Hi, 031987-ga:
By "Bronsted acid" one simply means a compound whose molecules are
capable of "donating" a hydrogen ion H+, e.g. to a reaction.
[Bronsted acid]
http://www.chem.purdue.edu/gchelp/gloss/bronacid.html
One can compare the relative "strengths" of such acids by the pH of an
aqueous solutions of given molarity (concentration of acid per unit of
water), because pH is the negative log of the concentration of
hydrogen ions. [Note that pure liquid water has about 10^-7 hydrogen
ions per molecule of water, for a pH of 7. This is considered
"neutral" between acids (pH < 7) and bases (pH > 7).]
H2SO3 is called sulfurous acid. H3PO4 is called phosphoric acid.
In the case of these two particular compounds it is possible to
predict which is the stronger acid on theoretical grounds. Both are
"ternary oxy acids" having formulas HpXOq for whole numbers p,q. Such
compounds' acidities can be compared based on charge and
electronegativity; see here:
[Ternary Oxy Acids: HpXOq]
http://bilbo.chm.uri.edu/CHM112/lectures/lecture13.htm#6
[Which is the strongest acid: H3PO4 or H2SO3?]
http://bilbo.chm.uri.edu/CHM112/lectures/HP-S.htm
Both compounds are neutral, so the issue is decided by
electronegativity.
Rewrite the chemical compounds like this:
Sulfurous acid: H2SO3 --> (HO)2 SO
Phosphoric acid: H3PO4 --> (HO)3 PO
The oxygen atoms without with hydrogens in these formulas are referred
to as "oxo" oxygens, and together with the "central atom" X are
responsible for the total electronegativity exerted upon the
"donatable" hydrogen ions.
Here both compounds contains a single "oxo" oxygen, so the comparison
comes down to the relative electronegativities of the two central
atoms, S and P.
From the fact that S is to right of P in the periodic table, we know S
has a greater electronegativity than does P. [Note that S is one row
below O while P is just before S, one row below N.]
Therefore sulfurous acid is a stronger acid than phosphoric acid.
regards, mathtalk-ga
Other Links of Interest:
Clicking on the P and S elements in this table brings up additional
information, including their respective Pauling electronegativities:
[Periodic Table of the Elements]
http://bilbo.chm.uri.edu/PeriodicTable/PeriodicTableoftheElements.htm |
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