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I performed an electrolysis of zinc, using two zinc electrodes and a zinc nitrate solution. However, after leaving the electrolysis to run for 30 min, at about 0.4 A and 6 V, I discovered that the cathode gained more mass than the electrode lost. I know that ideally the anode oxidises zinc into zinc ions, and the cathode reduces zinc ions into zinc metal, suggesting that total mass should be conserved.

A key observation I made was that a white substance was observed on the anode, which probably led to the extra weight. Also, bubbles were observed on one of the electrodes. I am not sure which one though.

Edit: My chemistry teacher suggested this could be related to zinc oxide or zinc hydroxide, but I can’t find much information on this.

Comparison of anode and cathode

enter image description here

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  • $\begingroup$ I would guess zinc oxide (or hydroxide), too, since you're presumably making hydrogen and oxygen. Unless something I'm not thinking of is making the bubbles. I guess it could be zinc reacting with water to make just hydrogen? $\endgroup$ – SendersReagent Mar 13 '16 at 22:36
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If the differences between the two electrode masses is small its probably due to excess electrolyte left on it or a measuring error. The white substance is dried up zinc nitrate.

Bubbles should have been observed at both electrodes since that would have been the hydrogen, oxygen and nitrous oxide dissociating form the diluted solution of zinc nitrate.

ie: $$\ce{3e¯ + 4H+ + NO3- -> NO + 2H2O}$$ $$\ce{O2(g) + 4H+(aq) + 4e -> 2 H2O}$$

So I'm pretty sure the bubbles didn't have anything to do with the additional mass gain. $%edit$

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  • $\begingroup$ Thanks. The electrodes are fairly consistently gaining more than they lose, although what you've said does make sense. Several do lose more than they gain, but on average they appear to gain about 0.1 more than they lose. e.g. gain 0.46 and lose 0.34. (for 4o minutes). Accuracy of scale is approximately ± 0.04 grams. It is also possible I just made a mistake somewhere though $\endgroup$ – JAS May 17 '15 at 8:05
  • $\begingroup$ Also, I could only see bubbles on one electrode, but this is just from one trial. I've attached a picture to my post $\endgroup$ – JAS May 17 '15 at 8:06
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The electrolysis of aqueous aluminum nitrate forms $\ce{N2O}$ and other products (including $\ce{OH-}$) along with some chemistry involving radical formation. I would expect similar with zinc nitrate. Note, $\ce{N2O}$ can form intermediate abducts with metal oxide (surface chemistry). Bottom line, be humble in the magnitude of complexity of this seemingly single experiment.

As background on a different forum (Sciencemadness.org) I shown how the net reaction ascribed to the reaction of $\ce{Cu(I)}$ aqueous, $\ce{H+}$ and $\ce{O2}$ could be express as the sum of several individual radical reactions. The associated quoted net cell reaction (which is actually a spontaneous electrochemical reaction as is the related better known reaction based on iron in place of copper) is also misleading, in my opinion, with even ascribed half cell reactions, which while depicting the electrochemical vantage and is certainly informative on energy measures, the mechanics like intermediate reactions and conditions (pH, temperature, ...) affecting the quoted net cell reaction are totally lost. Please do not associate such reductive simplicity with a true understanding of what is actually occurring, even though it is educationally convenient and you don't have to hire a true scientist to teach science.

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