Arrhenius Acid-Base Theory and NH3

[robert zellmer]

The textbook states NH3 is an Arrhenius base.

The book is technically incorrect.  The original definition stated
that OH- was part of the compound and that compound when put
in water released the OH-, making the solution basic (inc. the conc.
of OH-).  This theory wasn't able to correctly explain why NH3 was
a base.  Instead people came up with the nonexistent substance
ammonium hydroxide, NH4OH, which has an OH- in the formula.
This really doesn't exist.  If you put NH3 in H2O you get NH4+ ions
and OH- ions, which we can see using Bronsted-Lowry Theory.
However, if you allow the water to evaporate you don't get a compound
of ammonium hydroxide.  Instead what happens as the water
vaporizes is the NH3 slowly comes out of soln as a gas (which it is
to begin with) and you will eventually be left with nothing but air.

A looser definition, what the book uses, is an Arrhenius base is
any substance that increases the conc. of OH- in an aqueous soln.
NH3 does inc. the conc. of OH-.  But again, this does not strictly
fit the definition of an Arrhenius base.  Using this criteria any
anion that acts as a base would be considered an Arrhenius base
(such as F- and no books state this is so).

By the way, you will often see on a bottle of aqueous NH3 the name
ammonium hydroxide.  This is a misnomer (as I explained above) but
has kind of stuck for the name of an aqueous solution of NH3.

Here's a link which also describe this,

http://www.chemteam.info/AcidBase/Arrhenius-AcidBase.html

This technically applies to other things as well.  According to his
original theory the base had to have an OH (technically, OH-) in
the formula.  This means the answer in the solutions manual on
Carmen to 16.14(b) is technically not correct.

Dr. Zellmer
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