Download 9.2 Balancing Redox Reaction Equations

Balancing Redox Reaction Equations
Chapter 9.2
Two Methods for Balancing
1. The Half-Reactions Method
2. The Oxidation Numbers Method
• Both methods are based on the idea that electrons
are neither created or destroyed during a redox
reaction. They are simply transferred from the
species being oxidized to the species being reduced
• You must be able to balance redox reaction
equations using both methods
• You must be able to balance redox reaction
equations in both acidic solution and basic solution
The Half-Reactions Method
1. Identify and write down the unbalanced half-reactions (do this
by first assigning oxidation numbers)
2. Balance atoms and charges in each half-reaction (use H2O to
balance O, H+ to balance H, and e- to balance charge)
3. Multiply each half reaction by an integer such that the number
of electrons cancel
4. Add the half-reactions and cancel
5. For reactions that occur in basic solution follow this additional
step: Add OH- to both sides for every H+ present, combining to
yield H2O on the H+ side then cancel if necessary
The Half-Reactions Method
CuS(s) +NO3-(aq)
Cu2+(aq) +SO42-(aq) +NO(g)
The Oxidation Numbers Method
1.
Assign oxidation numbers and determine the atom that is oxidized and
the atom that is reduced
2.
Determine the number of electrons lost or gained per atom
3.
Determine the number of electrons lost or gained per reactant
molecule
4.
Multiply by an integer such that the number of electrons cancels
5.
Use these integers as coefficients for the reactants
6.
Balance atoms (use H2O to balance O and H+ to balance H)
7.
For reactions that occur in basic solution follow this additional step:
Add OH- to both sides for every H+ present, combining to yield H2O on
the H+ side then cancel if necessary
The Oxidation Numbers Method
BH4- +ClO3-
H2BO3- +Cl-
HOMEWORK
Required Reading:
p. 608-617
(remember to supplement your notes!)
Questions:
p. 613 #1-6
p. 616 #1,2
p. 617 #1-10