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Old Nov 28th 2008, 07:21 PM   #1
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Voltaic cell

I know this is a chemistry question, but I no a chemistry forum.

In an experiment, I found that voltage of Cu with Al is the lowest compared to Cu with Zn/Mg/Fe, but the $\displaystyle E^{o}$ of Al is more -ve than Zn,Fe. The lowest voltage should be Cu with Fe.
My lecture says that it is right, Al is a special case. She ask me to find it out. I had find all my reference books, Oxford Fajar, Pearson, McGraw Hill..., but I still can't get the answer. Help! Urgent!
Thanks.
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Last edited by topsquark; Apr 11th 2018 at 11:37 PM.
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Old Nov 29th 2008, 08:24 PM   #2
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Originally Posted by SengNee View Post
I know this is a chemistry question, but I no a chemistry forum.

In an experiment, I found that voltage of Cu with Al is the lowest compared to Cu with Zn/Mg/Fe, but the $\displaystyle E^{o}_{red}$ of Al is more -ve than Zn,Fe. The lowest voltage should be Cu with Fe.
My lecture says that it is right, Al is a special case. She ask me to find it out. I had find all my reference books, Oxford Fajar, Pearson, McGraw Hill..., but I still can't get the answer. Help! Urgent!
Thanks.
The reason behind is that aluminium forms aluminium oxide readily with oxygen. This layer is quite unreactive. Searching for the electronegativity series, aluminium should be at a higher position (i.e. a stronger reduing agent) than Fe. As a result the potential difference created should be greatest for aluminium amid all metals you tested
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Old Apr 11th 2018, 09:12 PM   #3
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Originally Posted by SengNee View Post
I know this is a chemistry question, but I no a chemistry forum.
There is one if you click at the "Chemistry Help" button under the banner of this site. It is being ran by the one who runs this forum, I think.
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Old Apr 12th 2018, 03:40 AM   #4
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Your question is not very clear , but this is the chart you want ....



The potential between any two metals is the difference found in this chart

Al is -1.66 ...Cu is + 0.16 the difference is 1.82V

Notice the most electronegative metal is Lithium , this explains the dominance of lithium batteries , they give high voltage , and hence high power.
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Old Apr 12th 2018, 03:25 PM   #5
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I think Werehk probably has the correct idea.
The table from Oz supports your original supposition that aluminium should give the higher potential.
But preparing and maintaining a "clean" aluminium surface is very difficult.
Any oxygen present will instantly react with the surface to create a thin non-conductive layer.
This (very) thin layer of aluminium oxide prevents further oxygen getting to the bulk of the aluminium, so preventing further oxidation.
Aluminium is actually very reactive, but aluminium oxide is very unreactive.
It is this layer of aluminium oxide that gives aluminium its excellent resistance to corrosion.
(see <wikipedia: passivation>)
Aluminium oxide is also non-conductive, which will therefore reduce the effectiveness of your voltaic cells.
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Last edited by Woody; Apr 12th 2018 at 03:27 PM.
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