1. 2.6.1 Oxidation Numbers
4/17/2019

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 P = 0, H2 = 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.
peter jackson
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
SO42- = -2 [total]
PO43- = -3
NH4+ = +1
NO3- = -1

8. Rule 5 Except in metal hydrides where it is -1
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 i.e Na, K, Li, Rb, Cs, Fr 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
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
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 K MnO4 K Mn O4 individual combined +1 +7 -2 +1 +7 -8 Overall
Sodium Oxide Na2 O
Overall
= 0 +1 -2
individual +2 -2
combined
K MnO4
K Mn O4
individual
combined
Overall
= 0 +1 +7 -2 +1 +7 -8

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

16. Work out the oxidation number of Mn in each of the following compounds
MnCl2
= +2
Manganese (II) chloride
MnO2
Manganese (IV) oxide
= +4
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 O
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
PbO2 is lead(IV) oxide

20. Oxidation numbers and Nomenclature
MnO2 is manganese(IV) oxide
MnO41- is manganate(VII)