Basic chemistry question: acid-base reactions

From: Jostein Abeltveit (abeltveit_at_hotmail.com)
Date: 11/08/04 

Date: 8 Nov 2004 10:07:32 -0800

Below is an excerpt from "Chemistry for Dummies" followed by my
questions related to this text. I would be grateful if someone would
bother taking a look:

"- Give me that proton: Bronsted-Lowery acid-base reactions

With the Arrhenius theory, acid-base reactions are neutralization
reactions. With the Bronsted-Lowery theory, acid-base reactions are a
competition for a proton [1]. For example, take a look at the reaction
of ammonia with water:

NH3(g) + H2O(l) <--> NH4+ + OH-

Ammonia is the base (it accepts the proton), and water is an acid (it
donates the proton) in the forward (left to right) reaction. But in
the reverse reaction (right to left), the ammonium ion is an acid and
the hydroxide ion is a base. If water is a stronger acid than the
ammonium ion, then there is a relatively large concentration of
ammonium and hydroxide ions at equilibrium. If, however, the ammonium
ion is a stronger acid, much more ammonia than ammonium ion is present
at equilibrium.

Bronsted and Lowery said that an acid reacts with a base to form
conjugate acid-base pairs [2]. Conjugate acid base pairs differ by a
single H+. NH3 is a base, for example, and NH4+ is its conjugate acid.
H2O is an acid in the reaction between ammonia and water, and OH- is
its conjugate base. In this reaction the hydroxide ion is a strong
base and ammonia is a weak base, so the equilibrium is shifted to the
left [3] -- there's not much hydroxide at equilibrium".

------------

Questions:

[1] Is this true only for equilibrium acid-base reactions? If, for
example, HCl and NaOH reacted, would this still be called a
neutralization reaction with the Bronsted-Lowery theory? It cannot be
called a "competition for a proton", now, can it?

[2] Again, is this only true for equilibrium acid-base reactions/weak
acids?

[3] I take it that when compared to H2O, NH4+ is the stronger acid.
However, for the sake of argument, let us imagine that H2O is in fact
the stronger acid. This should cause the above reaction to shift to
the right, correct? However, we know that OH- is a much stronger base
than NH3, so the reaction shifts to the left. Am I correct in assuming
that these two forces will, er, wrestle it out with each other, and
that in this case the force shifting the to left (due to OH-) will
"win" because the difference in strength between the two bases are
considerably larger than the difference between the two acids?

J.Abeltveit.

Relevant Pages

  • Basic chemistry: acid-base reactions
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  • Re: Basic chemistry question: acid-base reactions
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  • Re: acids
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  • Re: Copper (hydr)oxide and ammonium chloride
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