               96500        

∴         0.0002331606218 × 2(15.99)  =  0.007456476684g

                                            ( the mass of oxygen that should be liberated at the anode)

Because      C   +   O2            CO2

Therefore:

0.0002331606218 moles of oxygen   =   0.0002331606218 moles of carbon used from          

                                                                                                                           electrode

∴         0.0002331606218 × 12  =  0.002797927462g

                                            ( the mass of carbon that should be used up at the anode)

• Total mass lost from the anode should be 0.002797927462g
• Total mass gained at the cathode should be 0.0296113989g

Thus, total change in mass of electrodes should be: + 0.0296113989 – 0.00279792746

                                                                                =  + 0.02681347144g

RESULT for Experiment 1:

   Quite ironically, the electrodes actually lost 0.03g. Even though the electrodes should have gained about 0.027g, they lost 0.03g. It was then noticed that some of the carbon from the electrodes had settled at the bottom of the beaker containing aqueous copper sulphate. The reason identified was inevitable. This setting of particles of carbon at the bottom of the beaker cannot be prevented.

Experiment 2: Theoretical calculations 

Current = 2.5 Amps

Time = 60 × 15 = 900 seconds

Because: Cu²+  + 2e                    Cu  

Therefore:  2 × 96500 =  1   (moles of copper)

                   

∴                 96500  =  0.5

                900 × 2.5  =   ?

∴         0.5  ×  2250     =     0.01165803109  (moles of copper)

               96500        

∴         0.01165803109 × 63.5  =  0.7402849741g

                                            ( the mass of copper that should be deposited at the cathode)

Because: 4 OH-                            O2  + 2H2O + 4e

   Therefore: 4 × 96500 =  1  (moles of oxygen)

                   

∴                 96500  =  0.25

               900 × 2.5  =   ?

∴         0.25  ×  2250     =   0.005829015544 (moles of oxygen liberated)

               96500        

∴         0.005829015544 × 2(15.99)  =  0.1864119171g

                                            ( the mass of oxygen that should be liberated at the anode)

Because      C   +   O2            CO2

Therefore:

0.005829015544 moles of oxygen   =   0.005829015544 moles of carbon used from          

                                                                                                                           electrode

∴          0.005829015544 × 12  =  0.06994818653g

                                            ( the mass of carbon that should be used up at the anode)

• Total mass lost from the anode should be 0.06994818653g
• Total mass gained at the cathode should be 0.7402849741g

Thus, total change in mass of electrodes should be: + 0.7402849741 – 0.06994818653

                                                                               = + 0.6703367876g

RESULT for Experiment 2:

     Experiment 2 proved to be more successful than Experiment 1 as a gain in mass was observed. There was a gain in the mass of the electrodes by 0.34g. This value is not very close to the theoretical mass obtained, however, the fact that it shows a positive change in mass proves that it is in sync with the theory.

Data Table

Table showing the change in mass of electrodes and the % error

Conclusion: Overall, this experiment was not very successful. Some results were obtained; however, none of the reading showed exact similarity to the theoretical mass change and thus produced a high percentage of error. It has, however, been noticed that by increasing the current and getting higher theoretical masses to compare with; a better result can be obtained.

Evaluation: Some of the factors like settling of carbon at the bottom of the beaker were not under our control, however, other factors such as cleaning the electrodes carefully, weighing accurately etc could have been done better to produce more accurate results:

• A more accurate digital balance, giving weights up to 4d.p, should have been used to identify the slightest change in mass.
• The mass of the aqueous copper sulphate before and after the experiment could have been measure to see how much the carbon particles from the electrodes add to the weight of the solution after the mass of oxygen and copper liberated/deposited is subtracted.
• More variations in current could have been used.
• The effect of varying time could have also been seen