Oxidation-Reduction
(aka Redox)
Reactions
http://www.calgaryacademy.c
om/ICT/rr/redox1.html
These are electron- transfer reactions!
Oxidation Numbers
= ___________________________________________________
___________________________________________________
For example:
These are the oxidation numbers of
each atom. They keep track of where
electrons are.
Zn0 + Cu+2  Zn+2 + Cu0
Oxidation
= the _____ of electrons
Magnesium’s
oxidation # is 0
when it is a solid.
Mg
After oxidation,
magnesium’s oxidation
# increases …
0
Notice:
In an oxidation reaction, the
element’s oxidation number will
__________________..
+2
(s)
-
 Mg + 2e
because _______
_____________
Reduction
= the ________ of electrons
After reduction,
iron has an
oxidation # of 0…
An iron ion has
an oxidation # of
+3
+3
-
Fe + 3e 
0
Fe (s)
because _______
Notice:
In a reduction reaction, the element’s
oxidation number will
______________.
_____________
Hint for remembering:
Another hint for remembering:
LEO
LEO
the lion
Loss of electrons
is oxidation
says
GER
GER
Gain of electrons
is reduction
Oxidation or Reduction???
1.)
0
Br2 +
2e-
 2Br
-1
___________
2.)
Li0  Li+1 + e___________
Redox Reactions:
Oxidation & reduction reactions can’t happen alone!
0
+2
-
Oxidation: Mg (s)  Mg + 2e
+3
0
Reduction: Fe + 3e  Fe (s)
Redox Reaction:
3 Mg
Fe3+ is the
________ agent
The oxidation # increased.
Mg(s) was oxidized.
Mg lost electrons.
0
+3
0
(s) +
(s) + 2 Fe  2Fe
The oxidation # decreased.
Fe3+ was reduced.
Fe3+ gained those electrons.
Mg0 is the
________ agent
3 Mg
+2
http://www.calgaryacadem
y.com/ICT/rr/redox4.html
How can we identify a redox reaction?
Step 1: Assign oxidation numbers to each atom in the
reaction.
Rules for Assigning Oxidation Numbers
1.) Any atom that is uncombined and has a neutral charge has an
oxidation number of 0.
Ex: 2Na + Cl2  NaCl
2.) Ions have an oxidation number equal to their ionic
charge.
Ex: Chlorine ion: Cl-
3.) Group I elements always have an oxidation number
of +1 in compounds, while the Group II elements
always have an oxidation number of +2 in
compounds.
Ex: K2SO4  Each K will have an oxidation # of ____
Ex: BaCl2  Ba will have an oxidation # of ____
4.) Fluorine is always -1 in compounds. The other halogens are also
-1 when they are the most electronegative element in a
compound.
5.) Hydrogen is +1 in compounds…
EXCEPT if it is combined with a metal.
Ex: HCl  hydrogen will be ___
LiH  hydrogen will be ___
6.) Oxygen is usually -2 in compounds…
EXCEPT when it is combined with fluorine, it
becomes +2….and
EXCEPT when it is in the peroxide ion (O22-), it
becomes -1.
7.) The sum of the oxidation numbers in all compounds must be 0.
Rule 6:
-2
Ex:
Rule:
+1
1H  ____
1N  ____
3O  ____
HNO3
8.) The sum of oxidation numbers in polyatomic ions must be
equal to the charge on the ion.
Rule 6:
-2
Ex:
Cr2O72-
2Cr  ____
7O  ________
How can we identify a redox reaction?
Step 1: Assign oxidation numbers to each atom. (Use rules!)
MnO2 + 4HCl  MnCl2 + Cl2 + 2H2O
How can we identify a redox reaction?
Step 2: Identify whether there are any changes in oxidation number
for a particular atom between the reactant and product sides.
