Chapter 21.docx

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14 Apr 2012
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Chapter 21
- Redox reactions and assignment of oxidation numbers, keeping track of
electrons in overall stoichiometry of balanced reaction
oxidization: losing electrons
reduction: gaining electrons
LEO says GER
“losing electrons oxidation” “gaining electrons reduction”
These oxidation numbers are assigned using the following rules:
1. The convention is that the cation is written first in a formula,
followed by the anion.
For example, in NaH, the H is H-; in HCl, the H is H+.
2. The oxidation number of a free element is always 0.
The atoms in He and N2, for example, have oxidation numbers of 0.
3. The oxidation number of a monatomic ion equals the charge of the
ion.
For example, the oxidation number of Na+ is +1; the oxidation number of
N3- is -3.
4. The usual oxidation number of hydrogen is +1.
The oxidation number of hydrogen is -1 in compounds containing
elements that are less electronegative than hydrogen, as in CaH2.
5. The oxidation number of oxygen in compounds is usually -2.
Exceptions include OF2, since F is more electronegative than O, and
BaO2, due to the structure of the peroxide ion, which is [O-O]2-.
6. The oxidation number of a Group IA element in a compound is +1.
7. The oxidation number of a Group IIA element in a compound is +2.
8. The oxidation number of a Group VIIA element in a compound is -1,
except when that element is combined with one having a higher
electronegativity.
The oxidation number of Cl is -1 in HCl, but the oxidation number of Cl is
+1 in HOCl.
9. The sum of the oxidation numbers of all of the atoms in a neutral
compound is 0.
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