Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: roro on November 19, 2017, 04:20:18 PM
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Is it possible that in the experiment we had in class we used electrodes that are not pure
(we used zinc, iron, aluminum and copper). The teacher said nothing about this.
And if they are not pure, may this affect the voltage measures?
We did an experiment and the results are illogical to the theoretical voltage we are calculating.
thank you all :)
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They are definitely not exactly pure (really pure metals cost fortune), which doesn't necessarily mean that's why observed potentials are off.
Hard to comment further not knowing the results you got.
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The problemtic ones were al-cu we used CuCl2 0.1M and AlCl3 0.1M we got E=0.66v the theortical is 2.01v and al-zn we used AlCl3 0.1M and ZnCl2 0.1M we got E=0.34v, the theortical is 0.9v.
Thank you
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These cells have one thing in common.
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Aluminium is a very poor choice for electrodes. Its oxide layer makes erratic voltages. In addition, aluminium gets corroded also without an external current.
Pure aluminium eases the corrosion problem a bit but worsens the effectiveness of the oxide layer.
To my limited knowledge, no battery uses aluminium with an aqueous electrolyte. Aluminium batteries are scarce, I believe still in the research labs, and use exotic electrolytes.
Zn is much easier for the protective oxide layer despite it grows one too. But at commercial Zn batteries, an initial strong discharge current can be necessary to spoil the oxide layer before the cell gives its usual voltage. Possibly an explanation for your unexpected voltage - or not.
More generally, commercial batteries tend to use quite pure metals, like 99.9%. This does not change the output voltage, but it changes everything about the self-discharge, that is, the corrosion without an external current. Sometimes they use alloys for this same goal: decades ago, Zn was alloyed with about 0.1% Hg, which implied that batteries had to be collected to preserve the environment; presently, purer Zn avoids the addition of Hg.