U9 RedOx Rxns PPT
Assigning oxidation states and identifying
oxidizing and reducing agents
After today you will be able to…
• Identify oxidation states
• Identify oxidizing agent and
the reducing agent
• Write half reactions
Oxidation-reduction (redox)
is a type of chemical
reaction.
Originally, oxidation was defined as any
process in which oxygen is added to
another substance.
Reduction was defined as loss of oxygen
from a reactant.
•If one substance is oxidized another
substance in the same reaction is reduced.
•By today’s definition of a redox reaction,
not all of these reactions involve oxygen,
but they do involve a transfer of
electrons.
Oxidation: loss of electrons
Reduction: gain of electrons
LeO the lion says GeR!
Lose electrons Oxidized
Gain electrons Reduced
In order to write and balance
redox reactions, oxidation
numbers must be assigned to
the individual elements.
Oxidation Numbers
Signifies the number of charges
an atom would have in a
molecule or ionic compound if
the electrons were transferred
completely.
Rules for Assigning Oxidation Numbers
1. Each atom in a pure element has an
oxidation number of zero (0).
Example: I2, Cu = 0
Rules for Assigning Oxidation Numbers
2. Monatomic ions have oxidation
numbers equal to the charge on the
ion.
Example: Mg+2=+2, H+1=+1
Rules for Assigning Oxidation Numbers
3. Group 1 elements have an oxidation
number of +1, group 2 have +2, and
group thirteen have +3.
Example: NaCl, Na=+1, Cl=-1
Rules for Assigning Oxidation Numbers
4. Fluorine always has an oxidation
number of -1 in compounds with
other elements.
Rules for Assigning Oxidation Numbers
5. The oxidation number of H is +1 and
for O is -2 in most compounds.
a) Exception #1: When H forms a
compounds with a metal it will
have an oxidation number of -1.
+2 -1
Example: CaH2
Rules for Assigning Oxidation Numbers
5. The oxidation number of H is +1 and
for O is -2 in most compounds.
b) Exception #2: When O forms a
peroxide it will have a oxidation
number of -1.
+1 -1
Example: H2O2
Rules for Assigning Oxidation Numbers
6. The sum of oxidation numbers must be
zero (0) for a neutral compound, and
for a polyatomic ion it must be equal
to the ion charge.
7. Cl, Br, and I always have an oxidation
number of -1 except when combined
with oxygen or fluorine.
+1 -2
Example: ClO-1
Rules for Assigning Oxidation Numbers
8. Oxidation states do not have to be
integers.
Example: I3-1, I = -1/3
Examples: Assign oxidation numbers to the
following compounds:
+3
-2
a.Al2O3
Helpful hint: Assign
+1 +5 -2
b.H3PO4
oxidation
numbers
to
+6
-2
c.Cr2O7-2 the elements that you
+1 +6 -2
know
first!
Then
the
d.H2SO4
+4 -2
rest
will
work
-2
e.CO3
themselves out!
+5 -2
f. NO2+1
Writing Half Reactions
Example: Identify which elements are being oxidized
and reduced and write the half reactions for each.
losing 2 electrons
0
0
+2
Helpful
hint:
Helpful
You
hint:
must
+1
2Ag
+ Cu  2Ag + Cu+2
gaining 1 electron
balance
LeO each
the lion
halfsays
reaction
GeR!
Cu
Ag
Oxidized:____
for
both mass Reduced:____
and charge!
+1
Ag
Oxidizing agent: ____
Cu
Reducing agent: ____
Writing Half Reactions
Example: Identify which elements are being oxidized
and reduced and write the half reactions for each.
0
Mg
losing 2 electrons
+1 -1
+ 2HCl
Mg
Oxidized:____
H
Oxidizing agent: ____
+2 -1
 MgCl2 +
gaining 1 electron
0
H2
H
Reduced:____
Mg
Reducing agent: ____
U9 RedOx Rxns WKSHT
due Thurs at 7:30am