What is the difference between oxidation number and oxidation state?

1 Answer
Jun 3, 2016

Oxidation number might be easier to understand at the high school level, but oxidation state and oxidation number are synonymous in many cases.

The only time I can think of where they are different are in coordination complexes.

In #["Ti"("CO")_6]^(2-)#, or hexacarbonyltitanate(II), the oxidation state is #-2# but the oxidation number is #"II"#. The "-ate" indicates the negative overall charge of the complex.

On the other hand, #["Ti"("CO")_6]^(2+)#, or hexacarbonyltitanium(II), would have the same oxidation number but a #+2# oxidation state.


OXIDATION STATE EXAMPLE

The oxidation state is the hypothetical charge if the bond that the atom makes with a second atom is 100% ionic in character, and if after accounting for electronegativities, the more electronegative atom in the pair has the more negative oxidation state.

It isn't ever exactly the case for a real bond to have 100% ionic character (i.e. to transfer 100% of the electron density into another atom's valence orbitals), but oxidation states turn out to be a good charge accounting scheme.

Consider the following #E^@# vs. #"pH"# (Pourbaix) diagram.

https://upload.wikimedia.org/

For example, if we are at #"pH"# #2#, then we can apply some positive voltage to #"Fe"^(3+)# at #"1 atm"# pressure, #"273.15 K"#, and constant #"pH"# as follows:

#\mathbf(stackrel(color(blue)(+3))("Fe"^(3+))(aq) + 4stackrel(color(blue)(+1))("H")_2stackrel(color(blue)(-2))("O")(l)-> stackrel(color(blue)(+6))("Fe")stackrel(color(blue)(-2))("O"_4^(2-))(aq) + 8stackrel(color(blue)(+1))("H"^(+))(aq) + 3e^(-))#

#E_"ox"^@ > "0.8 V"#

(We started in the #"Fe"^(3+)# region on the upper left side of the graph, and we moved upwards to oxidize.)

For this half-reaction, we've oxidized iron(III) #(stackrel(+3)("Fe"))#to iron(VI) #(stackrel(+6)("Fe"))# in acidic conditions.

So, iron attains a #+6# oxidation state, and its oxidation number is represented as #"VI"#.


OXIDATION NUMBER VS. STATE EXAMPLE

One difference I can find is in #["Ti"("CO")_6]^(2-)#. This is called hexacarbonyltitanate(II).

The oxidation number of titanium here is written as #"II"#, but the oxidation state of titanium is actually #-2# (making it a #d^6# metal). The negative overall charge of the complex is conveyed by the "-ate" in the name, and the negative oxidation state is deduced.

This compound has a #t_(2g)^6# configuration, being low spin, and is a stable 18-electron complex (as it should be for a coordination complex with six #pi#-acceptor ligands).