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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Tangera on July 06, 2008, 10:53:46 AM

Title: Why does Aluminium chloride give an acidic solution when dissolved in water?
Post by: Tangera on July 06, 2008, 10:53:46 AM
Hello!

My question is why does aluminium chloride give an acidic solution when dissolved in water? I am aware that AlCl3 is a covalent molecule that ionizes in water into Al3+ and Cl-...does this happen to have anything to do with it giving an acidic solution in water?? My guess is that H3O+ ions may be formed as well, but why then, will H3O+ ions give an acidic solution?

Thank you for helping!
Title: Re: Why does Aluminium chloride give an acidic solution when dissolved in water?
Post by: Borek on July 06, 2008, 11:24:56 AM
What form does the Al3+ cation take in water?
Title: Re: Why does Aluminium chloride give an acidic solution when dissolved in water?
Post by: manyuen_wong on July 08, 2008, 11:29:08 AM
Hello!

My question is why does aluminium chloride give an acidic solution when dissolved in water? I am aware that AlCl3 is a covalent molecule that ionizes in water into Al3+ and Cl-...does this happen to have anything to do with it giving an acidic solution in water?? My guess is that H3O+ ions may be formed as well, but why then, will H3O+ ions give an acidic solution?

Thank you for helping!

Al3+, having a high charge/surface area ratio charge density, polarizes the water ligands and ejects out H+ ions.
As a result of polarization by Al3+, there is a drift of electron density towards Al3+ and this weakens the OH bonds
Title: Re: Why does Aluminium chloride give an acidic solution when dissolved in water?
Post by: cliverlong on July 09, 2008, 04:43:46 PM
Hello!

My question is why does aluminium chloride give an acidic solution when dissolved in water?

I am aware that AlCl3 is a covalent molecule that ionizes in water into Al3+ and Cl-
My textbook, Facer, A2 Chemistry and the following link

http://www.chemguide.co.uk/inorganic/period3/chlorides.html#top

states that anhydrous Aluminium Chloride is hydrolysed by water, through a few intermediates, ending up producing H3O+ and hence the acidity, not the simple ionisation that you have written.

Quote
I am aware that AlCl3 is a covalent molecule

again Chemguide gives more detailed information on the structure of Aluminium (III) Chloride and shows, depending on temperature, the dative bonds, not just covalent bonds, leading to a molecular formula Al2Cl6

Quote
I am aware that AlCl3 is a covalent molecule
Facer states that adding water to hydrated Aluminium Chloride , deprotonates the AlCl3 H2O complex producing H3O+ and hence the acidity.

Clive