Acids (1 Viewer)

[velox]

In the excel books it says this but i dont really understand

If two acids exist, the lower oxidation state is -ous and the higher is -ic.
e.g. H2s04, sulfuric acid
h2s03, sulfurous acid
thanks!

 

[xiao1985]

lolz, i was just replyin to one of cm_tutor's post... before they ionise, it is a neutral molecule... there are no oxidation state on the molecule... once they ionises, both anions would have same oxidation state i think, because both of them will be 2- charged...

 

[CM_Tutor]

Sorry xiao1985, but no. You are talking about net charge. As neutral molecules, they have no net charge, and after ionisation they end up with the same net charge (as the sulfate ion, SO₄^2-, and the sulfite ion, SO₃^2-, both have a -2 charge). However, net charge is only the same as oxidation state for monatomic ions, whereas these are molecules (before ionisation) and polyatomic ions (after ionisation). When we talk of oxidation state, it is of usually for an element within a species (sometimes even for different atoms of the same element in the species, but lets not go there at the moment).

In this specific case, both sulfuric acid, H₂SO₄, and the the sulfate ion, SO₄^2-, have sulfur in the +6 oxidation state, whereas both sulfurous acid, H₂SO₃, and the sulfite ion, SO₃^2-, have sulfur in the +4 oxidation state.

Wrx, a much easier way to remember this is to memorise formulae for inorganic acids named .....ic acid, and then remember the following rules:

adding 'per' to an 'ic' acid means add one O.

eg. perchloric acid is HClO₄, as chloric acid is HClO₃

changing 'ic' to 'ous' means remove one O, so long as at least one O remains

eg. nitrous acid is HNO₂ as nitric acid is HNO₃
sulfurous acid is H₂SO₃ as sulfuric acid is H₂SO₄
chlorous acid is HClO₂ as chloric acid is HClO₃

adding 'hypo' to an 'ous' acid means remove one O, so long as at least one O remains

eg. hypochlorous acid is HClO as chlorous acid is HClO₂

As the names change per...ic ---> ...ic ---> ...ous ---> hypo...ous, there is a decrease in oxidation state of the central atom in the acid.

So, perchloric acid, HClO₄, has chlorine in oxidation state +7
chloric acid, HClO₃, has chlorine in oxidation state +5
chlorous acid, HClO₂, has chlorine in oxidation state +3
hypochlorous acid, HClO (or more usually, HOCl), has chlorine in oxidation state +1

If you need to figure out the names of the ions:

...ic acids give ...ate ions
...ous acids give ...ite ions
hydro...ic acids give ...ide ions