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Redox Reactions
What is redox?
 Redox reactions involve a transfer of electrons.
 Oxidation – involves losing electrons (increase in oxidation
number)
 Reduction – involves gaining electrons (reduction in oxidation
number)
 LEO the lion says GER
 Losing electrons = oxidation
 Gaining electrons = reduction
How do you know if a reaction is redox
or not?
 Oxidation numbers can be assigned to all atoms based
on electronegativity relative to the other atoms to
which they are bonded.
 If oxidation numbers change from one side of the
equation to the other, that indicates that the reaction is
a redox reaction.
Rules for Assigning Oxidation Numbers
1.
The oxidation number of an atom in the elemental state is zero.
Example: Cl2 and Al both are 0
2.
The oxidation number of a monatomic ion is equal to its charge.
Example: In the compound NaCl, the sodium has an oxidation number
1+ and the chlorine is 1-.
3.
The algebraic sum of the oxidation numbers in the formula of a compound is zero.
Example: the oxidation numbers in the NaCl above add up to 0
4.
The oxidation number of hydrogen in a compound is 1+, except when hydrogen
forms compounds called hydrides with active metals, and then it is 1-.
Examples: H is 1+ in H2O, but 1- in NaH (sodium hydride).
5.
The oxidation number of oxygen in a compound is 2-, except in peroxides when it
is 1-, and when combined with fluorine. Then it is 2+.
Example: In H2O the oxygen is 2-, in H2O2 it is 1-.
6.
The algebraic sum of the oxidation numbers in the formula for a polyatomic ion is
equal to the charge on that ion.
Example: in the sulfate ion, SO42-, the oxidation numbers of the sulfur
and the oxygens add up to 2-. The oxygens are 2- each, and the sulfur is
6+.
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Examples
 NaCl
 HNO3
 CaCO3
 PO4-3
Redox Reactions
 Identify which of the following equations represent
redox reactions.
 For each redox equation identified, write the oxidation
state above each element.
 Indicate the element that has been oxidized and the
one that has been reduced.
examples
 KClO3  KCl + O2
 NaOH + HCl  NaCl + H2O
k
 CO2 + H2O  C8H12O2 + O2
 NaCl + AgNO3  NaNO3 + AgCl