The document discusses oxidation and reduction reactions. It defines oxidation as the loss of electrons or an increase in oxidation state, and reduction as the gain of electrons or a decrease in oxidation state. Redox reactions involve both oxidation and reduction occurring simultaneously. Oxidation states can be used to identify what is oxidized and reduced in a reaction. The document provides examples of calculating oxidation states in various compounds and ions using rules like the sum of oxidation states equaling the overall charge.

1. IB Chemistry Power Points
Topic 09
Oxidation and Reduction
www.pedagogics.ca

2. REDOX
A guide for A level students
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
Much taken from

3. OXIDATION & REDUCTION – Simplified Definitions
OXIDATION

4. OXIDATION & REDUCTION – Simplified Definitions
OXIDATION
is a GAIN OF OXYGEN
2Mg + O2 ——> 2MgO
magnesium has been oxidised as it has gained oxygen

5. OXIDATION & REDUCTION – Simplified Definitions
OXIDATION
is a GAIN OF OXYGEN
2Mg + O2 ——> 2MgO
magnesium has been oxidised as it has gained oxygen
is the REMOVAL (LOSS) OF HYDROGEN
C2H5OH ——> CH3CHO + H2
ethanol has been oxidised as it has ‘lost’ hydrogen

6. OXIDATION & REDUCTION – Simplified Definitions
REDUCTION

7. OXIDATION & REDUCTION – Simplified Definitions
REDUCTION
is a GAIN OF HYDROGEN
C2H4 + H2 ——> C2H6
ethene has been reduced as it has gained hydrogen

8. OXIDATION & REDUCTION – Simplified Definitions
REDUCTION
is a GAIN OF HYDROGEN
C2H4 + H2 ——> C2H6
ethene has been reduced as it has gained hydrogen
is the REMOVAL (LOSS) OF OXYGEN
CuO + H2 ——> Cu + H2O
copper(II) oxide has been reduced as it has ‘lost’ oxygen
However as chemistry became more sophisticated, it
was realised that another definition was required

9. ...
OXIDATION AND REDUCTION IN TERMS OF ELECTRONS
Oxidation and reduction are not only defined as changes in O and H
OXIDATION
REDUCTION
REDOX
OXIDATION & REDUCTION – Better Definitions

10. ...
OXIDATION AND REDUCTION IN TERMS OF ELECTRONS
Oxidation and reduction are not only defined as changes in O and H
OXIDATION Removal (loss) of electrons ‘OIL’
species will get less negative or more positive
REDUCTION
REDOX
OXIDATION & REDUCTION – Better Definitions

11. ...
OXIDATION & REDUCTION – Better Definitions
OXIDATION AND REDUCTION IN TERMS OF ELECTRONS
Oxidation and reduction are not only defined as changes in O and H
OXIDATION Removal (loss) of electrons ‘OIL’
species will get less negative or more positive
REDUCTION Gain of electrons ‘RIG’
species will become more negative or less positive
REDOX When reduction and oxidation take place

12. OIL - Oxidation Is the Loss of electrons
RIG - Reduction Is the Gain of electrons

13. OXIDATION NUMBERS (STATES)
Used to... tell if oxidation or reduction has taken place
work out what has been oxidised and/or reduced
construct half equations and balance redox equations
FOR ATOMS AND SIMPLE IONS
The number of electrons which must be added or removed to become neutral
Atoms Na = 0 neutral already ... no need to add any electrons
Cations Na+ = +1 need to add 1 electron to make Na+ neutral
Anions Cl¯ = -1 need to take 1 electron away to make Cl¯ neutral
Q. What are the oxidation states of the elements in the following?
a) C b) Fe3+ c) Fe2+
(0) (+3) (+2)
d) O2- e) He f) Al3+
(-2) (0) (+3)

14. OXIDATION STATES
ELEMENTS H in H2 = 0 both are the same and must add up to Zero
COMPOUNDS C in CO2 = +4
Explanation
MOLECULES
The SUM of the oxidation states adds up to ZERO
O in CO2 = -2 1 x +4 and 2 x -2 = Zero
• because CO2 is a neutral molecule, the sum of the oxidation states must be ?
• for this, one element must have a positive OS and the other must be ?
HOW DO YOU DETERMINE THE VALUE OF AN ELEMENT’S OXIDATION STATE?
• from its position in the periodic table and/or
• the other element(s) present in the formula (oxygen is almost always -2 etc)
HOW DO YOU DETERMINE WHICH IS THE POSITIVE ONE?
• the more electronegative species will have the negative value

