A study of strength of oxoacid

| May 20, 2015

A study of strength of oxoacid

Generally, oxoacids are simply polyatomic ions with a positively polarized hydrogen, which can be split off as a cation(ion) .
A main class of inorganic protic acids is the  ternary oxoacids. Mononuclear oxoacid contain hydroxyl group(s) (-OH) and oxo group(s)(=O) attached to a central heteroatom conforming to the general formula HmXOn.
In a very simplified manner, the structure of an oxoacid molecule can be described as M-O-H, where, however, even other atoms or atom groups can be connected to the central atom M. In a solution, such a molecule can be dissociated to ions in two distinct ways:
•    M-O-H <=> (M-O)- + H+
•    M-O-H <=> (M)+ + OH-
All oxoacids have the acidic hydrogen bound to an oxygen atom, so bond strength (length) is not a factor, as it is with binary nonmetal hydrides. Rather, the electronegativity of the central atom (E) and the number of O atoms determine oxoacid acidity. With the same central atom E, acid strength increases as the number of oxygen attached to E increases. With the same number of oxygens around E, acid strength increases with the electronegativity of E.

Some oxyacids :-
H2SO3 (aq)    sulphurous acid    oxo
H2C2O4 (aq)    oxalic acid    Oxo
H3PO4 (aq)    phosphoric acid    Oxo

H2CO3 (aq)    carbonic acid    Oxo
HNO3 (aq)    nitric acid    Oxo
HClO4 (aq)    perchloric acid    Oxo
HClO (aq)    hypochlorous acid    oxo

These compounds are acidic when X is a nonmetal or a metalloid and very widely  in strength,from a atom(m), number of O atoms (N),and HClO
Number of H atoms (m), number of O atoms(n), and electronegativity or formal charge of the central atom (x)  influence the strength of oxoacids. While relating these factors to trends in acidity hold well qualitatively for all oxoacids,attempts to estimate actual Ka values with reasonable accuracy lead to only limited  success.
The Ricci version for compounds having the general formula ( HmXOn) used that
Equation : Pka =8-9f+4 (n-m)

References:-

1.    Ricci ,J .E.Am. chem soc. 1948 ,70,109;pauling, l.college chemistry, 3rd ed.;freeman ;san Francisco ,1964;p.540 .
2.    Kivinen, Mäkitie: Kemia, p. 202-203, chapter=Happihapot
3.    “Hapot”. Otavan iso Fokus, Part 2 (El-Io). Otava. 1973. p. 990. ISBN 951-1-00272-4.
4.    Otavan suuri Ensyklopedia, s. 1606, art. Happi
5.    Otavan suuri Ensyklopedia, s. 1605, art. Hapot ja emäxet
6.    Red Book 2005, s. 124, chapter IR-8: Inorganic Acids and Derivatives
7.    Kivinen, Mäkitie: Kemia, p. 459-461, chapter Kemian nimistö: Hapot
8.    Red Book 2005, p. 129-132, table IR-8-1
9.    Red Book 2005, p. 132, note a

Methodology:-

Via MS-Excel, a compilation of literature values were made in order to prepare various comparisons among the previously stated factors . Regression equations  for the relationship between the peak values and given factors , m, n, and electronegativity , were evaluated to determine correlation . A composite equation relating oxoacids strength to their respective m, n, and electronegativity values was derived. Empirical parameters in the composite equation were evaluated and related to the position of X and trends in the periodic table

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