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Relative acidity of HClO vs HClO2 etc.

Posted: Tue Jul 30, 2019 8:12 am
by Katherine Fitzgerald 1A
Can someone provide more of the "why" in this explanation pasted below for the relative acidity of HClo, HClo2, etc.? My comments/questions are in italics

"In oxyacids, the H is always bonded to one of the oxygens. Any factor that weakens the O-H
bond, makes the substance more acidic."

This makes sense, because a more weakly bonded H will be given off more easily

"Within an oxyacid series such as HClO, HClO3,HClO2 and HClO4, as the number of oxygens bonded to the central atom increases,
the oxidation number of the central atom increases causing a weakening of the O-H bond strengthand an increase in the acidity.

How do additional oxygens change the oxidation number of the central atom, more fundamentally? Why does an increase in the oxidation number of Cl weaken the O-H bond?

Re: Relative acidity of HClO vs HClO2 etc.

Posted: Tue Jul 30, 2019 12:35 pm
by Chem_Mod
In general, adding more oxygen atoms to the central atom in an oxyacid helps to distribute the negative charge of the conjugate base over a greater number of atoms. If a proton is less strongly attached to any one of the oxygens, then you get a stronger acid. As the number of oxygens increases as you go from HClO to HClO4, the oxidation number of Cl increases.
The oxidation number for Cl in HClO is +1, +3 for HClO2, +5 for HClO3, and +7 for HClO4. The oxidation number represents the number of electrons Cl loses. As the oxidation number increases, the charge density of that atom increases such that the atom will strongly pull electron density toward itself (electron withdrawing) from other species near it. This is why HClO4 will be the strongest acid. Once it loses its hydrogen, the central Cl will strongly pull electron density toward itself, leaving us with a conjugate base that is more stable than the conjugate bases of HClO, HClO2, and HClO3.