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What chemical compounds might be present in drinking water?
What chemical compounds might be present in drinking water?

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4.1 Aqueous chemistry of aluminium

Aluminium is a metal forming an aqueous cation, unlike the non-metal boron found above it in Group 13. For example, hydrates of aluminium sulfate, Al2(SO4)3, can be made by dissolving bauxite, Al2O3.3H2O, in sulfuric acid and evaporating the solution. Dissolving such sulfates in water forms Al3+(aq) (Figure 16):

Al2(SO4)3(s) = 2Al3+(aq) + 3SO42−(aq)
(Equation 36)
Described image
Figure 16 The hexaaquaaluminium(III) complex ion.
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Figure 16 This shows a ball and stick model of the complex. In the centre is a grey sphere with six sticks attached to it to form an octahedral shape. The sticks are coloured grey at the end adjoining this sphere and red at the other end. Attached to the red ends of each stick is a red sphere. Each red sphere is also attached to two sticks coloured red and white. These are arranged in a V-shape. At the white end of each of these sticks is a white sphere. The model can be rotated.

Figure 16 The hexaaquaaluminium(III) complex ion.

  • What is the coordination geometry of aluminium in Figure 16?

  • Al3+(aq) is shorthand for the octahedral coordination complex Al(H2O)63+.

Careful addition of aqueous sodium hydroxide to this solution will first precipitate insoluble aluminium hydroxide:

Al3+(aq) + 3OH−(aq) = Al(OH)3(s)
(Equation 37)

The oxide is produced by filtering and heating the hydroxide:

2Al(OH)3(s) = Al2O3(s) + 3H2O(g)
(Equation 38)

Both oxide and hydroxide are unusual in being amphoteric. Hence they will first dissolve in and neutralise acids:

Al(OH)3(s) + 3H+(aq) = Al3+(aq) + 3H2O(l)
(Equation 39)

  • Watch Video 5 to determine how else an amphoteric hydroxide (or oxide) behaves.

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    Transcript: Video 5 The reaction of aluminium(III) ions with hydroxide ions.

    NARRATOR
    This is an aqueous solution of potassium aluminium sulfate, often referred to as 'alum'. It contains the aluminium three plus ion. If we add dilute sodium hydroxide, we precipitate aluminium hydroxide, an insoluble base used in indigestion tablets. However, in excess alkali, we form the soluble aluminate ion.
    End transcript: Video 5 The reaction of aluminium(III) ions with hydroxide ions.
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    Video 5 The reaction of aluminium(III) ions with hydroxide ions.
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  • It will dissolve in an alkali and will neutralise it.

As excess sodium hydroxide is added to the precipitate that is initially formed in Equation 37, the precipitate dissolves to form the tetrahydroxyaluminate ion:

Al(OH)3(s) + OH−(aq) = [Al(OH)4]−(aq)
(Equation 40)

This shows a close resemblance of Al(OH)3 to Be(OH)2, which is also amphoteric. Note that the bases ammonia and sodium carbonate are not strong enough to cause this dissolution.