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Explanation
In general, the oxide of a metal dissolved in water gives a base and the
oxide of a nonmetal dissolved in water gives an acid. The oxides of some metalloids and
transition metals give amphoteric compounds when dissolved in water.
Let's look at why it is that the oxides of metals give basic solutions and
the oxides of nonmetals give acidic solutions.
| When the oxides react with water they form compounds containing hydrogen
as well as oxygen and the other element. In general, if that other element is a metal the
compound is basic. On the other hand, if that other element is a nonmetal, the compound is
generally acidic. |
MgO + H2O  Mg(OH)2 base |
SO2 + H2O H2SO3 acid |
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| Even though magnesium hydroxide is an ionic compound, it is represented
here using an electron dot diagram. The Mg-O bond is ionic the O-H bond is covalent. H has
a stronger attraction (bond) to the electrons around O than does Mg because H is more
electronegative than Mg. Therefore, when the compound splits H remains attached to O and
OH separates away from Mg. The electrons from Mg go with O giving Mg2+ and OH-.
The OH- makes the solution basic. |
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··
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H : O : Mg : O : H
·· ·· |
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H : O : ¯
·· |
Mg2+ |
··
¯ : O : H
·· |
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| Next we have H2SO3 which is a covalent
compound. S and H are both nonmetals. Therefore, they both share electrons with O. S is
more electronegative than H, so when splitting occurs it is the S that stays attached to
the electrons on O and the H separates away from O, leaving its electrons behind and
becoming an H+ ion. The H+ makes the solution acidic. The
second H+ can also come off, but it is more difficult to remove because that
involves pulling it away from something that already has a negative charge.
Of course the H+ doesn't just leave, it is pulled off by water
or some other proton acceptor. |
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·· ·· ··
H : O : S : O : H
·· ·· ··
: O :
·· |
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·· ·· ··
H : O : S : O : ¯
·· ·· ··
: O :
·· |
H+ |
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| H+ |
·· ·· ··
¯ : O : S : O : ¯
·· ·· ··
: O :
·· |
H+ |
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E-mail instructor:
Eden Francis
Clackamas Community College
©1998, 2002 Clackamas Community College, Hal Bender
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