Metal, Non-Metal Oxides and pH
Date: Spring 2010
Why are metal oxides basic and non-metal oxides acidic? I
think that this may have something to do with the bonding in the
oxides, as metal oxides are ionic and non-metal oxides covalent
It might help to remember that while the Arrhenius definition of
acids and bases are based solely on the compound structure (whether
the compound has an H[+] or OH[-], for example), the Bronsted-Lowry
definition tried to expand the way acids and bases are defined by
looking at how the compound reacts. In this case, a compound like
NH3 which does not have an obvious OH[-] is still considered a base
because when it reacts with water: NH3 + H2O --> NH4[+] + OH[-] it
accepts a proton. Consequently, in this case, the water is acting
like an acid since it supplies a proton.
Now look at a metal oxide: MO. It will react with water in general,
in the following way:
MO + H2O --> MOH + OH[-].
For example: MgO reacts with water: MgO + 2H2O --> Mg(OH)2 + 2OH[-]
Thus, you can see that the MO received a proton and is therefore, by
the Bronsted-Lowry definition, a base.
Nonmetal oxides, on the other hand, produce things like carbonates
(if C) or sulfates (if S), and so on. For example:
CO2 + H2O --> H2CO3. And in such cases --you will have to look
at the Lewis Structures to see this-- you can see that the nonmetal
accepts a lone-pair, which according to the Lewis definition makes it an acid.
Now, as to why a metal oxide would react differently from a nonmetal
oxide, you would have to look at the relative stabilities of the
conjugate acids or bases that are produced. Remember that a reaction
happens spontaneously because there is either a gain in entropy or a
loss in enthalpy.
Greg (Roberto Gregorius)
Click here to return to the Chemistry Archives
Update: June 2012