Are Aluminium Ions Actually Acidic?

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    Aluminium Ions
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Discussion Overview

The discussion revolves around the acidity of aluminium ions, particularly in the context of a textbook passage that suggests aluminium ions are acidic. Participants explore the implications of this statement, questioning whether it refers specifically to Al(3+) and discussing the nature of aluminium compounds in relation to acidity.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions how aluminium ions can be considered acidic, specifically asking if Al(3+) is meant to be interpreted as an acid.
  • Another participant suggests that the passage might refer to aluminium compounds such as aluminates or oxides, which can exhibit amphoteric behavior, neutralizing both acids and bases.
  • A different viewpoint proposes that the high charge density of Al(3+) affects water ligands, leading to the formation of a complex that can deprotonate, depending on the concentration of sulfuric acid.
  • Some participants express uncertainty about whether "aluminium ions" specifically refers to Al(3+) or a more complex hydrated form, such as [Al(H2O)n]3+.
  • There is a suggestion that the author may not have intended to imply that Al(3+) can donate a proton, as this would typically not occur outside of nuclear reactions.
  • One participant recommends considering the Lewis definition of acidity, which could provide a broader perspective on the discussion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the interpretation of aluminium ions' acidity, with multiple competing views and uncertainties remaining about the nature of aluminium in this context.

Contextual Notes

The discussion highlights ambiguities in the original passage, including the lack of precision in describing aluminium ions and their behavior in acidic environments. The dependence on definitions of acidity and the specific forms of aluminium present in solution are also noted as potential limitations.

pivoxa15
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In a paragraph in the textbook "When CSIRO scientists were called into investigate, they discovered that the cause was the oxidation of iron pyrites in these area, much of which had been exposed as a result of draining swamps for grazing cattle and the use of floodgates to drain cane fields. This released sulfuric acid, which in turn mobilised the aluminium ions in the clays. These ions are also acidic."

How can aluminium ions be acidic? Is it saying Al(3+) is an acid?
 
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I'm not sure exactly what the passage means, but it's probably just saying that the aluminates (or oxides or hydroxides) in the clay are acidic. Many aluminium compounds are known to be amphoteric (notably the oxide and hydroxide) - they will neutralize acids as well as bases, by complexing with the free H+ or OH- in solution. Others are slightly acidic and some are slightly basic.

And when it says "aluminium ions", I'm not sure it necessarily means Al(3+) but it might. Furthermore, in acidic media the ions in solution are probably more like [Al(H2O)n]3+. In any case, that passage is written too casually (lacking any kind of precision of description) to be taken seriously.
 
i think it implies [Al(H_{2}O)_{6-n}(OH)_{n}]^{3-n}

the way i understood it, the high charge density of the "Al3+" ion dragged in electron density off the water ligands, so the water ligands could be deprotonated to form the above complex (though the number of deprotonations depends on the sulphuric acid concentration) and hydronium ions.
 
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So it is talking about Aluminium when part of a compound and that compound being an acid? I know that Al(3+) cannot donate a proton (hence not an acid) unless its a nuclear reaction which I don't think was on the author's mind.
 
pivoxa15 said:
So it is talking about Aluminium when part of a compound and that compound being an acid? I know that Al(3+) cannot donate a proton (hence not an acid) unless its a nuclear reaction which I don't think was on the author's mind.
Look for Lewis definition of acidity (more general).
 

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