Electrochemistry #1
Redox Review

A reaction in which electrons are
transferred from one atom to another is
called an oxidation–reduction
reaction.
Example

Determine what is oxidized, reduced, the
oxidizing agent, & reducing agent

HCl + HNO3  HOCl + NO + H2O
Half Reaction

Half reaction – one of the two parts of a
redox reaction

One half will be oxidation

One half will be reduction
Step to Balancing Half Reaction
Method
Separate the reactions into 2 half reactions
(one for oxidation & one for reduction)
2. Balance each ½ reaction un the following
order:
1.
a)
b)
c)
d)
Balance elements other than H & O
Balance O by adding H2O
Balance H by adding H+
Balance the charge to make each side equal
Step to Balancing Half Reaction
Method
3.
4.
5.
6.
7.
Multiply each ½ reaction by a number
to make the number of electrons
gained = the number of electrons lost
Add the ½ reactions
Simplify
Check your work
Tip – Break things into ions or binary
compounds & keep weak acids &
bases together!
Example
•Fe + CuSO4  Cu + Fe2(SO4)3





Fe  2Fe +3
2Fe  2Fe +3
0  +6
0  +6 (+6e-)
2Fe  2Fe +3 +6e-
Cu +2  Cu
 +2  0
 +2 + 2e-  0
 (Cu +2 + 2e-  Cu ) 3
 3Cu +2 + 6e-  3Cu

2Fe + 3Cu +2 + 6ex  2Fe +3 +6ex + 3Cu
2Fe + 3Cu +2  2Fe +3 + 3Cu
Another Example
AsO4 -3 + Zn  H3As + Zn +2




AsO4 -3  H3As
AsO4 -3  H3As + 4H2O
11 H+ +AsO4 -3  H3As + 4H2O

+11 -3
+8
(+8 e-)

11 H+ +AsO4 -3 + 8e-  H3As + 4H2O

0
0
0
Zn  Zn +2
 0
+2

(+2 e-)
( Zn  Zn +2 +2 e- )4
 4Zn  4Zn +2 +8 e
11H+ +AsO4 -3+ 8e- + 4Zn H3As + 4H2O + 4Zn +2 + 8 e11H+ +AsO4 -3+ 4Zn H3As + 4H2O + 4Zn +2
Try This one…
-2
 Mn +2 + CO2

MnO4 -1 + C2O4

2MnO4 -1+16 H++ 5C2O4-2  2Mn+2 +8H2O + 10CO2
Last one…

KMnO4 + HCl  KCl + MnCl2 + H2O + Cl2

6 H+ + 2MnO4- + 10 HCl  2Mn +2 +8H2O + 5Cl2
Balancing Basic

Balancing in basic solution is just like
acidic with one additional step

When balancing in acidic you will have
H+ at the end

In basic you don’t want H+, you want OH-
To change to basic

To change to basic, simply add an
equivalent number of OH- to each side
and simplify
Example

Balancing the following by using the ½ reaction
method in a basic solution
 AsO4-3 + Zn  H3As + Zn +2
 Acidic:

11H+ + AsO4-3 + 4Zn  H3As + 4 H2O + 4Zn+2

Basic:

11H+ + AsO4-3 + 4Zn  H3As + 4 H2O + 4Zn+2
+11 OH+ 11OH-



11 H2O + AsO4-3 + 4Zn  H3As + 4 H2O + 4Zn+2+ 11OH7 H2O + AsO4-3 + 4Zn  H3As + 4Zn+2+ 11OH-
Another Example
Ag + NO3-  Ag + + N+2
 Acidic:
 3Ag +6 H+ + NO3-  3Ag + + N+2+ 3H2O
 Basic:
 3Ag +6 H+ + NO3-  3Ag + + N+2+ 3H2O

+ 6OH+ 6OH


3Ag +6H2O + NO3-  3Ag + + N+2+ 3H2O + 6OH3Ag +3H2O + NO3-  3Ag + + N+2 + 6OH-
One More
Mn(NO3)2 + NaBiO3 + HNO3  NaMnO4 +
Bi(NO3)3 + H2O + NaNO3
 Acidic:

2Mn+2 + 14H+ + 5 BiO3-  2MnO4- + 5Bi +3 + 7H2O
 Basic:
 2Mn+2 + 14H+ + 5 BiO3-  2MnO4- + 5Bi +3 + 7H2O
+ 14 OH
+ 14 OH


2Mn+2 + 14H2O + 5 BiO3-  2MnO4- + 5Bi +3 + 7H2O+ 14 OH2Mn+2 + 7H2O + 5 BiO3-  2MnO4- + 5Bi +3 + 14 OH-
MEMORIZE!
Redox Reaction Types

Watchout for:

Keywords "acidified solution" or an acid included in the
reactants.

Anytime you see a neutral element, Cu°, O2, H2, etc. it
must be redox.

When you recognize great oxidizers like Cr2O72-,
MnO4-, and MnO2
Reaction Example 1

Solid copper reacts with dilute nitric acid
solution
Reaction Example 2

A solution of potassium permanganate is
mixed with an alkaline solution of sodium
sulfite
Reaction Example 3

Hydrogen peroxide is added to a solution
of iron (II) sulfate