Transcript Electrochemistry

Balancing Redox
Equations
Balancing Redox Equations

In balancing redox equations, the # of
electrons lost in oxidation (the increase in
ox. #) must equal the # of electrons
gained in reduction (the decrease in ox. #)

There are 2 methods for balancing redox
equations:
1. Change in Oxidation-Number Method
2. The Half-Reaction Method
1. Change in Oxidation-Number Method:

based on equal total increases and decreases in
oxidation #’s
Steps: page 645 in textbook
1. Write equation and assign oxidation #’s.
2. Determine which element is oxidized and which is reduced,
and determine the change in oxidation # for each.
3. Connect the atoms that change ox. #’s using a bracket; write
the change in ox. # at the midpoint of each bracket.
4. Choose coefficients that make the total increase in ox. # = the
total decrease in ox. #.
5. Balance the remaining elements by inspection.
Example
+4
3
S
0
+
4
So, 3 coefficient x +4 = +12
HNO3 
+1 +5 -2
Oxidation
numbers
SO2 + 4 NO
+4 -2
-3
Step 1:
3
Step 2:
+2 -2
+
2
H 2O
+1
-2
So, 4 coefficient x -3 = -12
S. By how much?
0 to +4 = change of +4
Which was oxidized?
Which was reduced?
N. By how much?
+5 to +2 = change
of -3
If needed, reactions that take place in acidic or
basic solutions can be balanced as follows:
Acidic:
Basic:
add H2O to the side
needing oxygen
balance as if in acidic sol’n
then add H+ to balance the add enough OH- to both
hydrogen
sides to cancel out each
H+ (making H2O) & then
cancel out water as
appropriate
Example:
Balance the following equation,
assuming it takes place in acidic solution.
Page 648 in textbook
+1
ClO4
+7
-2
Step 1:
Oxidation
numbers
-
+
8
I
+8 H+
-1
-8
Step 2: Which was
oxidized?
Which was reduced?
Step 5: Balance acid soln with
water…

Cl
+
-1
Iodine, -1 to 0 = +1
Chlorine, +7 to -1 = -8
4
I2
0
+ 4 H2 O
Example:
Balance the following equation,
assuming it takes place in basic solution.
ClO4
-
+
I

Cancel H+ by adding OH- to both sides.
H+ + OH- = H2O
Cancel H2O
Cl
+
I2
2. The Half-Reaction Method:

separate and balance the oxidation and
reduction half-reactions.
Steps:
1. Write equation and assign oxidation #’s.
2. Determine which element is oxidized and which is reduced,
and determine the change in oxidation # for each.
3. Construct unbalanced oxidation and reduction half reactions.
4. Balance the elements and the charges (by adding electrons
as reactants or products) in each half-reaction.
5. Balance the electron transfer by multiplying the balanced halfreaction by appropriate integers.
6. Add the resulting half-reaction and eliminate any common
terms to obtain the balanced equation.
Example: Balance the following
using the half-reaction method:
HNO3 + H2S 
NO
+
S
+
H 2O
Example: Balance the following
using the half-reaction method:
2
HNO3 + 3 H2S 
+1 +5 -2
Step 1: oxidation
numbers
+1 -2
2
NO
+3 S
+2 -2
S. -2 to 0 = +2
Which was
reduced?
N. +5 to +2 = -3
4
H 2O
0
+1 -2
Step 3: unbalanced half-rxns
x3 S2-  S + 2 eStep 2: Which was
oxidized?
+
3S2-  3S + 6e-
x2 N5+ + 3 e- N2+ 2 N5+ + 6e-  2 N2+
Step 4: balance the
half – rxns by adding
electrons
---------------------------------3S2- + 2N5+  3S + 2N2+
Step 6: Add half-rxns and
Step 5: balance electron cancel any common terms
transfer by multiplying by to obtain a balanced eq.
appropriate integers
Now, balance the eq.
w/coefficients
If needed, reactions that take place in acidic
solutions can be balanced as follows:
Acidic:
1.
2.
3.
4.
5.
Write separate eq’ns for the oxidation & reduction half-rxns
For each half-rxn:
a)
Balance all the elements except H and O
b)
Balance O using H2O
c)
Balance H using H+
d)
Balance charge using elections
If necessary, multiply one or both balanced half-rxns by an integer
to equalize the number of electrons transferred in the two half-rxns.
Add the half-reactions and cancel the identical species (those
appearing in reactants and products)
Check that the elements and charges are balanced
If needed, reactions that take place in basic
solutions can be balanced as follows:
Basic:
1.
Balance as if in acidic sol’n (follow ALL steps for acidic redox)
2.
Add enough OH- to both sides to cancel out each H+ (making H2O)
& then cancel out water as appropriate
3.
Check that the elements and charges are balanced
HOMEWORK:
Balance the following using the
half-rxn method…
In acidic sol’n:
a) Cu + NO3-  Cu2+ + NO
b) Cr2O72- + Cl-  Cr3+ + Cl2
c) Pb + PbO2 + H2SO4  PbSO4
In basic sol’n:
a) Al + MnO4-  MnO2 + Al(OH)4b) Cl2  Cl- + OClc) NO2- + Al  NH3 + AlO2-