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OXIDATION AND REDUCTION
Chapter 14
Oxidation and Reduction
Oxidation - addition of Oxygen.
Reduction - removal of Oxygen.
In terms of electrons
Oxidation is the loss of electrons.
(OILRIG)
Reduction is the gain of electrons
Oxidising and reducing
agents
An oxidising agent causes another substance to become
oxidised and, in the process, the oxidising agent is
reduced. Common oxidising agents include O2, Cl2, ClO-,
KMnO4 and H2O2
A reducing agent causes another substance to become
reduced and, in the process, the reducing agent is
oxidised. Common reducing agents include CO, SO2,
SO32Oxidising agents are useful in Sterilising and Bleaching.
Experiments: (a) Halogens as Oxidising agents
(b) Displacement reactions of
Metals.
(See Book Pages 184-186)
Oxidation Numbers
The oxidation number of an atom is the charge that the
atom has or appears to have when electrons are
distributed according to certain rule.
Rules
The ON of an element is 0.
The ON of an ion is the charge that it has.
The ON of F is always -1.
The ON of O IS -2 except OF2 (+2) and H2O2 (-1).
The ON of H is +1 except with metal hydrides (-1)
Oxidation and reduction in terms of
Oxidation numbers
Oxidation is an increase in Oxidation number.
Reduction is a decrease in Oxidation number.
 Balancing Redox reactions (See Book Page 187-188) +
HANDOUT
2.6.2
Balancing
equations
using
Oxidation
Numbers
 Chemical reactions are all about the




movement of electrons
When an equation is balanced the
electron movements must balance
Sometimes it is possible to balance an
equation in more than one way
It is therefore possible to make the
numbers of atoms balance but not the
electron movement
Using oxidation numbers prevents this
Sequence of events
 Write out the formulae of all reactants and






products
Assign oxidation numbers to all atoms
Pick out the atoms whose oxidation numbers
change
Write down the electron movements
Balance the electron movements
Enter the numbers from balanced electron
movement into the overall equation
Balance equation by inspection - [do H last]
1. Assign oxidation
2. Select those which
numbers
change
3. (a) Assign Oxidation – increase in
Ox. No
(b) Assign Reduction – decrease
o
+
2+
in
Ox.
N
H = Mn + 5 Fe3+ +4 H O
MnO4- + 5 Fe2+ + 8
+7 -2
+2
+1
+7
+2
Reduction
+2
+2
+3
+3
+1 -2
Oxidation
5. Write electron gains and
losses
Mn + 5 e- = Mn2+
+7
2
+2
6. Balance electron
transfers
5 Fe2+- 51 e- = 5 Fe3+
+2
7. Put numbers up into the
equation
8. Balance equation by inspection - leave H+ till last
+3
Sequence of events







Write down reactants and products
Assign oxidation numbers
Identify oxidation and reduction
Write down changes and electrons involves
Balance e- change
Put changes into equation
Balance equation by inspection [H last]
1. Assign oxidation
2. Select those which
numbers
change
2-
3 C2H5OH +2 Cr2O7
H2O
-2-2 +1-2+1 +6+6 -2
3. (a) Assign Oxidation – increase in
Ox. No
3++
Assign
Reduction
–Cr
decrease
+16 H+ =(b)
COOH
+
3 CH
4
11
3
o
in Ox. N
2
Oxidation
5. Write electron gains and
losses
-
24 e = 3 C2 x3
3 C2 - 12
-2
+1
+3
00 +100 -2-2+1 +3
Reduction
+1-
6. Balance electron
transfers
0
2 Cr2O72- + 12
63 e- = 42 Cr3+ x2
+6
+3
7. Put numbers up into the
equation
8. Balance equation by inspection - leave H+ till last :- O 3+(2*7) =
(3*2) + x  x=11
Examples
1. C2H5OH + MnO41- + H+ = CH3CHO + Mn2+ + H2O
2. C2H5OH + MnO41- + H+ = CH3COOH + Mn2+ + H2O
3. Cr2O72- + Fe2+ + H+ = Cr3+ + Fe3+ + H2O
4. MnO41- + C2O42- + H+ = CO2 + Mn2+ + H2O
5. Fe2+ + Cl2 = Fe3+ + Cl6. MnO41- + H2O2 + H+ = Mn2+ + O2 +H2O
7. Cr2O72- + CH3OH + H+ = Cr3+ + HCHO + H2O
8. Cr2O72- + CH3CHO + H+ = Cr3+ + CH3COOH + H2O
Answers
1. 5C2H5OH + 2MnO41- + 6H+ = 5CH3CHO + 2Mn2+ + 8H2O
2. 5C2H5OH + 4MnO41- + 12H+ = 5CH3COOH + 4Mn2+ +
11H2O
3. Cr2O72- + 6Fe2+ + 14H+ = 2Cr3+ + 6Fe3+ + 7H2O
4. 2MnO41- + 5C2O42- + 16H+ = 10CO2 + 2Mn2+ + 8H2O
5. 2Fe2+ + Cl2 = 2Fe3+ + 2Cl6. 2MnO41- + 5H2O2 + 8H+ = 2Mn2+ + 5O2 +8H2O
7. Cr2O72- + CH3OH + H+ = Cr3+ + HCHO + H2O
8. Cr2O72- + CH3CHO + H+ = Cr3+ + CH3COOH + H2O