Why is the oxidising power of oxyacids of chlorine in the order
HClO > HClO2 > HClO3 > HClO4 ?
I think the order should be opposite because in HClO4, chlorine is present in its highest oxidation state so it will be easier for it to get reduced
to any lower oxidation state easily.woelen - 28-2-2011 at 07:45
High oxidation state is not always the most oxidizing. Other things must be taken into account as well, such as symmetry of the ion, shielding of
charge, distribution of charge, et.c.
In perchlorate ion, the chlorine atom is completely covered by its 4 oxygen atoms and there is no lone pair of electrons on the chlorine atom. Charge
of the complete ion is evenly spread over all four oxygen atoms, making it a well-balanced resonance-stabilized ion. Perchlorate ion for this reason
is quite inert, especially at room temperature. Covalent perchlorates do not benefit from this stabilization and for that reason, covalent
perchlorates indeed are extremely strong oxidizers and extremely reactive (think of anhydrous HClO4 or even worse, CH3CH2-O-ClO3).
A similar effect is known for nitrate and nitrite. In aqueous solution, nitrite is much more reactive than nitrate. But the anhydrous acid HNO3 again
is an insanely reactive and strong oxidizer. To a lesser extent the same is true for H2SO4.
In aqueous solution, the lower oxoacids anions of chlorine are thermodynamically unstable as they are stronger oxidizers than the oxoacid which is at
the right of them. This leads to disproportionation:
- hypochlorite disproportionates to chlorate and chloride (this can be done with bleach by heating it at 70 C or so)
- chlorite disproportionates to chlorate and chloride (requires long boiling and is a slow process)
Chlorate also is capable of disproportionation, but that requires higher temperatures (e.g. 300 C or so, in aqueous solution this process is VERY VERY
slow and takes millenia). In that case it forms chloride and perchlorate. In the past, this was a well-known method for making perchlorates. Very pure
potassium chlorate was carefully melted such that it just becomes a mobile liquid. Then the liquid is kept at a constant temperature and slowly it
solidifies again, due to conversion of KClO3 to KCl and KClO4. The KCl can be leached out with water and KClO4 remains behind. This method of making
KClO4 is somewhat dangerous though. Impurities (especially organic ones) in the KClO3 could lead to ignition and/or explosion. Nowadays this method of
making perchlorates is replaced by electrolytic production.Bot0nist - 28-2-2011 at 08:35
High oxidation state is not always the most oxidizing. Other things must be taken into account as well, such as symmetry of the ion, shielding of
charge, distribution of charge, etc.
In perchlorate ion, the chlorine atom is completely covered by its 4 oxygen atoms and there is no lone pair of electrons on the chlorine atom. Charge
of the complete ion is evenly spread over all four oxygen atoms, making it a well-balanced resonance-stabilized ion. Perchlorate ion for this reason
is quite inert, especially at room temperature. Covalent perchlorates do not benefit from this stabilization and for that reason, covalent
perchlorates indeed are extremely strong oxidizers and extremely reactive (think of anhydrous HClO4 or even worse, CH3CH2-O-ClO3).
Thank you woelen, that was extremely helpful! I have a much better understanding of oxidizer's properties and how they effect their performance. I'm
off to read up on the molecular properties of my other favorite oxidizers. Thanks again. Claisen - 28-2-2011 at 08:50
A similar effect is known for nitrate and nitrite. In aqueous solution, nitrite is much more reactive than nitrate. But the anhydrous acid HNO3 again
is an insanely reactive and strong oxidizer. To a lesser extent the same is true for H2SO4.
You mean HNO3 is a stronger oxidising agent than HNO2 or the opposite? woelen - 2-3-2011 at 11:00
In dilute solutions HNO2 is a better oxidizer, and it also is much more reactive. At high concentration, on the other hand, HNO3 is very reactive.
This is due to the presence of non-ionized molecules of HNO3, which are not resonance stabilized.DJF90 - 2-3-2011 at 11:03
Might I suggest you find/create a Frost diagram for these species...Claisen - 4-3-2011 at 12:43
