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Name: Youlin
Status: student
Grade: 9-12
Country: Malaysia
Date: Spring 2010


Question:
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 molecules.



Replies:
Youlin,

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)
Canisius College



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