8
Reactions in which electrons are transferred between substances
Use of Oxidation numbers in determining redox reactions is
basically a bookkeeping method for keeping track of electrons
You must be able to identify an oxidation-reduction reaction. But first, we must learn the rules for
assigning oxidation #’s to different species.
Rules for oxidation numbers1) Atoms in elemental form are 0.2) Cation (positive metal ion); oxidation numbers:
Group 1 (+1), Group 2 (+2), Group 13 (+3).3) Nonmetals; usually negative numbers.
a.) oxygen = -2 unless a peroxide O2-2 then O= -1
b.) Hydrogen +1 with nonmetals, -1 with metalsc.) Oxidation numbers: (Group 17) (-1), Group 16 (-2), Group 15 (-3) unless bonded to oxygen then (+) oxidation as in a polyatomic ion (Ex: ClO3
-; Cl = +5)
4) Sum of oxidation numbers must = 0 for neutral compound or equal to charge of polyatomic ion.
5) Most electronegative (furthest to right and up) element gets a negative charge.
1) Atoms in elemental form are 0.
ExamplesAgPbCl2
O2
Oxidation # = 0 for 7 diatomic elements and for allother elements when by themselves.
2) Binary Ionic Compounds- charge of the ion based on Group number.
ExamplesAgCl Ag = +1 Cl = -1PbI2 Pb = +2 I = -1Fe2O3 Fe = +3 O = -2
3) Nonmetals; usually negative numbers.a.) oxygen = -2 unless a peroxide = -1b.) Hydrogen +1 with nonmetals, -1 with metals c.) Oxidation numbers: (Group 17) (-1), Group 16 (-2), Group 15 (-3) unless bonded to oxygen then (+) oxidation as in a polyatomic ion (Ex: ClO3-; Cl = +5)
ExamplesPbO oxygen = -2 Na2O2 oxygen = -1H2S hydrogen = +1 NaH hydrogen = -1KI iodine = -1 KIO2 iodine = + 3
Determine Oxidation # of element red element in each of the following:
MnO2
+4
KMnO4
+7
BrO2-
+3
BrO3-
+5
Br2
0
HClO4
+7
H2SO4
+6
PO33-
+3
CaH2
-1
SO42-
+6
Na2S
-2
Mg(NO3)2
+5