**If there is a change in oxidation number for particular type of atom,
the reaction is redox.***
+4
-2
+1
-1
+2
-1
0
+1
MnO2 + 4HCl  MnCl2 + Cl2 + 2H2O
-2
Another Example
PbO2 + 4HI  I2 + PbI2 + 2H2O
Half-Reactions
= show either oxidation or reduction portion of the
redox reaction
-2
+4
+1
-1
0
+2
-1
+1
-2
PbO2 + 4HI  I2 + PbI2 + 2H2O
Oxidation:
Reduction:
***There must be the same number of atoms on each side of the
reaction, and the net charge must be the same on both sides.***
Oxidation: I1-  I20
Reduction: Pb4+  Pb2+
Another Example:
Cu + AgNO3  Cu(NO3)2 + Ag
Oxidation:
Reduction:
Electrochemical Cells
= _______________________________________
- 2 types:
(1) Voltaic cell = _________________________________________________
(2) Electrolytic cell = ______________________________________________
______________________________________________
- Electrode = ____________________________________________
____________________________________________
 anode = where _____________ occurs
 cathode = where _____________ occurs
AN OX:
RED CAT:
Anode is the site of
oxidation
Reduction occurs at the
cathode
Voltaic Cells (aka galvanic cells)
**_______________________________________________________.**
(2) The electrons leave the zinc anode and pass
through the external circuit to the copper rod.
(1) Electrons are
produced at the zinc
rod according to the
oxidation halfreaction:
(3) Electrons enter the
copper rod and
interact with copper
ions (Cu+2) in solution.
There, a reduction
half-reaction occurs:
B/c zinc is oxidized at
the zinc rod, the zinc
rod is the anode. The
anode (in a voltaic
cell) is a negative
electrode.
(4) To complete the circuit, both positive and
negative ions move through the aqueous
solutions via the salt bridge.
B/c the copper ions
are reduced at the
zinc rod, the zinc rod
is the cathode. The
cathode (in a voltaic
cell) is a positive
electrode.
Animation of Voltaic Cell:
http://www.mhhe.com/physsci/chemistry/essentialchemis
try/flash/galvan5.swf
Virtual Lab:
http://www.chem.iastate.edu/group/Greenbowe/sections/
projectfolder/flashfiles/electroChem/voltaicCellEMF.html
Electrolytic Cells
**________________________________________________________________
_____________________________________________________________.**
Example: Electroplating
(1) Silver (Ag) is oxidized to
produce silver ions (Ag+1) in
the following oxidation halfreaction:
Since the silver rod is the site
of oxidation, it is the anode.
However, in electrolytic cells,
the anode is the positive
electrode.
(2b) Positive silver ions migrate
away from anode, towards the
cathode, which, in electrolytic
cells is the negative electrode.
(2a)The electrons produced by
the reduction of Ag are moved
through the external wire by a
power source.
(3) Electrons flowing through
the spoon cause the reduction
of silver ions in the following
reduction half-reaction:
Since reduction is occurring at
the spoon, the spoon is the
cathode. When silver ions
become reduced, they plate
(cover) the spoon.
Voltaic Cells
Similarities
Electrolytic Cells
**To determine which substance is the anode/cathode
(1) Check Table J: Activity Series
(2) The metal that is higher on the chart will be oxidized and thus is the
anode.
(3) The metal that is lower on the chart will be reduced and this is the
cathode.
**In a voltaic cell:
 Anode = negative electrode
 Cathode = positive electrode
FAT CAT
Electrons flow
From Anode To CATthode.
Salt bridge:
- connects the 2 containers & provides a path for a flow of ions between the
two beakers.
-As electrons leave one half of a galvanic cell and flow to the other, a
difference in charge is established. If no salt bridge were used, this charge
difference would prevent further flow of electrons. A salt bridge allows the
flow of ions to maintain a balance in charge between the oxidation and
reduction vessels while keeping the contents of each separate. With the
charge difference balanced, electrons can flow once again, and the reduction
and oxidation reactions can proceed.