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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: FouRRaW on June 05, 2006, 08:41:09 PM

Title: Weakness of Arrenhius definition
Post by: FouRRaW on June 05, 2006, 08:41:09 PM

I don't really know what this means??

The arrenhius definition of acids and bases did not account for cases such as NH₃ producing basic solutions because it did not account for the reaction with the H₂O molecule.

Could anyone clarify??

Title: Re: Weakness of Arrenhius definition
Post by: wereworm73 on June 05, 2006, 09:42:36 PM

The Arrhenius definition of acids & bases is strictly limited to substances that dissociate in water to form H₃O⁺ or OH^-.  Unfortunately, this overlooks a lot of other substances that can also neutralize acids or bases.  NH₃, for instance, can neutralize acids, but it doesn't have an -OH group so the Arrhenius definition overlooks this one as a base.

Title: Re: Weakness of Arrenhius definition
Post by: hmx9123 on June 05, 2006, 09:43:31 PM

First off, start with what is the Arrhenius definition.  What does it say, explicitly?  Then ask yourself how the reaction of water with NH₃ doesn't fit.  Try it and let us know the results.

As I was typing, wereworm73 posted, so some of this may be moot, but read it and ask the questions anyway.

Title: Re: Weakness of Arrenhius definition
Post by: FouRRaW on June 05, 2006, 09:57:38 PM

well, NH₃ + H₂O = OH^- + NH4⁺

so doesn't neutralization occur?  and b/c there are OH- ions than arrenhius would say this is a base?

But he is wrong because Neutralization occurs, correct??

Title: Re: Weakness of Arrenhius definition
Post by: wereworm73 on June 06, 2006, 05:43:06 PM

In that reaction, the OH^- ions are coming from the water, not the NH₃ molecules.  So the Arrhenius definition doesn't count NH₃ as a base.

These below are Arrhenius bases because the OH^- ions are the result of the molecules dissociating (breaking up) in water.

NaOH --> Na⁺ + OH^-
Ca(OH)₂ --> Ca²⁺ + 2 OH^-

Back in the late 1800's, Arrhenius figured out how some of the acids & bases work (which was quite a breakthrough back then) but not all of them (because his definition of them was too narrow).  Later on, Bronsted & Lowry broadened (and improved) the definition to include all substances which can either donate or accept protons.

Title: Re: Weakness of Arrenhius definition
Post by: hmx9123 on June 06, 2006, 11:48:06 PM

Further down the line, you get the Lewis definition of acids and bases, which is even broader.

Title: Re: Weakness of Arrenhius definition
Post by: swati on June 07, 2006, 07:23:19 AM

 :)  Hello everyone
Also we have solvent system definition ( by Cady & Elsey ) which defines acids & bases not only in aqueous solvents but other solvents also.

According to it

An acid is a solute that either by direct dissociation or by reaction with the solvent increase the concentration of cation characteristic of the solvent .

An base is a solute that either by direct dissociation or by reaction with the solvent increase the concentration of anion characteristic of the solvent .