July 08, 2020, 07:20:58 AM
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Topic: Electrolysis of Zinc Sulphate with a change in concentration -> weird results  (Read 330 times)

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Offline fishblipblop

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I did an experiment investigating how a change in the concentration of zinc sulphate in electrolysis affect the zinc collected at the cathode. I used zinc electrodes for the experiment and did it with 0.1,0.2,0.3,0.4,0.5,0.6, 1.0M. I expected the lower concentration, the less products formed. But instead the lower the concentration the higher the amount of zinc produced.

Concentration/M   Initial mass / g   Final mass / g   Change / g   Current / A Percentage error
0.10   0.911   0.986   0.075   0.14    778%
0.20   0.937   1.053   0.116   0.29    555%
0.30   0.928   1.008   0.080   0.33    297%
0.40   0.933   1.029   0.096   0.65    142%
0.50   0.968   0.983   0.114   0.74    152%
0.60   0.893   0.966   0.073   0.63    90%
1.00   1.18       1.229   0.049   0.93    -13%

Would this be because of overpotential or simply high errors and would this have anything to do with the Nernst equation? I saw that cell potential depends on concentration. If we reduce the concentration of the solution, it reduced the equilibrium value and makes the equation more spontaneous and hence more products formed ideally. Does this mean that in electrolysis of zinc sulphate using zinc electrodes if I decrease the concentration of zinc sulphate from 1.0M to 0.5M, I will get more zinc produced at the cathode? This confuses me because I thought that the lower the concentration, the less ions in solution and so less product produced. Thanks!!

Offline Borek

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First things first: what was the theoretical mass of zinc deposited in each case, calculated from the Faraday's law?
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Offline fishblipblop

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Concentration/M   Current/A      Exptal mass   Theoretical mass   
M   A   DA    /g +/– 0.002g   m   Dm
0.10   0.14   0.01   0.075   0.00853799   0.00065729
0.20   0.29   0.01   0.116   0.017685837   0.000708111
0.30   0.33   0.01   0.080   0.020125263   0.000721663
0.40   0.65   0.01   0.096   0.03964067   0.000830082
0.50   0.74   0.01   0.114   0.045129378   0.000860575
0.60   0.63   0.01   0.073   0.038420957   0.000823306
1.0   0.93   0.01   0.049   0.05671665   0.000924949

Offline Borek

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Tables are unreadable, can you try to format them using spaces or the table BBCodes?

Assuming last two columns of numbers are experimental mass and theoretical mass (there are 6 columns of numbers and only 4 headers, so this can be a wrong guess) your results are several orders of magnitude wrong, there is no way to make any sense from the experiment/calculations.
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Offline fishblipblop

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I attached my results (better formatted) with this post.

Also I agree that the results are way off. However, I plotted the percentage error against the concentration of zinc sulphate. There is an exponential correlation to the results. The diagram is also attached.

Offline Borek

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What is DA?

No idea what you are doing but I am almost sure you are not doing what you think you are doing ;)

I mean - there is no way for the current efficiency to be over 100%. In good conditions it can be very close to 100%, but never higher. If it is higher, that means you deal not with electrodeposition, but with some process of which electrodeposition is only a part.

Plus, I doubt you can get high current efficiency in this case, as zinc is highly reactive and will happily react with water, lowering the produced mass. Not to mention water electrolysis that probably can't be entirely eliminated (but can be definitely controlled with pH).
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Offline fishblipblop

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DA is the uncertainty for the ammeter.

I am confused about it myself so I am putting all the uncertainties on the production of zinc hydroxide which trap a large proportion of water even though this does not fully support my data. So even though I dried it with a hairdryer, there is still some water trapped within.

Regardless, thanks for the help.

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