1. 3/9/2014

2. Introduction
 Oxidation is Loss of Electrons
 Reduction is Gain of Electrons
 OIL RIG
 Substances that cause oxidation are
called oxidising agents
 E.g. O2, Cl2 [halogens], MnO41-, Cr2O72 Substances that cause reduction are
called reducing agents
 E.g. C, CO, H2

3. The number of electrons
an atom appears to
have gained or lost.
Man made system so anomalous
results can be obtained.

4. Rule 1
In free elements the oxidation
number is 0
e.g. Na = 0,
Cu = 0,
P = 0,
H2 = 0,
S8 = 0
Cl2 = 0
 A free element is an element on its own
 It is neutral so it will have neither lost nor
gained electrons

5. Rule 2
The sum of all the oxidation
numbers in a molecule is 0.
E.g
CaCO3 = 0
(NH4)2SO4 = 0

6. Rule 3
The oxidation number of a simple
ion is the charge on that ion.
E.g.
Cl- is -1, Na+ is +1
O2- is -2 Ca2+ is +2
Al3+ is +3
N3- is -3
S2- is -2

7. Rule 4
The sum of oxidation numbers in a
complex ion is the charge on the ion
2-
SO4 = -2 [total]
PO43- = -3
NH4+ = +1
NO3- = -1

8. Rule 5
In compounds containing H the Oxidation
Number of H is +1
e.g. H2O, HCl, NH4+, CH3COOH
Except in metal hydrides where it is -1
e.g. NaH, KH, CaH2 where it is -1

9. Rule 6
In compounds containing oxygen the
Oxidation Number of oxygen is -2
Except in
(i) peroxides where it is -1
e.g. H2O2, (Na2O2 and BaO2)
(ii) When bonded to F when it is +2

10. Rule 7
The Alkali Metals [Group I]
are always +1 in a compound
i.e Na, K, Li, Rb, Cs, Fr

11. Rule 8
The Alkaline Earth Metals [Group II]
are all +2 in compounds
Group II
Be, Mg, Ca, Sr, Ba, Ra

12. Rule 9
The Halogens [Group VII]
are all -1
F, Cl, Br, I
Except
when bonded to a more electronegative
element
e.g in Cl2O the Cl is +1
and in ClO2 the Cl is +4

13. Rule 10
 Oxidation is a decrease in oxidation
number
 Reduction is a gain of oxidation number
Mg + Cu2+ = Mg2+ + Cu
0
+2
+2
0
 Mg has been oxidised Ox. No. zero to +2
 Loss of 2 electrons
 Cu has been reduced Ox. No. +2 to zero
 Gain of 2 electrons

14. Examples
Sodium Oxide
Na2 O Overall
=0
+1 -2 individual
+2 -2 combined
K MnO4 Overall= 0
K Mn O4
+1 +7 -2 individual
+1 +7 -8 combined

15. Examples #2
Na2 S2 O4
?
+1 +3 -2
+2 +6 -8
Na2 S2 O3
+1 +2 -2
?
+2 +4 -6
C12 H22 O11
0 +1 -2
?
0 +22 -22

16.  Work out the oxidation number of Mn in
each of the following compounds
MnCl2
= +2 Manganese (II) chloride
MnO2
= +4
Manganese (IV) oxide
MnO4-
= +7
Manganate (VII)
Mn can have different oxidation numbers depending
on the elements it is combined with.
These are called Oxidation States
When naming transition metal compounds it is normal
to include the oxidation state in the name

17. Transition Elements
 Have variable valency
 Variable oxidation numbers [oxidation states]
 Definite colours associated with oxidation
states
 Fe2+ iron (II)
= Green [+2]
FeCl2
 Fe3+ iron (III)
= Yellow [+3]
FeCl3
 Manganese (VII) = Pink [+7]
MnO4 Manganese(IV) = Brown [+4]
MnO2
 Manganese(II) = Colourless [+2] MnCl2
 Catalytic properties [as do their compounds.]

18. Anomalies
C6 H12 O6
+1 -2
+12 -12
C appears to have
gained or lost no
electrons
Calculate the oxidation number of S in Na2S4O6
S is + 2.5
Clearly it can’t lose half an electron so this is an
anomaly.

19. Oxidation Numbers and Nomenclature
 Compounds containing two elements
end in ide
 if in a compound an element has more
than one oxidation state then the
oxidation state of the least
electronegative element is stated.
 e.g. Cu2O is copper(I) oxide
 CuO is copper (II) oxide
 PbO is Lead(II) oxide
 PbO is lead(IV) oxide

20. Oxidation numbers and Nomenclature
MnO2 is manganese(IV) oxide
1- is
MnO4
manganate(VII)