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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Faisal on May 11, 2008, 01:25:57 AM

Title: reaction of Al(OH)3 with OH-
Post by: Faisal on May 11, 2008, 01:25:57 AM

Can anyone tell me why the reaction of Al(OH)₃ with OH^- is Al(OH)₄?I knw tht Al(OH)₃ acts as an acid here so shudn't it give water cuz of neutralization?Thx in advance

Title: Re: reaction of Al(OH)3 with OH-
Post by: Astrokel on May 11, 2008, 04:14:14 AM

Water don't have to be the side product of reaction between a base and an acid because at times, the relative acidity or basicity are being compared.

For example, in the nitration of benzene,

HNO3 acts as a base due to presence of stronger acid H2SO4.

Would you consider HNO3 as a base or an acid? Do you think the reaction gives you water despite HNO3 acts as a base?

The underlying principal in neutralization, is that H+ + OH- --> H2O

So, do you think you have a source of H+ from the reaction you gave?

Title: Re: reaction of Al(OH)3 with OH-
Post by: Borek on May 11, 2008, 04:47:14 AM

Al(OH)₃ + OH^- = Al(OH)₄^-

H₃AlO₃ + OH^- = H₂AlO₃^- + H₂O

or even

H₃AlO₃ = HAlO₂ + H₂O

HAlO₂ + OH^- = AlO₂^- + H₂O

At least to some extent it is a matter of convention. Al³⁺ has a tendency to react with water and OH^- creating very complicated ions. First reaction is a little bit closer to the reality, second helps understand Al amphotericity and is at least partially correct, as aluminates can be isolated by drying out the solution. However, as long as there is enough water, you will not see things like AlO₂^- as they immediately hydrolize.

Title: Re: reaction of Al(OH)3 with OH-
Post by: Faisal on May 11, 2008, 07:28:30 AM

Quote from: Astrokel on May 11, 2008, 04:14:14 AM

Water don't have to be the side product of reaction between a base and an acid because at times, the relative acidity or basicity are being compared.

For example, in the nitration of benzene,

HNO3 acts as a base due to presence of stronger acid H2SO4.

Would you consider HNO3 as a base or an acid? Do you think the reaction gives you water despite HNO3 acts as a base?

The underlying principal in neutralization, is that H+ + OH- --> H2O

So, do you think you have a source of H+ from the reaction you gave?

So if there is no source of H+...neutraization doesn't take place...so in every amphoteric acid with base results in something other than neutralization?
Are there any general rules for amphoteric reactions...there aren't any amphoteric reactions in my book

Title: Re: reaction of Al(OH)3 with OH-
Post by: Astrokel on May 11, 2008, 10:24:21 AM

Hmm, it really depends on your definition of neutralization. Most people define neutralization as a reaction that gives salt and water only. For example, metal carbonates + acid gives you another by product CO2, hence, some wouldn't consider it as a neutralization process.

hmm as for amphoteric reaction, maybe this help, http://en.wikipedia.org/wiki/Amphoteric

;)

Title: Re: reaction of Al(OH)3 with OH-
Post by: Faisal on May 11, 2008, 04:09:41 PM

ohk got it...thank you