Oxidation
Process in which atoms or ions attain a
more + oxidation state
Ex.
Na → Na+ + e
Cl- → Cl + e
Ca+ → Ca2+ + e
Reduction
Atoms or ions of elements attain a more
negative oxidation state
Ex.
Cl + e → ClMnemonic: “OIL RIG”
Oxidation involves loss, reduction
involves gain
Oxidation Number
Uncombined elements have oxidation # of 0
Monoatomic ions have oxidation # equal to
their ion charge
F is always –1 in a compound
O is almost always –2 in a compound
H is +1 in all compounds except those with
metals (then its –1)
More electronegative element is assigned a
number equal to its ion charge
Algebraic sum of oxidation numbers in a
compound is always zero
Practice
Determine the oxidation numbers for each
element in the compounds below
NaCl
H2SO4
NO3CO2
SF6
AgNO3
Redox Reactions
Redox reactions are among the most
common of all chemical processes
Combustion, synthesis, decomposition,
and single replacements are all redox
reactions
Definition
Reaction in which a change in oxidation
number occurs
Oxidation – becomes more positive
Reduced – becomes more negative
Oxidation and reduction always come in
pairs
Electrons are transferred from the
substance being oxidized to the
substance being reduced
Example
In the reactions below, identify what is
oxidized and what is reduced
2PH3 + 4O2  P2O5 + 3H2O
2H2O + Al + MnO4-  Al(OH)4- + MnO2