15. OXIDATION STATES
COMPLEX IONS
The SUM of the oxidation states adds up to THE CHARGE
- sum of the oxidation states = - 1
2- sum of the oxidation states = - 2
+ sum of the oxidation states = +1
2- the oxidation state of S = +6 there is ONE S
in SO4
O = -2 there are FOUR O’s
+6 + 4(-2) = -2 so the ion has a 2- charge
e.g. NO3
SO4
NH4
Example SO4
2-

16. OXIDATION STATES
- sum of the oxidation states = - 1
2- sum of the oxidation states = - 2
+ sum of the oxidation states = +1
What is the oxidation number of Mn in MnO4¯ ?
• the oxidation state of oxygen in most compounds is - 2
• there are 4 O’s so the sum of its oxidation states - 8
• overall charge on the ion is - 1
• therefore the sum of all the oxidation states must add up to - 1
• the oxidation states of Mn four O’s must therefore equal - 1
• therefore the oxidation state of Mn in MnO4¯is +7
+7 + 4(-2) = - 1
COMPLEX IONS
The SUM of the oxidation states adds up to THE CHARGE
e.g. NO3
SO4
NH4
Example

17. OXIDATION STATES
CALCULATING OXIDATION STATE - 1
Many elements can exist in more than one oxidation state
In compounds, certain elements are used as benchmarks to work out other values
HYDROGEN +1 except 0 atom (H) and molecule (H2)
-1 hydride ion, H¯ in sodium hydride NaH
OXYGEN -2 except 0 atom (O) and molecule (O2)
-1 in hydrogen peroxide, H2O2
+2 in F2O
HALOGENS -1 except 0 atom (X) and molecule (X2)
Q. Give the oxidation state of the element other than O, H or F in...
+ IF7 Cl2O7
SO2 NH3 NO2 NH4
2- S2O3
NO3¯ NO2¯ SO3
2- S4O6
2- MnO4
2-
2- ?
What is odd about the value of the oxidation state of S in S4O6

18. OXIDATION STATES
A. The oxidation states of the elements other than O, H or F are
SOO = -2 2 x -2 = - 4 overall neutral S = +4
2 NHH = +1 3 x +1 = +3 overall neutral N = - 3
3 NOO = -2 2 x -2 = - 4 overall neutral N = +4
2 NH+ H = +1 4 x +1 = +4 overall +1 N = - 3
4
IFF = -1 7 x -1 = - 7 overall neutral I = +7
7 ClOO = -2 7 x -2 = -14 overall neutral Cl = +7
27 NO¯ O = -2 3 x -2 = - 6 overall -1 N = +5
3NO¯ O = -2 2 x -2 = - 4 overall -1 N = +3
2SO2- O = -2 3 x -2 = - 6 overall -2 S = +4
3
2- O = -2 3 x -2 = - 6 overall -2 S = +2
S2O3
2- O = -2 4 x -2 = - 8 overall -2 Mn = +6
MnO4

19. OXIDATION STATES
CALCULATING OXIDATION STATE - 2
The position of an element in the periodic table can act as a guide
METALS • have positive values in compounds
• value is usually that of the Group Number Al is +3
• where there are several possibilities the
values go no higher than the Group No. Sn can be +2 or +4
Mn can be +2,+4,+6,+7
NON-METALS • mostly negative based on their usual ion Cl usually -1
• can have values up to their Group No. Cl +1 +3 +5 or +7

20. OXIDATION STATES
CALCULATING OXIDATION STATE - 2
The position of an element in the periodic table can act as a guide
METALS • have positive values in compounds
• value is usually that of the Group Number Al is +3
• where there are several possibilities the
values go no higher than the Group No. Sn can be +2 or +4
Mn can be +2,+4,+6,+7
NON-METALS • mostly negative based on their usual ion Cl usually -1
• can have values up to their Group No. Cl +1 +3 +5 or +7
Q. What is the theoretical maximum oxidation state of the following elements?
Na P Ba Pb S Mn Cr
What will be the usual and the maximum oxidation state in compounds of?
Li Br Sr O B N +1

21. OXIDATION STATES
CALCULATING OXIDATION STATE - 2
The position of an element in the periodic table can act as a guide
A. What is the theoretical maximum oxidation state of the following elements?
Na P Ba Pb S Mn Cr
+1 +5 +2 +4 +6 +7 +6
What will be the usual and the maximum oxidation state in compounds of?
Li Br Sr O B N
USUAL +1 -1 +2 -2 +3 -3 or +5
MAXIMUM +1 +7 +2 +6 +3 +5

22. OXIDATION STATES
THE ROLE OF OXIDATION STATE IN NAMING SPECIES
To avoid ambiguity, the oxidation state is often included in the name of a species
manganese(IV) oxide shows that Mn is in the +4 oxidation state in MnO2
sulphur(VI) oxide for SOS is in the +6 oxidation state
3 dichromate(VI) for CrO2- Cr is in the +6 oxidation state
27
phosphorus(V) chloride for PCl5 P is in the +5 oxidation state
phosphorus(III) chloride for PCl3 P is in the +3 oxidation state
Q. Name the following... PbO2
SnCl2
SbCl3
TiCl4
BrF5

