[Samoski]

Hi all. First time poster here, so be nice.

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I created an Electrochemical cell (that was to be used for electroplating) out of:

Aluminium Sulphate + Aluminium      or   Al2(SO4)3 (aq) + Al (s)
and
Potassium Permanganate + Carbon   or   KMnO4 (aq) + C (s)

Theoretically, each cell should generate 3.2v of energy. However, as the battery was constructed in a series of two. Therefore the theoretical energy the battery should generate is 6.4V.

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The change in energy potentials can be calculated by:

Products - [Al3+] = 1.0M
Reactant - [MnO4-] = 0.1M

Q = [Products] / [Reactants]
   = [1]5 / [0.1]3
   = 1 / 0.001
   = 1000

Eocell = 3.2 – (0.0591 / 15) x 3
         = 3.2 – 0.01182
         = 3.18818v

So basically the sell should have produced 6.376v..

However, when hooked up to a Voltmeter, I received a reading of just 0.7v!

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The 500ml of 1.0M Aluminium Sulphate solution (split into two beakers for the cell) was created using 333.225gm of Aluminium Sulphate. All of it was able to dissolve.

I am assuming/been told that the cell didn't work to its potential because their was too much "salt" present in the solution ("salt" meaning metal salt I think). This large amount of salt conteracted with the K2SO4 Salt bridge (which was compeletly saturated with a large amount of unsaturated Potassium Sulphate present also - they were in glass salt bridges with cotton balls).

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So basically my question is, can someone help explain the CHEMISTRY behind the significant difference in predicted and actually Voltages?

Cheers,
Sam

[Borek]

Draw your experimental setup and write equations of reactions your are expecting to take place.

[Samoski]

The reactions that should occur are:

Reduction:
MnO4- (aq) + 8H+ + 5e- -> Mn2+ (s) + 4H2O (l)
= 3MnO4- (aq) + 24H+ + 15e- -> 3Mn2+ (s) + 12H2O (l)       (balanced)

Oxidation:
Al (s) -> Al3+ (aq) + 3e-
= 5Al (s) -> 5Al3+ (aq) + 15e-            (balances)

I will try and draw the setup later...but if you read my first post you should be able to get an idea of it. A beaker with KMnO4, a beaker with Al2(SO4)3, another with KMnO4 and another with Al2(SO3)4 (in that order). First beaker has a carbon rod attached to a wire. The 2nd has a piece of aluminium attached to a wire which leads to the 3rd beaker and attaches to another piece of carbon. The last beaker has a piece of aluminium attached to a wire. Salt bridges link: 1st - > 2nd, 2nd-3rd, 3rd-4th.

[Borek]

Permanganate needs highly acidic solution (look at the reaction equation). Al gets passivated in water with Al₂O₃. To remove oxides it is enough to use acidic or basic solution, but then Al will react with the solution directly, without need for oxidizing agent.