Oxidation - Reduction
RedOx
Oxidation States
. . . of an element are determined from
the number of electrons that are ______
other atoms
Gained from
Lost to
AND
Shared with
Rules of assigning
oxidation states
Atoms have a negative oxidation state if
they have the higher e-neg in the bond
NH3
N=-3
H=+1
Oxidation states on
all the atoms of a molecule & compound
must add up
to equal zero
Uncombined elements
free state
Elements that are not bonded to a
different element have oxidation states of
zero.
Al(s) Cu(s) Ag(s) Cl2 O2 O3 Ne
NOW More complicated . . .
H2S
sulfur has a -2 oxidation state
H2SO3
sulfur has a +4 oxidation state
H2SO4
sulfur has a +6 oxidation state
Assigning oxidation states
to atoms in a polyatomic
ion
Oxidation states on individual atoms must
equal the charge on the ion
+6
-2-2) )
(S(S
(SO
OO
)
44
+6
-2-2
-8 = =
=
-2
Try these . . .
hydroxide
dichromate
ammonium
Assign oxidation states to
each atom
NaOH
MnCrO4
(NH4)2SO4
K2O2
Balancing RedOx reactions
using oxidation states
Zn + HNO3
Zn(NO3)2 + NO2 + H2O
Go through the steps on paper…
Losing e oxidation
+4
+3
+2
+1
0
-1
-2
-3
-4
Gain e- Reduction
determine the number
of e- lost and gained
-
1. Assign oxidation
states to all atoms
2. Select product coefficients that balance
the e- HINT: switch #’s
balance major elements with reactants
3. Balance leftovers with inventory
*coefficients must be lowest possible whole #s
Balance - use oxidation states
Cu + HNO3
Cu(NO3)2 + H2O + NO
3Cu + 8HNO3
3Cu(NO3)2 + 4H2O + 2NO
K2Cr2O7 + H2O + S
SO2 + KOH + Cr2O3
2K2Cr2O7 + 2H2O + 3S
3SO2 + 4KOH + 2Cr2O3
Balancing net ionic rxns
Mg + Al3+
Mg2+ + Al
Balance for mass
and charge
Al3+ and Mg+2 are ions
if they give you the charge use it!
Its part of a net ionic rxn
 they have removed the spectator ions.
Balance . . .
Cu2+ + K
for mass and charge
K+ + Cu
RedOx reactions
Chemical reactions involving an
electron
transfer
between reactants
Recognizing RedOx
assign oxidation states to the individual
elements in reactants and the products
if the oxidation state changes for some of
the particles it is considered a RedOx reaction
Single
Replacement
Reactions
areare
ALWAYS
Double
Replacement
Reactions
NEVERRedOX
RedOX
In a RedOx reaction
If one atom is being oxidized another
must be reduced.
In other words . . .
oxidation and reduction always occur
together
If electrons are lost by
one species in a reaction they
all MUST be
gained by another!
+3
Oxidation
+2
+1
Oxidation results in an increase
0
in oxidation state
-1
-2
-3
Atoms that Lose e lectrons undergo
Oxidation
Metals tend to undergo oxidation
+3
Reduction
+2
+1
Reduction results in a decrease
0
in oxidation state
-1
-2
-3
Atoms which Gain e lectrons undergo
Reduction
Non-metals tend to undergo reduction
GeR
RedOx: Yes or NO?
HCl + NaOH  HOH + NaCl
Mg + 2HCl  MgCl2 + H2
MnO2 + 4HBr  MnBr2 + Br2 + 2H2O
Predicting Products of
Single Replacement Reactions
Al + CuCl2
?
Write a balanced equation based on
these reactants.
Is this a RedOx reaction?
What species is oxidized?
Reduced?
Roaring Animals Often
Attack
Elements that undergo oxidation cause
reduction
Reducing agent
Elements that undergo
reduction cause oxidation
Oxidizing agent
LEO GER
RA OA
2Al + 3CuCl2
2AlCl3 + 3Cu
What is the Oxidizing Agent?
Cu+2 because it gets reduced
What is the Reducing Agent?
Al because it gets oxidized
These answers always come
from the reactant side
MnO2 + 4HCl
MnCl2 + Cl2 + 2H2O
Assign oxidation states to all atoms
What species is oxidized?
How do you know?
What species is reduced?
How do you know?
What species is the oxidizing agent?
Mn+4 because it gets reduced
What species is the reducing agent?
Cl- because it gets oxidized
-
Remember . . .
+4
+3
+2
+1
0
-1
-2
-3
-4
Gain e- Reduction
Losing e oxidation
RedOx Practice
Writing half-reactions
A half reaction shows either oxidation or
reduction of a RedOx reaction.
The electrons being lost (oxidation) or
gained (reduction) are also shown.
Oxidation half-reaction
Fe(s)
Fe+3(aq) + 3e-
oxidation number increases
electrons are lost
 products
conservation of mass AND CHARGE
charges on both sides equal each other
Reduction half-reaction
Sn+4 + 2e-
Sn+2
oxidation number decreases
electrons are gained
reactants
conservation of mass AND CHARGE
charges on both sides equal each other
Balancing RedOx using
1/2 reactions
1.
2.
3.
4.
ASSIGN OXIDATION STATES . . .
Write the oxidation 1/2 reaction
Write the reduction 1/2 reaction
Balance the two half reactions so that the
number of electrons transferred is equal
5. Use these coefficients to balance the RedOx
atoms
6. Balance leftover atoms by inventory
. . . use 1/2 reactions
KMnO4 + HCl  MnCl2 + KCl + Cl2 + H2O
ZnS + O2  SO2 + ZnO
The End
I came . . . I saw . . .
I RedOxed