Balancing
Redox
Equations
1. Identify The Following Reaction
as Redox or Non-Redox
Na+ + Cl- + Ag+ + NO3- → Na+ + NO3- + AgCl
This reaction is precipitation, therefore,
it is not a redox reaction.
We have learned that the following
reaction types are redox:
Synthesis Single replacement
Combustion
Decomposition
2. Identify the following as redox or
non-redox.
SO2 + H2O → H2SO3
Synthesis, but NOT redox
To be a redox reaction, electrons
must be exchanged and there must be
a change in oxidation states. You
must always look at the oxidation
states of all reactants and products.
Balancing Redox Reaction
When a reaction describes a chemical
change that involves a change in oxidation
states, the number of atoms AND the
charges of reactants and products must
balance.
Example:
Ce4+(aq) + Sn2+(aq) → Ce3+(aq) + Sn4+(aq)
The number of atoms balance easily,
however, the charges do not.
Redox Reactions
Some redox reactions are easily
balanced by inspection but most
require a more systematic method.
Before balancing a redox reaction, a
molecular equation must be written
and the environment must be
identified as either acidic or basic
Steps in Balancing
Redox
Reactions by the HalfReaction Method
Before balancing a redox reaction,
you must first identify the oxidation state
changes for the oxidized and reduced
species. (These common oxidation state
chances should be learned from your
reaction reference) Next, you must
decide whether it occurs in an acid or
basic solution
•
Acidic = hydrogen cations
available in solution
• Basic = hydroxide ions
available in solution
Step 2For each half-reaction:
 Balance all elements except
H and O
 Balance O by adding H2O
 Balance H by adding H+
 Balance charge by adding
electrons
Step 4Add the half-reactions
together and cancel
identical species from
both sides.
Reactions in Acidic Solution
Step 1Write separate equations
for the oxidation and
reduction half reactions
from the net ionic
equation
Step 3Equalize the number of
electrons in the 2 half
reactions by multiplying
one or both balanced half
reactions by an integer
Step 5Check that all ELEMENTS
and CHARGES balance
Example:
Cu(s) + NO3-(aq)

H

Cu2+(aq) + NO2(g)
Remember the Chem. Of Cu Lab?
Reactions in Basic Solution
Step 1Completely balance the
reaction as described
previously for reactions
in an acidic solution
Step 3Cancel water molecules
appearing on both sides
of the reaction.
Step 2 –
Add OH- ions to both sides
equal to the number of H+
ions present (goal is to
add hydroxide to one side
and make waters on the
other side)
Step 4Check that all ELEMENTS and
CHARGES balance
Example:
Am3+(aq) + S2O82-(aq) → AmO2+(aq) + SO42-(aq)
Practice:

AuCl4-(aq) + AsH3(g) OH


H3AsO3(aq) + Au(s) + Cl-(aq)
By knowing some common
oxidation state changes, products in
redox reactions can be predicted.
Example:
What happens when chlorine gas is
bubbled through a strong base?
(rxn. ref.)