Galvanic cells (1 Viewer)

[crammy90]

in galvanic cell's. (ill use question 5 HSC 2005 as my example)
so the Al oxidises: 2Al-->2Al3+ + 6e-
then the copper reduces 3Cu2+ + 6e- --> 3Cu
my problem: so when this al oxidises, the al3+ ions will move through the salt bridge towards the beaker containing the cu (i.e. the cathode) to prevent polarisation. This will occur as the Cu2+ are being reduced, so the plus charge is lost and so the al3+ come over to replace what is lost during reduction. (Atleast that is my understanding).
QUESTIONS: why is is that if the al3+ come over they are not also reduced.
why is it that as the charge is being lost the sulphate from the copper sulphate doesnt just migrate through the salt bridge over towards the oxidising anode?
and lastly, when the al3+ come over, do they react with the SO4 to form aluminium sulphate :S

as you can see i am a bit confused
this would really be helpful if someone could answer this
ive been asking all year but my teacher doesnt give a good explanation and i cant find it in the text books, they just seem to identify the salt bridge as the medium for ion migration :S

 

[brenton1987]

2Al-->2Al3+ + 6e-
3Cu2+ + 6e- --> 3Cu

QUESTIONS: why is is that if the al3+ come over they are not also reduced.
why is it that as the charge is being lost the sulphate from the copper sulphate doesnt just migrate through the salt bridge over towards the oxidising anode?
and lastly, when the al3+ come over, do they react with the SO4 to form aluminium sulphate

If the aluminium ions were to reduce the overall cell reaction would be:
Al-->Al³⁺ + 3 e^- +1.68
Al³⁺ + 3 e^- --> Al -1.68

Of course each reaction would cancel the other out and the reaction will cease.

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The question does not specifically mention the sulfate anion.
As the reaction proceeds beaker one will become positively charged and beaker two will become negatively charged. Both cations and anions move through the salt bridge from high concentration to low concentration (Law of Diffusion).

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Aluminium sulfate will form but it wont precipitate as it has a solubility of 870 g^.L^-1
Precipitation is not the best thing for a Galvanic cell which is why nitrate is usually used in the electrolyte.