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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: p3t3r1 on April 23, 2008, 05:24:39 PM

Title: Why does the OH react with HA and not H3O+?
Post by: p3t3r1 on April 23, 2008, 05:24:39 PM
In titrating HA, the weak acid with a strong base, why does the OH- ions from the strong base react with the HA atom (and neutralize it) rather than say react with the H3O+ to form complete neutralization? Just curious. Thanks.
Title: Re: Why does the OH react with HA and not H3O+?
Post by: Borek on April 23, 2008, 05:41:05 PM
Please elaborate: what do you mean by

react with the H3O+ to form complete neutralization?

What is the source of H3O+?
Title: Re: Why does the OH react with HA and not H3O+?
Post by: SchoolBoyDJ on April 24, 2008, 09:05:23 PM
Please let me know if this is correct.

HA+OH- ---> H30+A-

If this is the rxn you're talking about I think you're confused.  HA is a weak acid meaning it will not react with water (or very very little will), meaning there are no H30+ available to react.  Now if you're talking about a "2-stepper"...that's only part of the explanation. 
Title: Re: Why does the OH react with HA and not H3O+?
Post by: agrobert on April 24, 2008, 11:37:15 PM
Please let me know if this is correct.

HA+OH- ---> H30+A-

If this is the rxn you're talking about I think you're confused.  HA is a weak acid meaning it will not react with water (or very very little will), meaning there are no H30+ available to react.  Now if you're talking about a "2-stepper"...that's only part of the explanation. 

Both of you are confused.

A weak acid does not dissociate completely.

It is in equilibrium

HA + H2O <--> H30+ + A-

Notice the double sided arrow notation.

HA represents a general protic acid.

HCl is a strong acid.

HCl + H2O --> H30+ + Cl-

NaOH is a strong base.

When you titrate an acidic solution with NaOH you are neutralizing the solution.  Typically titration is used in an analytical matter to determine solution strength or concentration.

The equation is

-OH + H3O+ --> H2O + NaCl

Please ask more questions if you do not understand.