This the reaction that explains the loose or gain oxygen, hydrogen, electron transfer and the increase or decrease of oxidation number. In this slide, we also talk about the oxidation number: how it is being calculated, examples of element in a compound with their oxidation number

1. OXIDATION-REDUCTION
REACTION

4. TERMS USED IN OXIDATION-
REDUCTION
Term
Oxidation
Reduction
Meaning
To combine with oxygen
To lose hydrogen
To lose electrons
To increase in oxidation number
To lose oxygen
To combine with hydrogen
To gain electrons
To decrease in oxidation number

5. OXIDATION
The term oxidation was originally used to
describe reactions in which an element combines
with oxygen.
Example: The reaction between magnesium
metal and oxygen to form magnesium oxide
involves the oxidation of magnesium.
2Mg(s) + O2(g)  2MgO(s)

6. REDUCTION
The term reduction comes from the Latin
stem meaning "to lead back." Anything
that that leads back to magnesium metal
therefore involves reduction.
MgO(s) + C(s)  Mg(s) + CO(g)

7. THE ROLE OF OXIDATION NUMBERS IN
OXIDATION-REDUCTION REACTIONS
 Chemists eventually extended the idea of
oxidation and reduction to reactions that
do not formally involve the transfer of
electrons.
 CO(g) + H2O(g)  CO2(g) + H2(g)
 What changes in this reaction is the
oxidation state of these atoms. The
oxidation state of carbon increases from
+2 to +4, while the oxidation state of the
hydrogen decreases from +1 to 0.

9. Determine the oxidation
state of…
• H in H2O
• N in NH4
+
• S in S2O3
2-

10. Determine the oxidation state of…
Sum of all oxidation states of all atoms = Overall
of molecule / ion / atom
• H in H2O Let the oxidation state of H be x.
Thus, in HO, 2x + (-2) = 0
x = 1
Sum of all oxidation states of all atoms = Overall
of molecule / ion / atom
• N in NH4+ Let the oxidation state of N be x.
Thus, in NH4+, x + 4(+1) = +1
x = -3
Sum of all oxidation states of all atoms = Overall
of molecule / ion / atom
• S in S2O3
2- Let the oxidation state of S be x.
Thus, in S2O3
2-, 2x + 3(-2) = -2
x = +2

11. Questions: Identify the oxidation numbers of the atoms in
• H in HCO3
-
• N in N2
• Fe in Fe3+
• N in NO2
-
• O in H2O
• Cl in NaOCl
• Mn in MnO4
-
• V in VO2
+

12. PRACTICE QUESTIONS
 Calculate the oxidation number of Cu in
CuO(s) + H2(g)  Cu(s) + H2O()
 Calculate the oxidation number of C in
CaCO3(s)  CaO(s) + CO2(g)
 Calculate the oxidation number of S in
H2S + Cl2  2HCl + S
 Calculate the oxidation number of Mn in
MnO4
-(aq) + 5 Fe2+(aq) + 8 H+(aq)  Mn2+(aq) + 4 H2O() + 5 Fe3+(aq)

13. REMEMBER THIS PHRASE: LEO THE LION SAYS GER.
LEO = Loss of Electrons is Oxidation
GER = Gain of Electrons is Reduction
Another way is to simply remember that reduction is to
reduce the oxidation number.
Therefore, oxidation must increase the value.
Oxidizing Agent - that substance which oxidizes
somebody else. It is reduced in the process.
Reducing Agent - that substance which reduces
somebody else. It is oxidized in the process.

15. Oxidizing and Reducing Agents
Remember:
• An oxidizing agent is itself REDUCED
when it oxidizes something.
• A reducing agent is itself OXIDIZED when
it reduces something.
2Mg + O2 → 2MgO
• Mg is oxidized, and thus is the reducing
agent.
• O2 is reduced, and thus is the oxidizing
agent.

17. LIST OF COMMON OXIDIZING AND
REDUCING AGENTS
Oxidizing agents Reducing agents
Oxygen (O2) Hydrogen (H2O)
Chlorine (Cl2 → Cl-) Carbon (C)
Hydrogen peroxide (H2O2 → H2O) Hydrogen peroxide (H2O2 → O2)
Iron (III) salts (Fe3+ → Fe2+) Carbon monoxide (CO→ CO2)
Concentrated sulphuric acid (H2SO4 → SO2) Iodide to Iodine (I-→I2)
Dichromate (VI) ion in acid (Cr2O7
2-→ Cr3+) Sulphate (IV) to Sulphate (VI) (SO3
2- → SO4
2-)
Manganese (VII) ion in acid (MnO4
-→ Mn2+) Iron (II) salts (Fe2+ → Fe3+)

18. HOW TO IDENTIFY OXIDIZING AND REDUCING
AGENT IN A REACTION
Points to remember
The oxidizing and reducing agents are always on the
reactant side of the equation.
Consider H and O oxidation number when there are
no more atoms left for consideration.

19. Sample Question: Consider the redox reaction as represented by the following equation:
I2 + 2S2O3
2-  2I- + S4O6
2-
Which of the following is oxidized?
I2  I-
2I = 0 I = -1
I = 0 I = -1
0 to -1 (Decrease in oxidation number)
Therefore, I2 is the oxidizing agent. Therefore, reduced.
S2O3
2-  S4O6
2-
(2S) + (-6) = -2 (4S) + (-12) = -2
2S – 6 = -2 4S – 12 =-2
2S = 6 – 2 4S = 12 - 2
S = +2 S = +2.5
+2 to +2.5 (Increase in oxidation number)
Therefore, S2O3
2- is the reducing agent. Therefore, oxidized.

20. Sample Question: Consider the reaction represented by the following equation:
2Al + Fe2O3  Al2O3 + 2Fe
The oxidizing agent is?
Al  Al2O3
Al = 0 (2Al) + (-6) = 0
2Al – 6 = 0
2Al = 6
Al = 0 Al = +3
0 to +3 (Increase in oxidation number)
Al is the reducing agent.

21. Practice Questions
 Zn + 2Ag+  Zn2+ + 2Ag
Which substance in the reaction above is (i) Oxidized (ii) Reduced
 Zn + CuSO4  ZnSO4 + Cu
In the reaction above, identify the (i) Oxidizing agent (ii)
Reducing agent