Unit 5:
OxidationReduction
Reactions
By Brittany Behrens &
Joanna McConnell
What is Redox?
An oxidation-reduction (redox) reaction is a
chemical reaction in which
electrons are transferred between reactants.
LEO says GER:
Loss of Electrons is Oxidation
Gain of Electrons is Reduction
Oxidation and reduction cause a
change in the oxidation number of each element
involved in the redox reaction.
Rules for Redox: Oxidation Numbers
1. The oxidation number of an atom in an uncombined element is zero.
2. The oxidation number of a monatomic ion is its ionic charge.
3. The algebraic sum of the oxidation numbers of all of the atoms in a
neutral compound is zero.
4. The algebraic sum of the oxidation numbers of all of the atoms in a
polyatomic ion is the net ionic charge.
5. Fluorine = -1
Elements that form monatomic ions often have their
charge = oxidation number
Group 1 = +1
Group 2 = +2
Group 7 = -1
H is usually +1
O is usually –2
Peroxides are a notable exception: in H2O2, H=+1 and O=-1.
Oxidation Number Example
Consider this compound: K2Cr2O7
Step 1:
Use oxidation number rules to assign numbers
to as many elements as possible.
Group 1 elements, including K, have oxidation number +1
Oxygen has oxidation number -2
The neutral compound’s sum of oxidation numbers = 0
Step 2:
Solve for the oxidation number of Cr.
2(+1) + 2(x) + 7(-2) = 0  2x = 12 
x=6
Thus, the oxidation number of K=+1, O=-2, and Cr=+6.
Balancing Redox Equations
Consider this redox reaction:
Dichromate ions and Iron (II) ions react in acid.
Step 1:
Write a half-reaction for each of the reactants.
Cr2O72-  Cr3+
Fe2+  Fe3+
Balance for Mass
Step 2:
Since the reaction is being balanced in acid,
add H+ and H2O as needed to make sure
that the same number of each element is
present on each side of each half-reaction.
14H+ + Cr2O72-  2 Cr3+ + 7H2O
Fe2+  Fe3+
Balance for Charge
Step 3:
Since this is a redox reaction,
add electrons (e-) as needed to make sure
that the net charge of each half-reaction is ZERO.
6e- +14H+ + Cr2O72-  2Cr3+ + 7H2O
Fe2+  Fe3+ + e-
Add the Half-Reactions
Step 4:
Multiply the half-reactions by
coefficients to cancel out the electrons.
Add the two half-reactions to determine
the equation for the net reaction.
6e- + 14H+ + Cr2O72-  2Cr3+ + 7H2O
6 Fe2+ 6Fe3++ 6 e14H+ + Cr2O72- + 6Fe2+  2Cr3+ + 6Fe3+ + 7H2O
Check your Work
Word Equation:
Dichromate ions and Iron (II) ions react in acid.
Balanced Equation:
14H+ + Cr2O72- + 6Fe2+  2Cr3+ + 6Fe3+ + 7H2O
Is mass conserved?
Hydrogen:
Chromium:
Oxygen:
Iron:
14 atoms on the left, 7(2)=14 atoms on the right
2 atoms on the left, 2 atoms on the right
7 atoms on the left, 7 atoms on the right
6 atoms on the left, 6 atoms on the right
Is charge conserved?
14(+1) + 1(-2) + 6(+2) = 2(+3) + 6(+3) + 7(0)
14-2+12 = 6+18  24 = 24
Basic Balancing
Step 5:
If the reaction is to be balanced in a basic solution,
follow the steps for balancing in acid,
then add OH- as needed.
14H+ + Cr2O72- + 6Fe2+  2Cr3+ + 6Fe3+ + 7H2O
+14OH-
+14OH-
14H2O + Cr2O72- + 6Fe2+  2Cr3+ + 6Fe3+ + 7H2O + 14OH7
7H2O + Cr2O72- + 6Fe2+  2Cr3+ + 6Fe3+ + 14OH-
Applications: Electrochemistry
AnOx
RedCat
e- flow
Voltmeter
Anode
Oxidation
Loses Mass
Cathode
Reduction
Gains Mass
Salt Bridge (KNO3)
Al
Ag
K+
NO3-
Look up Eo values in a
table of standard
reduction potentials.
Al3+
Al(s)  Al3+(aq) + 3eAg+(aq) + e-  Ag(s)
Ag+
LEO
GER
If Eorxn is
positive, the
reaction is
spontaneous
Eo = +1.66 V
Eo = +0.80 V
Al(s) + 3Ag+(aq)  3Ag(s) + Al3+(aq)
Eorxn = +2.46 V
The Nernst Equation
Standard
Reduction
Potential (V)
Nonstandard
Reduction
Potential (V)
o
E=E
Moles of etransferred
RT
– nF
8.31 J/Kmol
Temperature (K)
lnQ
Reaction Quotient:
[products]
[reactants]
96,500 C/mol e-
Remember…
For Oxidation-Reduction Reactions,
 A redox reaction is a chemical reaction in which
electrons are transferred between reactants.
 LEO says GER
 AnOx, RedCat
 The Nernst Equation (given on the AP exam)
Remember to Study for the AP Exam
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