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
0
