Both Al(OH)3 and Al2(SO4)3 are relatively insoluble in pure
water, and may form precipitates.
http://en.wikipedia.org/wiki/Al(OH)3
http://en.wikipedia.org/wiki/Al2(SO4)3
However, as dkcx pointed out, since Al(OH)3 is
amphoteric (since it has both covalent and ionic character, due to
the high charge density of Al3+ ion), it is soluble in (ie. it
can react to form soluble compounds with) both acids and
alkalies.
Which begets the next question : would the electrolyte
solution be acidic or alkali as the electrolysis carries on? There
are a few factors/processess responsible for H+ (or H3O+) and
OH- concentration in the electrolyte :
1) H+ (or H3O+) from H2SO4 (the original electrolyte).
2) OH- from H2O as H+ (or H2O) is reduced at the cathode.
3) H+ (or H3O+) from the hydrolysis of Al3+ cation (high charge
density on Al3+ polarizes electron density of the O-H bond in water
ligands, allowing for ready proton dissociation).
So the pH of the solution at any given time, depends on
several factors. Without further data (including the molarity
of the H2SO4 acid used as the initial electrolyte) and
further calculations (beyond the scope of both 'O' and
'A' levels), it is impossible to say for sure which, or how much of
each, insoluble precipitate, will be present.
This is not a fair questions (at either 'O' or 'A' levels),
and so you shouldn't encounter such questions in your exam.
However, because of its amphoteric nature (ie. it can react with
both acids and alkalis to form soluble compounds). it is a
safer bet to assume, as dkcx did, that the ppt is Al2(SO4)3 rather than
Al(OH)3 (which could also be present in some quantity at some
stage, before being reacted/dissolved away).
And both anpanman and dkcx have partially (but
incompletely) identified the reason why the ppt (whichever it
may be) stops the current from flowing - the buildup of positive
charge at the anode (now coated with a ppt that acts as a physical
barrier) prevents electrons from flowing from the anode to the
electric cell to the cathode.
anpanman said : >>> ppt surrounding
the anode thus causing no more anions to be discharged at anode
<<<. Are you sure anions from solution are discharged? If
so, what is the anion? Try focusing on the (reactive) electrode
instead (the passivation layer of Al2O3 must first be removed
for the Al metal pieces to function as electrodes). And
remember - Red Cat riding An Ox (Reduction at Cathode, at Anode
Oxidation occurs).
dkcx said : >>> the reason the
reaction stop is that the layer of salt acts as a conductor and
prevents further exchange of e- between the metal the solution.
<<< "Acts as a conductor"? Solid precipitates cannot
conduct electricity, as the ions are immobile. "Exchange of e-
between metal and solution"? That takes place at the cathode, not
anode. According to anpanman's question, the ppt coats the anode
(which makes sense since that's where Al3+ ions are generated); and
at the anode, e- is transferred from the metal to the electric cell
to the cathode, not to (any ions in) the solution.