23. OXIDATION STATES
THE ROLE OF OXIDATION STATE IN NAMING SPECIES
To avoid ambiguity, the oxidation state is often included in the name of a species
manganese(IV) oxide shows that Mn is in the +4 oxidation state in MnO2
sulphur(VI) oxide for SOS is in the +6 oxidation state
3 dichromate(VI) for CrO2- Cr is in the +6 oxidation state
27
phosphorus(V) chloride for PCl5 P is in the +5 oxidation state
phosphorus(III) chloride for PCl3 P is in the +3 oxidation state
Q. Name the following... PbO2 lead(IV) oxide
SnCl2 tin(II) chloride
SbCl3 antimony(III) chloride
TiCl4 titanium(IV) chloride
BrF5 bromine(V) fluoride

24. REDOX REACTIONS
OXIDATION AND REDUCTION IN TERMS OF ELECTRONS
Oxidation and reduction are not only defined as changes in O and H
REDOX When reduction and oxidation take place
OXIDATION Removal (loss) of electrons ‘OIL’
species will get less negative or more positive
REDUCTION Gain of electrons ‘RIG’
species will become more negative or less positive
+7
+6
+5
+4
+3
+2
+1
0
-1
-2
-3
-4
R
E
D
U
C
T
I
O
N
O
X
I
D
A
T
I
O
N

25. REDOX REACTIONS
OXIDATION AND REDUCTION IN TERMS OF ELECTRONS
Oxidation and reduction are not only defined as changes in O and H
REDOX When reduction and oxidation take place
OXIDATION Removal (loss) of electrons ‘OIL’
species will get less negative or more positive
REDUCTION Gain of electrons ‘RIG’
species will become more negative or less positive
REDUCTION in O.N. Species has been REDUCED
e.g. Cl is reduced to Cl¯ (0 to -1)
INCREASE in O.N. Species has been OXIDISED
e.g. Na is oxidised to Na+ (0 to +1)
+7
+6
+5
+4
+3
+2
+1
0
-1
-2
-3
-4
R
E
D
U
C
T
I
O
N
O
X
I
D
A
T
I
O
N

26. REDOX REACTIONS
OXIDATION AND REDUCTION IN TERMS OF ELECTRONS
REDUCTION in O.S. INCREASE in O.S.
Species has been REDUCED Species has been OXIDISED
Q. State if the changes involve oxidation (O) or reduction (R) or neither (N)
Fe2+ —> Fe3+
I—> I¯
2 F—> FO
2 2CO2- —> CO24
2
HO—> O22 2
HO—> HO
22 2CrO2- —> Cr3+
27
Cr2O7
2- —> CrO4
2-
SO4
2- —> SO2

27. REDOX REACTIONS
OXIDATION AND REDUCTION IN TERMS OF ELECTRONS
REDUCTION in O.S. INCREASE in O.S.
Species has been REDUCED Species has been OXIDISED
Q. State if the changes involve oxidation (O) or reduction (R) or neither (N)
Fe2+ —> Fe3+ O +2 to +3
I2 —> I¯ R 0 to -1
F2 —> F2O R 0 to -1
C2O4
2- —> CO2 O +3 to +4
H2O2 —> O2 O -1 to 0
H2O2 —> H2O R -1 to -2
Cr2O7
2- —> Cr3+ R +6 to +3
Cr2O7
2- —> CrO4
2- N +6 to +6
SO4
2- —> SO2 R +6 to +4

28. OXIDATION STATES - Review
CALCULATING OXIDATION STATE – MOST IMPORTANT
Q. What is the oxidation state of each element in the following compounds/ions ?
CH4
PCl3
NCl3
CS2
ICl5
BrF3
PCl4
+
H3PO4
NH4Cl
H2SO4
MgCO3
SOCl2

29. OXIDATION STATES
CALCULATING OXIDATION STATE - 2
Q. What is the oxidation state of each element in the following compounds/ions ?
CHC = - 4 H = +1
4 PClP = +3 Cl = -1
3 NClN = +3 Cl = -1
3 CSC = +4 S = -2
2 IClI = +5 Cl = -1
5 BrFBr = +3 F = -1
3 PCl+ P = +5 Cl = -1
4
H3PO4 P = +5 H = +1 O = -2
NH4Cl N = -3 H = +1 Cl = -1
H2SO4 S = +6 H = +1 O = -2
MgCO3 Mg = +2 C = +4 O = -2
SOCl2 S = +4 Cl = -1 O = -2