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Aluminium is actually a very reactive metal, but under normal circumstances
aluminium does not react with water because an impermeable protective
layer composed of aluminium oxide (Al2O3)
either forms within seconds or is already in place. This means that
strong nitric acid can be transported in aluminium containers quite
safely.
Solid sodium hydroxide doesn't react with aluminium (sodium is
more reactive than aluminium and is already bonded to the hydroxide
ion), but the layer of aluminum oxide previously formed by passive
corrosion can be dissolved with sodium hydroxide in solution.
Al2O3(s)
+ 2 NaOH(aq) + 3 H2O(l)
→ 2 Na+(aq) + 2 [Al(OH)4]-(aq)
This allows the reaction of aluminium and water to take place,
but until the protective layer has been removed it occurs relatively
slowly. Aluminium hydroxide, Al(OH)3, Alum, is the most
stable form of aluminium in normal conditions.
2 Al(s) + 6 H2O(l)
→ 2 Al(OH)3(s) + 3 H2(g)
Aluminium hydroxide is amphoteric. It dissolves in acid, forming
[Al(H2O)6]3+ (hexaaquaaluminate)
or its hydrolysis products. It also dissolves in strong alkali,
with the OH- ion (in this case from the sodium hydroxide)
forming the soluable [Al(OH)4]- (tetrahydroxoaluminate).
Al(OH)3 + NaOH(aq)
→ Na+(aq) + [Al(OH)4]-(aq)
So with plenty of water the overall reaction is
2 Al(s) + 6 H2O(l)
+ 2 NaOH(aq) → 2 NaAl(OH)4(aq)
+ 3 H2(g)
The aluminium completely dissolves and the water acts here also
as an acid. With less water the aluminium makes a solid.
2 Al(s) + 6 H2O(l)
+ 2 NaOH(aq) → 3 H2(g)
+ 2 NaAl(OHOHOHOH)(aq)
2 Al(s) + 2 H2O(l)
[ + 4 H2O(l) ] + 2 NaOH(aq)
→ 3 H2(g) + 2 NaAlO2(aq)
[ + 4 H2O(l) ]
2 Al(s) + 2 H2O(l)
+ 2 NaOH(aq) → 2 NaAlO2(s)
+ 3 H2(g) |