OXIDATION AND REDUCTION Lesson Note
LEARNING OBJECTIVES
This SS2 Chemistry lesson note covers OXIDATION AND REDUCTION for First Term, Week 4.
Lesson Note
LEARNING OBJECTIVES
At the end of class, students should be able to
1) define oxidation and reduction in term of oxygen, hydrogen and electron gain and loss
A. DEFINITION
REDOX simply means Reduction and Oxidation. They occur together, when one is oxidized, another is reduced.
IINTRODUCTION TO OXIDATION
Is the process which involves the addition of oxygen or any other electronegatives element or a process which involve the removal of electropositives element or hydrogen.
B. DEFINITION OF OXIDATION
Oxidation can simply be define in four ways
In term of Oxygen:
Oxidation is addition of oxygen.
Example : 2Mg + O2 2 MgO i;e Mg is oxidized
Reduction is removal of oxygen.
Example: CuO + H2 Cu + H2O i:e CuO is reduced
In terms of Hydrogen:
Oxidation is removal of hydrogen.
Example: H2S + Cl2 2HCl + S i:e H2S is oxidized
Reduction is addition of hydrogen.
Example: C2H4 + H2 C2H6 i:e C2H4 is reduced
In term of Electron transfer.
Oxidation is loss of electron by an atom, ion or molecule. Therefore oxidation is also called De-electronation.
Example: Na Na+ + e-
Cu+ Cu2+ + e-
Pb2+ Pb4+ + 2e-
In the following reaction, positive charge on the species increases
In the following reaction, negative charge decreases.
2Cl- Cl2- + 2e-
S2- S + 2e-
Reduction is the gain of electron by atom, ions and molecules. Reduction is called electronation
Example: Na+ + e- Na
Cu2+ + 2e- Cu
Pb4+ + 2e- Pb2+
Fe3+ + e- Fe2+
In the following reaction, positive charge on the species decreases.
In the following reaction, negative charge increases
+ e- O2+
S + 2e- S2-
C. OXIDATION AND REDUCTION IN NATURE AND DAILY LIFE
i) Rusting of Iron: 4Fe + 3O2+ 2H2O ----- 2Fe2O3.xH2O
Iron is oxidized by oxygen in presence of water. This is a redox reaction.
ii) Combustion/Burning: Burning of wood, petrol, kerosene, candle is oxidation. Fuel is oxidized to CO2 and H2O.
iii) Photosynthesis: Plants reduce CO2 to glucose. 6CO2 + 6H2O ------- C6H12O6 + 6O2- Carbon is reduced.
iv) Respiration in Body: Glucose is oxidized in our body to produce energy. C6H12O6 + 6O2 ----> 6CO2 + 6H2O + Energy
v) Bleaching: Bleach (sodium hypochlorite) oxidizes coloured materials to colourless.
vi) Batteries/Phones: Rechargeable batteries work on redox reactions - charging and discharging involves oxidation and reduction at electrodes.
vii) Decay of food: Fats and oils are oxidized leading to rancidity.
D. EXAMPLES OF REDOX EQUATIONS
H2S + FeCl3 → 2FeCl2 + 2HCl + S
E. OXIDIZING AND REDUCING AGENTS
1. Oxidizing Agent: A substance which can readily accept electrons from other substances. It gains electrons, gives oxygen, removes hydrogen. Good oxidizing agents: O2, Cl2, Br2, HNO3, KMnO4, K2Cr2O7, H2O2.
NB. Oxidising agent carries out oxiation itself. Reduced in a chemical reaction
2. Reducing Agent: A substance which can readily lose electron to other substance. It itself is OXIDIZED. It loses electrons, gives hydrogen, removes oxygen. Good reducing agents: H2, C, CO, H2S, SO2, Metals like Na, Zn, Fe.
Hint: The agent is the opposite.
If X is oxidized, X is the reducing agent.
If Y is reduced, Y is the oxidizing agent.
In Zn + CuSO4 reaction:
Zn is oxidized -> Zn is Reducing Agent
CuSO4 (or Cu2+) is reduced -> Oxidizing Agent
F. CALCULATION OF OXIDATION NUMBER
Oxidation number of an element is defined as the charge which an atom of the element has in its ion or appears to have when present in the combined state with other atoms.
Example: Calculate the oxidation number and IUPAC of
S in H2SO4 :
Solution: To determine the oxidation number of S in H2S . The valence electron of hydrogen is 1 and oxygen is -2
H2SO4 = 1×2 + S + (-2 × 4) = 0
2 + S -8 = 0
2 + S = -8
S = -8 + 2
S = ± 6
IUPAC: Hydrogen tetraoxosulphate (iv) acid
Evaluation
OBJECTION QUESTIONS
1. Oxidation is defined as:
A) Gain of electrons
B) Loss of electrons
C) Gain of hydrogen
D) Decrease in oxidation number
2. Which of these is an example of oxidation in daily life?
A) Photosynthesis
B) Rusting of iron
C) Freezing of water
D) Dissolving sugar
3. In the reaction CuO + H2 -> Cu + H2O, CuO is:
A) Oxidized
B) Reduced
C) Oxidizing agent
D) None
4. The oxidation number of free elements like O2, Na is:
A) +1
B) -2
C) 0
D) +2
5. The oxidation number of oxygen in H2O2 (hydrogen peroxide) is:
A) -2
B) -1
C) +1
D) 0
6. A reducing agent is a substance that:
A) Gains electrons
B) Is reduced itself
C) Is oxidized itself
D) Gives oxygen
7. Find the oxidation number of S in H2SO4:
A) +4
B) +6
C) -2
D) +2
8. Which of the following is a good oxidizing agent?
A) H2
B) CO
C) KMnO4
D) Zn
9. In the reaction 2Fe2+ + Cl2 -------> 2Fe3+ + 2Cl-, Fe2+ is:
A) Reduced to Fe3+
B) Oxidized to Fe3+
C) Reducing agent
D) Both B and C
10. The oxidation number of Cr
in K2Cr2O7
A) +3
B) +6
C) +7
D) +2
Theory Questions
1. a) Define oxidation and reduction in terms of:
i) electrons
ii) oxygen
b) In the reaction: Zns + CuSO4(aq) → ZnSO4(aq) + CuS
i) Which element is oxidized? Why?
ii) Which element is reduced? Why?
iii) Identify the oxidizing agent and reducing agent.
2. a) State 6 rules for assigning oxidation numbers.
b) Calculate the oxidation number of the elements in:
i) MnO2
ii) Na2SO3
iii) Cr2O7
iv) NH4